Reproductive system of the lancelet. Study of cross sections of the lancelet body

Studying the object

Examine the external structure of the lancelet using a magnifying glass on whole adult fixed specimens.

The lancelet lives in the coastal strip of the seabed. Usually it lies on the ground. The body length is 3-8 cm. It buries itself in the sand with its rear end. Body color is whitish. The ventral side is wider, and the dorsal side is narrow. Toward the rear end, the entire body is pointed in the shape of a lancet, hence the name of the animal. The dorsal fin bends around the rear end of the body and forms the caudal fin and passes on the ventral side into a short ventral fin. The oral tentacles are clearly visible at the anterior end.

From the oral tentacles, two clearly visible metapleural folds stretch along the sides of the ventral side - right up to the ventral fin. In the place where they come into contact with the ventral fin, there is an opening of the peribranchial cavity, called atrioporom.

Leather formed by single-layer mucous epithelium and dermis, located under a thin layer of gelatinous connective tissue.

The entire body of the lancelet is transparent and many organs are clearly visible in the transmitted light of a microscope (Fig. 1).

The muscular system of the lancelet is metameric (a characteristic of invertebrates), consisting of muscle segments myomer. Myomeres are visible through the skin and it is clear that they are separated from each other by thin partitions - myoseptami. A layer of transverse muscles lies across the ventral side of the lancelet.

Rice. 1. General form and the location of the internal organs of the lancelet:

1 – tactile tentacles, 2 - preoral funnel, 3 – velar tentacles, 4 – chord, 5 – neural tube, 6 - pharynx with gill slits, 7 – hepatic outgrowth, 8 - intestine, 9 – atriopor, 10 – subcaudal fin, 11 – metapleural fold, 12 - gonads, 13 - muscles, 14 – myomer, 15 – myosepta, 16 – caudal fin, 17 - Hesse's eyes, 18 - anal opening.

Rice. 2. The head section of the lancelet:

1 – chord, 2 – neural tube, 3 - olfactory fossa, 4 – sail (ring-shaped fold separating the oral cavity from the pharynx), 5 – velar tentacles, 6 – preoral tentacles, 7 - Hesse's eyes.

On the dorsal side of the body, find chord, which goes far to the head end. The notochord, or dorsal string, is the axial skeleton of the lancelet’s body. It represents a light, vertically striated rod that stretches along the dorsal side from the anterior end of the body to the posterior.

Located above the chord neural tube, there is a cavity inside it - neurocoel.

Heads and skulls the lancelet does not (hence the name skullless). Place the specimen under a low magnification microscope and examine the dotted black dots on the neural tube – Hessian eyes(photosensitive organs) (Fig. 2).

In the anterior half of the body, under the chord, there is peribranchial cavity outward opening atrioporom.

Digestive and respiratory system lancelet are closely related. The walls of the pharynx are pierced by numerous (up to 150 pairs) obliquely located gill slits. The pharynx extends to about half of the lancelet's body. Water is driven by the tentacle first into the preoral funnel, then into the oral cavity and pharynx, through the gill slits into the peribranchial cavity, and finally exits through the atriopore to the outside.

Food brought into the pharynx with a stream of water does not exit with water through the gill slits. At the bottom of the pharynx there is a subbranchial groove - endostyle. Food lumps, once in the pharynx, are enveloped in mucus and are carried to the bottom of the pharynx. Thanks to the work of the ciliated epithelium of the endostyle, food lumps move deeper along it and enter the midgut. The hindgut is a tube that ends in the anus on the left side of the back of the lancelet's body.

The cecum arises from the lower part of the middle intestine hepatic outgrowth. Find it by changing the lighting. On a preparation of a whole specimen, the hepatic outgrowth is noticeable in the form of a yellowish body visible through the gill section. When water passes through numerous gill slits, gas exchange occurs - oxidation of venous blood in the vessels located in the gill septa.

Consider circulatory system lancelet according to the diagram (Fig. 3). It is closed, there is no heart, there is only one circulation. The blood is colorless. The function of the heart is performed by the abdominal aorta, located under the pharynx. Venous blood collected in it from all over the body is pushed into the branchial arteries by contractions of the walls of the abdominal aorta. Gas exchange occurs in them. Blood enriched with oxygen flows through the efferent gill arteries into the paired epibranchial vessels - aortic roots.

The arrows show the direction of blood flow; the veins and abdominal aorta are colored black.

Rice. 3. Diagram of the circulatory system of the lancelet:

1 - abdominal aorta, 2 – gill arteries, 3 - roots of the aorta, 4 – carotid arteries, 5 – dorsal aorta, 6 - anterior cardinal veins, 7 – posterior cardinal veins, 8 – ducts of Cuvier, 9 – venous sinus, 10 - subintestinal vein, 11 – portal system of the hepatic outgrowth, 12 – hepatic vein, 13 - tail vein.

Paired pairs extend from them into the anterior section carotid arteries. In the second half of the body, the roots of the aorta merge, forming an unpaired dorsal aorta, which extends to the caudal section. Arteries and capillaries extend from the dorsal aorta, through which cellular gas exchange and metabolism occur. The waste blood enters the veins through capillaries. From the anterior part of the body, venous blood collects in paired anterior cardinal veins. In them, blood flows from the front end to the back. The veins of the posterior end of the body form paired posterior cardinal veins, in which the blood moves to the anterior end of the body. Somewhat behind the pharynx, the anterior and posterior cardinal veins merge through the ducts of Cuvier and through the venous sinus the blood again enters the abdominal aorta.

In the posterior part of the lancelet's body, in addition to the posterior cardinal veins, there is an azygos intestinal vein. It forms a capillary network in the hepatic outgrowth, which is called portal system of the hepatic outgrowth. The capillaries of the hepatic outgrowth unite to form the hepatic vein, which flows into the abdominal aorta.

Excretory system lancelet – paired nephridia with solenocytes. Nephridia in the number of 90 pairs are located above the pharynx and open at one end as a whole, and at the other into the atrial cavity. Nephridia are not visible on conventional study preparations. Look at them in the picture.

Lancelets are dioecious. Find the gonads, 25–26 pairs located on the sides of the body in the form of dark round or oval spots. They are visible through the abdominal wall of the body. In immature lancelets, the gonads are not visible. Males have gonads with fine-grained contents, while females have coarse-grained contents. Gonads do not have excretory ducts. The germ cells enter the atrial cavity through a rupture in the walls of the gonads and the walls of the body and are expelled with water through the atriopore. Fertilization is external. Embryonic development proceeds very quickly. Development with metamorphosis. The larva is mobile, up to 3 mm. The number of gill openings is 14 pairs. The larval stage lasts about 3.5 months.

Study of cross sections of the lancelet body

On a cross-section of a lancelet in the pharynx area, examine the relative position of the organs and the structural details of the animal under low magnification under a microscope (Fig. 4). On a preparation of a cross-section of a lancelet in the intestinal area (Fig. 5), examine the structural features of the notochord, neural tube, connective tissue membrane, intestine, coelom and compare with the previous preparation.

When studying sections, it is necessary to compare them with the drawing. Please note that the body of the lancelet is covered with a single layer epithelium(as in invertebrates). The epithelium (epidermis) is covered on top cuticle. Under the epithelium is cutis. The epithelium and cutis make up the skin of the lancelet. On the dorsal side, find the dorsal fin, examine the myomeres separated by myoseptae. Between the myomeres the notochord is located in the form of a large oval. A section of the neural tube with a neurocoel is visible above the notochord.

Visible under the chord pharynx, consisting of gill septa separated by gill slits.

On the ventral side of the pharynx it is clearly visible endostyle, lined with glandular and ciliated cells. On some preparations, a hepatic outgrowth is visible on the side of the pharynx.

Determine the sex of the lancelet based on the contents of the gonads. The coarse contents are the eggs of females. In males, the gonads are filled with numerous small reproductive cells. The abdominal side is represented by metapleural folds and an unsegmented transverse muscle located under the skin.

Control questions

1. Name the characteristics that distinguish chordates from representatives of other types.

2. Describe the external structure of the lancelet and explain its adaptations to its habitat.

3. What is the skin of the lancelet formed by?

4. How does reproduction occur in the lancelet?

Rice. 4. Cross section of a lancelet in the pharynx area:

1 – dorsal fin, 2 – metapleural folds, 3 – epidermis, 4 – cutis, 5 – chord, 6 – neural tube, 7 - Hesse's eyes, 8 – gelatinous membrane of the notochord, 9 – myosepta, 10 – myomer, 11 - pharyngeal cavity, 12 - gill slit, 13 – interbranchial septum, 14 – endostyle, 15 - epibranchial groove, 16 – hepatic outgrowth, 17 – gonad, 18 - atrial cavity, 19 – coelomic cavity, 20 - transverse muscles.

Rice. 5. Cross section of lancelet in the intestinal area:

1 – dorsal fin, 2 – subcaudal fin, 3 – epidermis, 4 – cutis, 5 – chord, 6 – neural tube, 6a– neurocoel, 7 – eyes of Hesse, 8 – jelly shell of notochord, 9 – myosepta, 10 – myomere, 11 - intestinal wall 12 - intestinal cavity, 13 – coelomic cavity.

The lancelet is a representative of the phylum Chordata. It looks like a simple translucent small (up to 10 cm) fish. It got its name for its resemblance to a lancet (scalpel). There are about 30 species of lancelets; they live mainly in the seas of tropical and temperate climates in sandy shallow waters (they are bottom animals). The lancelet exposes only the front part of its body. It feeds on small organisms (plankton) and organic particles that sink to the bottom.

The lancelet is important to science because it allows us to understand the origins of vertebrates. Most likely a close relative of their ancestor.

Throughout its life, the lancelet retains the basic characteristics of the Chordata type. First of all, this is the notochord, which in more highly organized representatives is replaced by the spine at the stage of embryonic development. The lancelet neural tube does not differentiate into the spinal cord and brain.

Lancelets belong to the subtype Skullless. These animals do not have a skull. This subtype is otherwise called Cephalochordates. This is due to the fact that the notochord (in the absence of a skull) extends into the head section (although the head is not separate).

The lancelet is a rather primitive animal compared to other chordates belonging to the Vertebrate (or Cranial) subphylum. It has a number of characteristics that unite it with invertebrates (especially annelids). However, on the other hand, the lancelet has all the main characteristics of the Chordata type, which is why it is classified as the latter.

The body of the lancelet is laterally compressed and pointed at the ends. The caudal fin is quite large and resembles the cutting part of a lancet. The dorsal fin (fold of skin) is low. The lancelet does not have paired fins, like fish. Bending its body from side to side, it swims and buries itself in the sand. The preoral opening is surrounded by small tentacles.

The skin forms a single-layer mucous epithelium and a layer of connective dermis.

Notochord and overlying neural tube protected by connective tissue. There are sensory and motor nerves that arise in pairs from the neural tube. Light-sensitive cells are found in nervous tissue. The lancelet can only distinguish between light and dark. At the anterior end of the neural tube there is an olfactory fossa. The skin contains tactile cells.

From the chord on the sides there is a ribbon of muscles. Each band is vertically divided into parts by connective tissue. As a result, muscle metamers are formed - myomeres.

The oral opening is located at the anterior end of the body from below at the bottom of the preoral funnel. Following the mouth is the pharynx, which is lined above and below with ciliated epithelium, and on the sides has gill slits (about 100 pairs of them). The food, along with the flow of water, enters the pharynx. Water is removed through the gill slits. Food particles stick to the mucous membrane of the inner surface of the pharynx. Lumps of food stuck together with mucus then follow into the intestines. The intestine is straight, without bends. In the anterior part of the intestine there is a hepatic outgrowth.

1. Brain vesicle. 2. Chord. 3. Neural tube. 4. Caudal fin. 5. Anus. 6. The hind intestine is in the form of a tube. 7. Circulatory system. 8. Atriopore (opening of the peribranchial cavity). 9. Periopharyngeal cavity. 10. Gill slit. 11. Throat. 12. Oral cavity. 13. Perioral tentacles. 14. Preoral opening. 15. Gonads (ovaries/testes). 16. Eyes of Hesse. 17. Nerves. 18. Metapleural fold. 19. Blind hepatic outgrowth

The gills are protected by a pair of skin folds that form peribranchial cavity, which performs protective function(to prevent sand from getting in). On the ventral side of the body, the peribranchial cavity has an unpaired opening through which water and waste products exit.

At the lancelet hclosed circulatory system. The two main vessels are the abdominal (with venous blood) and the dorsal (with arterial blood). The body and especially the pharynx in the area of the gills are penetrated by smaller vessels. Gas exchange occurs by diffusion. Oxygen and carbon dioxide go where there is less of them. That is, carbon dioxide leaves the venous blood in the gills and oxygen enters. Blood is colorless because it contains no formed elements. Lancelets do not have a heart, and the role of a pump is performed by branches of the abdominal vessel near the gills.

Excretory organs -metanephridia. Similar to the excretory organs of annelids. These are tubes (about 100 pairs), inside they open as a whole, outwards - into the pericobranchial cavity. Located metamerically in the pharynx.

Lancelets pseparate sexes, breed in the warm season. The gonads (ovaries and testes) are paired. Eggs and sperm are swept into the peribranchial cavity, and from there into the external environment. Fertilization is external. From the fertilized egg, a larva develops, which floats in the water column and after some time settles and turns into an adult.

Lancelets serve as food for marine animals. There is a fishery for Asian lancelet.

CLASS LANCELANDS. LANCELET

Structurebodies. Fish-like shape, 4-8 cm long. At the head end there is a mouth with tentacles, a fin runs along the back, turning into caudal and sub-caudal fins. The skull is missing. The skeleton is internal, represented by a notochord (cord in a dense shell). The body is segmented, the muscles are well developed.

Cover. Single-layer epidermis, underneath is a thin layer of connective tissue.

Body cavity. Secondary.

Digestive system. Includes the mouth, oral cavity, pharynx, midgut, where the liver duct flows, hindgut, and anus. No stomach. It feeds on invertebrates that enter the mouth with a current of water.

Respiratory system. The gills are in the form of long oblique slits on the wall of the pharynx. The gills are protected by a peribranchial cavity, which has an opening on the ventral side. Water enters the mouth and exits into the peribranchial opening.

Circulatory system. Closed, represented by dorsal and abdominal vessels and capillaries. There is no heart; its role is played by the abdominal vessel, through which blood moves to the gills. The blood is colorless, there is no hemoglobin. Blood carries nutrients and gases (O2, CO2) throughout the body -

Excretory system. Excretory tubes arranged in segments. One end of each tube opens into the body cavity, the other into the peribranchial cavity. It is weakly connected to the circulatory system.

Nervous system. The central nervous system is in the form of a tube located on the dorsal side of the body above the notochord. There is a channel running inside the tube. In each segment of the body, a pair of nerves arise from the tube.

Sense organs. Very primitive. Light-sensitive cells are located along the neural tube; in the superficial layer of the skin there are nerve cells that perceive chemical irritations. Taste and smell are picked up by touch cells throughout the body.

Reproduction. Dioecious animals. Females have ovaries, males have testes, located in segments (25 pairs). The germ cells exit through the peribranchial cavity into the water, fertilization is external.

Development. Happens in water. A blastula develops from the zygote, then a gastrula, after which the larva emerges from the egg and develops for about three months. It actively feeds on invertebrate animals - zooplankton. After which it sinks to the bottom and as an adult leads a rather passive lifestyle, burrowing into the ground.

Rice. Lancelet: 1 - mouth with tentacles, 2 - pharynx with gill slits, 3 - liver, 4 - intestine, 5 - anus, 6 - muscles, 7 - notochord, 8 - neural tube

Using a stained total preparation of a lancelet, we will examine the structure and relative position of the main organs of the animal (Fig. 6).

A chord (axial skeleton) stretches along the entire body of the lancelet (approximately along the midline) from the head to the tail. Its transverse striations are clearly visible on the preparation. The notochord, together with the neural tube, is surrounded by a connective tissue membrane. The anterior end of the notochord protrudes far beyond the anterior edge of the neural tube, which is a specific feature of skullless animals.

Above the notochord is the central nervous system, represented by the already mentioned neural tube. By placing the specimen under a low magnification microscope, you can see numerous dark spots - Hessian eyes (Fig. 6), which are located in the walls of the neural tube closer to its inner surface.

At the anterior end of the neural tube, a cephalic extension of the neural tube can be seen, which is sometimes called the cerebral ventricle, although the anterior part of the lancelet neural tube, corresponding to the vertebrate brain, is not differentiated.

The digestive tube is located under the notochord. It begins with a preoral funnel, surrounded by numerous tentacles. The oral cavity is separated from the pharynx by a ring-shaped fold - parus. The walls of the pharynx are penetrated by numerous (up to 100) obliquely located gill slits (Fig. 6). The pharynx gradually passes into an undifferentiated intestinal tube, ending in the posterior part of the body with the anus.

From this tube, immediately behind the pharynx, a hepatic outgrowth extends forward, the main part of which is located on the right side of the pharynx.

Figure 7 – Cross section of a lancelet in the pharynx area

1 – epidermis; 2 – dorsal fin; 3 – metapleural folds;

4 – trunk muscles; 5 – abdominal muscle; 6 – chord,

7 – neural tube; 8 – neurocoel; 9 – neural tube slit;

10 – connective tissue membrane; 11 – myosepta;

12 – interbranchial septa; 13 – endostyle; 14 – epibranchial groove; 15 – coelomic channels; 16 – atrial cavity;

17 – hepatic outgrowth; 18 – gonads; 19 – roots of the aorta.

In mature lancelet specimens, gonads are visible (usually 25 pairs of them), which in the form of dark round spots are visible through the abdominal wall of the body.

Using a cross-section of a lancelet in the pharynx area, we will examine, under low microscope magnification, the relative position of the organs and the structural details of the animal (Fig. 7).

On the dorsal side, the cut of the low dorsal fin is clearly visible. On the sides of the body there are metapleural folds fused under the pharynx.

Approximately in the center of the preparation there is a chord, which has an oval shape when cut. Above the notochord lies the neural tube with a clearly visible neurocoel. The notochord and neural tube are surrounded by a connective tissue membrane, from which myosepta extend (several of them are visible in a cross section).

The lower part of the preparation is a section of the pharyngeal (branch) section of the digestive tube and the surrounding atrial cavity. In cross-section, the pharynx is pierced by a large number of gill slits. An endostyle is visible at the bottom of the pharyngeal region. On the dorsal side of the pharynx there is a suprabranchial groove. On both sides of the epibranchial groove, sections of two blood vessels - the roots of the aorta - are clearly visible. The hepatic outgrowth is visible only on those sections that are made closer to the posterior end of the pharynx. In sexually mature individuals, the gonads are located on the inner walls of the metapleural folds.

Using a preparation of a cross-section of a lancelet in the intestinal area (Fig. 8), we will consider the structural features of the notochord, neural tube, connective tissue membrane, intestine, coelom and compare the relative position of these organs with what was seen in the previous preparation.

Figure 8 – Cross-section of the lanceolate in the intestinal area

1 – epidermis; 2 – dorsal fin; 3 – metapleural folds;

4 – musculature; 5 – chord; b – neural tube; 7 – neurocoel;

8 – connective tissue membrane; 9 – intestines; 10 – dorsal aorta.

The study of the circulatory system of the lancelet should be carried out using tables and drawings (Fig. 9), since on preparations blood vessels not visible.

As already mentioned, the lancelet does not have a heart and is replaced by an unpaired abdominal aorta. Its walls are formed by striated muscles, which ensures the pulsation of the aorta. Blood moves from the abdominal aorta to the branchial arteries, which are located in the interbranchial septa. Venous blood, passing through the gill arteries, is oxidized directly through the thin walls of these vessels located near the surface of the gill slits. Blood enriched with oxygen is collected in the paired roots of the aorta. Some of the blood from them goes forward through the small carotid arteries, and the bulk goes to the tail. Approximately in the middle of the body, the roots of the aorta merge into the main trunk canal - the dorsal aorta, through which blood is carried throughout the body.

Figure 9 – Diagram of the circulatory system of the lancelet

1 – abdominal aorta; 2 – branchial arteries; 3 – aortic roots;

4 – carotid arteries; 5 – dorsal aorta; 6 – anterior cardinal veins; 7 – posterior cardinal veins; 8 – ducts of Cuvier; 9 – venous sinus; 10 – subintestinal vein; 11 – portal system of the hepatic outgrowth; 12 – hepatic vein

Venous blood from the head part of the body moves backward through the paired anterior cardinal veins, and from the tail part through the posterior cardinal veins forward. The anterior and posterior cardinal veins on each side of the body merge into the thin-walled duct of Cuvier. Both of these ducts empty into the venous sinus.

From the digestive organs, blood collects in the intestinal vein, which in the hepatic process breaks up into a network of capillaries, forming the portal system of the hepatic process. Through the short hepatic vein, blood flows into the venous sinus.

General characteristics. Skullless are marine, predominantly bottom-dwelling animals that retain the basic characteristics of the type throughout their lives. Their organization represents, as it were, a diagram of the structure of a chordate animal. The skullless are of great interest for solving the question of the origin of animals. Science owes its knowledge of the skullless primarily to the research of A. O. Kovalevsky.

Structure and vital functions. Of the relatively small number of representatives of skullless species (about 20 species), the most common and well studied is lancelet (Ampliio-xuslanceolatus)(Fig. 200). This small animal (up to 8 cm long) lives in the shallow waters of the seas, burrowing into the sand and exposing the anterior part of its body. It feeds on small food particles that sink to the bottom.

The body shape of the lancelet is elongated, laterally compressed, pointed in front and behind. A low longitudinal fold of skin stretches along the back - the dorsal fin. At the posterior end of the body there is a spear-shaped caudal fin. There are no paired limbs.

The skin is formed by a single-layer mucous epidermis and connective tissue dermis.

The skeleton is represented by a chord stretching along the body, thinning towards the ends. The notochord and the neural tube lying above it are surrounded by a connective tissue membrane.

The muscles stretch in ribbons on both sides of the body. These muscle bands are metamerically divided by thin connective tissue septa (myosepta) into a number of myomeres.

The central nervous system is of a primitive structure (Fig. 201). It has the appearance of a tube, the neurocoel of which in the anterior part forms a cavity representing the rudiment of the cerebral ventricle. From the central nervous system, dorsal - motor-sensory and abdominal - motor nerves depart in pairs, which do not connect into common mixed nerves, as in vertebrates.

Rice. 200. Lancelet (diagram):

/ - preoral funnel surrounded by tentacles; 2 ~- caudal fin; 3 - dorsal fin; 4 - subcaudal fin; 5 - opening of the peribranchial cavity;
V- gonads; 7 - number; 8 - myosepta

Rice. 201. Internal structure of the lancelet:

/ - longitudinal section of the body; // - cross-section of the body (in the pharynx and intestinal area);

/ - chord; 2 - spinal cord; 3 - dorsal; 4 - number; 5 - in general; b- pharynx; 7 - gill slit; 8 - interbranchial septum; 9 - endostyle; 10 - peribranchial cavity; // - opening of the peribranchial cavity; 12 - liver; 13 - intestine; 14 - nephridium; 15 - anus; 16 - dorsal aorta; 17 - subintestinal vein; 18 - gonads

The sense organs are primitive. Along the central nervous system there are light-sensitive cells - the eyes of Hesse. At the anterior end of the body there is an olfactory fossa. The perioral tentacles perform a tactile function.

The digestive system begins with a preoral funnel surrounded by tentacles. At the bottom there is a mouth leading into a large pharynx. The top and bottom of the pharynx are grooves lined with ciliated epithelium. By the movement of the cilia of the lower groove - eidostylium - food particles that enter the pharynx first move forward, and then along the dorsal groove of the pharynx - to the intestine. The endostyle in vertebrates is modified into the thyroid gland of internal secretion. The intestine stretches without bends or noticeable expansions from the pharynx to the anus. A hepatic outgrowth extends from the anterior part of the intestine, which is homologous to the liver of vertebrates.

The respiratory organs are the partitions between the numerous gill slits that cut through the walls of the pharynx. In some deep-sea forms of skullless fish, the gill slits open outward. In the common lancelet, which lives in the sand of shallow waters, they lead into a large circumbranchial cavity. The latter is formed in the embryo by fusion of two lateral folds of skin along the midline of the abdomen. Water entering through the gill slits from the pharynx into the peribranchial cavity is removed from it through an unpaired opening (antriopore) on the ventral side of the body.

The circulatory system is closed (Fig.

Subtype Skullless - Acrania Lancelet

202). There is one circle of blood circulation. There is no heart, and the blood moves due to the pulsation of some large vessels. The abdominal aorta stretches under the pharynx, from which the afferent branchial arteries extend in both directions, carrying venous blood to the interbranchial septa. Through the thin covers of the latter, the blood absorbs oxygen dissolved in water. Oxidized arterial blood through the efferent gill arteries enters the paired epibranchial vessels - the roots of the dorsal aorta, which merge behind the pharynx into the dorsal aorta. The dorsal aorta extends backward over the notochord, giving off branches< различным органам задней половины тела. Наджаберные сосуды продолжаются вперед сонными артериями, снабжающими кровью головной отдел животного.

Venous blood flows from the intestine through the intestinal vein to the hepatic outgrowth and breaks up in the walls into capillaries, forming the portal system of the liver. Blood leaves the liver through the hepatic vein, which flows into the venous sinus, which lies at the root of the abdominal aorta. The large ducts of Cuvier also flow into the sinus on the left and right. They are formed by the fusion of the anterior and posterior paired cardinal veins, which carry blood from the anterior and posterior parts of the body. From the venous sinus, blood enters the abdominal aorta. This closes the circle of blood circulation.

Rice. 202. Lancelet blood circulation diagram:

/ abdominal aorta; 2 expansion of the base of the afferent gill arteries;
,4 — branchial arteries; 4 —
roots of the spinal aorta; 5 - carotid arteries; 6 - dorsal aorta; 7 tail foam; Lnodknshechnan vein; 9 — portal system of the liver; 10 - peribranchial vein; // - anterior cardinal vein;
12 — rear cardinal foam; 13 - Cuvier influx

The excretory organs are represented by modified metanephridia, located metamerically in the pharynx. Their outer ends open into the peribranchial cavity.

The reproductive organs look like two rows of paired gonads. The ovaries of females and the testes of males form rows of tubercles on the walls of the body cavity in the region of the gill slits. Reproductive products are excreted into the peribranchial cavity.

Phylum Chordata

The subphylum unites lower chordates. The head section of the body is not separate, the skull is absent, the skeleton is represented by a notochord. The nervous system has the form of a tube, the sensory organs are primitive - there are only sensory cells in the skin and along the neural tube. The circulatory system is closed; the function of the heart is performed by a pulsating abdominal vessel. The entire body is segmented, including some internal organs, the excretory system and the gonads. Body size is small. Distributed mainly in the equatorial zone of the Pacific, Indian and Atlantic Oceans. Bottom animals feed on small planktonic and benthic organisms suspended in water. Some skullless fish are objects of local fishing, for example, in a number of countries the Asian lancelet is eaten.

Class cephalochordates

Includes about 35 species of small marine animals with body shapes resembling fish. The body length of adult animals is from 1 to 8 cm.

Lancelet (Branchiostoma lanceolatum)- a typical representative is a translucent warm-water animal 4-8 cm long, with smooth skin consisting of the epidermis and the skin itself (corium). Lives mainly on sandy areas of the bottom at depths of 10-30 m in the Black Sea, Atlantic, Indian and Pacific oceans. The animal buries itself in the sandy soil, with the anterior end of its head sticking out. One of the main features of skullless animals is the absence of a jaw apparatus; Passive nutrition is associated with this. The lancelet uses as food only those marine organisms that enter the mouth with water.

The body is lanceolate, laterally compressed and pointed at both ends. A skin fin fold runs along the body, in which a dorsal fin is distinguished, which passes into the lanceolate caudal and subcaudal (anal) sections.

The axial skeleton is represented by a chord that stretches from the anterior to the posterior end of the body. Muscle has a metameric structure. It is adjacent to the notochord and is divided into myomeres, consisting of striated muscles. Myomeres are separated from each other by connective tissue layers - myosepta. Contraction of myomeres leads to bending of the lancelet's body from the horizontal plane.

The neural tube is located above the notochord, it is shorter than the notochord, its anterior end slightly does not reach the end of the notochord (hence the name of the class - cephalochordates). The neural tube is not differentiated into the brain and spinal cord, but in the head region the neural tube forms a small extension - the rudiment of the brain. The central cavity of the tube (neurocoel) expanded in this place is called the ventricle. The anterior end of the lancelet's neural tube innervates the anterior end of the body and sensory organs, and also coordinates the life activities of the animal.

Sense organs are poorly developed. At the anterior end of the body there is a pigment spot, or an unpaired “eye” (it is assumed that this is a remnant of the balance organ); along the entire tube there are special light-sensitive pigment cells - the eyes. In addition to the ocelli, there are tactile cells on the oral tentacles and in the skin. The peripheral nervous system is represented by nerves extending from the brain tube.

The circulatory system of the lancelet is closed, there is only one circulation, there is no heart. Physiologically, it is replaced by a pulsating abdominal aorta, from which more than a hundred branchial arteries arise. As a result of vascular pulsation, blood from the abdominal aorta enters the branchial arteries. The latter do not break up into capillaries; gas exchange occurs through the walls of the arteries in the partitions between the gill slits. Oxidized blood first collects in the paired roots of the aorta, which pass into an unpaired vessel - the dorsal aorta, stretching backward under the chord. From the roots of the aorta to the anterior end of the body, blood flows through the carotid arteries, and the posterior end of the body receives it from the dorsal aorta. Venous blood collects in paired anterior and posterior cardinal veins, which unite in the middle part of the body to form the ducts of Cuvier. From the intestine, venous blood enters the intestinal vein, through which it moves from the posterior end to the anterior. The intestinal vein approaches the hepatic outgrowth and forms a capillary system in it - the portal system. Blood then flows from the hepatic vein and ducts of Cuvier into the abdominal aorta.

The digestive system is closely connected with the respiratory organs. Both of these systems begin with a preoral opening surrounded by a corolla of tentacles. It leads into the perioral funnel, at the bottom of which the mouth is located. The oral opening passes into the pharynx. The wall of the pharynx on the right and left is perforated by gill slits (over 100), which open into the atrial (peribranchial) cavity, which is connected to the external environment through an unpaired outlet (atriopore). The inside of the pharynx is covered with ciliated cells. Thanks to the movement of the cilia, water entering the pharynx penetrates through the slits into the peribranchial cavity, and from there out through the atriopore. Gas exchange occurs in the vessels of the interbranchial septa.

Along with water, algae, protozoa and other microscopic organisms enter the pharynx and settle on the endostyle, a groove lined with cilia located on the ventral side of the pharynx that secretes a sticky liquid. Food particles deposited on the endostyle are glued together by droplets of mucus and move into the posterior part of the pharynx - the intestine, which is a straight tube that opens outward through the anus. Behind the pharynx, a hollow, blind growth extends from the initial part of the intestine - the liver; the cells of its walls secrete digestive enzymes. Digestion of food occurs in the cavity of the hepatic outgrowth and in the intestines. Undigested residues are expelled through the anus.

Subtype Skullless. Class Lancelets

The excretory organs are represented by a large number (about 100 pairs) of nephridia located in the gill region and their structure is very similar to the metanephridia of annelids.

The genital organs have a metameric structure (segmented). Lancelet and other skullless dioecious species. Sexual dimorphism is not expressed. The release of mature eggs and sperm occurs immediately after sunset, fertilization is external (in water). Larvae about 3 months old. They live in the water column, feeding on planktonic animals. Then the larva sinks to the bottom. The lancelet reaches sexual maturity in the 2-3rd year of life.

The peculiarities of the embryonic development and structure of the lancelet were studied by A. O. Kovalevsky, who established the proximity of these animals to the most ancient ancestor of vertebrates.

Kovalevsky Alexander Onufrievich (1840-1901)- Russian zoologist-evolutionist, academician. Petersburg Academy of Sciences. One of the founders of comparative embryology and physiology of invertebrates and lower chordates. He substantiated the unity of origin of multicellular animals, developing the theory of germ layers, and established the nature of the formation of mesoderm and the secondary body cavity. Embryological studies of the development of lancelets, ascidians, ctenophores, and holothurians made it possible to draw conclusions about the position of these animals in the system of the animal world. Twice he was awarded the prize. K. M. Bera. At the end of his scientific career, he studied excretory organs and phagocytosis in invertebrate animals.

TYPE CHORDATA

SUBTYPE CRANIAL (ACRANIA)

CLASS cephalochordates (CEPHALOCHORDATA)

Lesson 1. EXTERNAL AND INTERNAL STRUCTURE OF THE LANCELETA

Systematic object position

Phylum Chordata

Subtype Skullless (Acrania)

Class Cephalochordata

Representative - lancelet (Branchiostoma lanceolatum Pall.)

Equipment and materials

1. Fixed specimens of adult lancelets, placed in watch glasses with water or in a transparent vessel with a fixing liquid (according to the number of participants).

2. Finished preparations: 1) total preparation of lancelet; 2) a transverse incision in the pharynx; 3) a transverse incision in the intestinal area (according to the number of participants).

3. Tables: 1) appearance lancelet; 2) general location of internal organs; 3) transverse incision in the pharynx; 4) transverse incision in the intestinal area; 5) circulatory system.

4. Microscopes with low magnification (according to the number of students).

5. Tabletop tripod magnifiers with 10x magnification (according to the number of students).

Introductory Notes

The lancelet is a representative of the skullless, most primitive chordates. All the main characteristics of the chordate type in skullless animals are well expressed and preserved for life. The notochord functions as an axial skeleton, the central nervous system is represented by the neural tube, and the pharynx is penetrated by gill slits.

There is a secondary mouth and a secondary body cavity - the whole. Metamerism persists in a number of organs. Skullless animals are characterized by bilateral (bilateral) body symmetry. All these features indicate a phylogenetic connection between skullless animals and certain groups of invertebrate animals (annelids, arthropods, echinoderms, etc.).

In addition, skullless animals, and in particular the lancelet, are characterized by a number of specific primitive features that distinguish them well from other chordates. These differences are as follows.

The epidermis of skullless animals is single-layered, covered with a thin cuticle. Among the cells of the epidermis there are individual sensory and glandular cells. Cutis is poorly expressed and is represented by a thin layer of gelatinous tissue.

The central nervous system is not differentiated into the brain and spinal cord. Due to the absence of a brain, there is no skull. The sense organs are poorly developed: there are tactile cells scattered over the surface of the body, light-sensitive formations - the ocelli of Hesse, located in the walls of the neural tube, and the olfactory fossa lying in the head section on the surface of the body.

The gill slits do not open outward, but into the atrial, or peribranchial, cavity, which arises as a result of the fusion of the lateral (metapleural) folds of the skin. The digestive system consists of a poorly differentiated tube, in which only two sections are distinguished - the pharynx and the intestine.

At the bottom of the pharynx there is a longitudinal groove - endostyle, lined with ciliated epithelium and glandular cells (Fig. 1). At the oral opening, the endostyle bifurcates and, bending around it on both sides with two grooves, rises to the upper side of the pharynx, where it passes into the epibranchial groove directed towards the intestine. The function of the endostyle is to extract food particles from the water. The latter, entering the pharyngeal intestine along with the flow of water, settle to the bottom of the pharynx and become enveloped in mucus, which is secreted

Rice. 1. Endostyle (cross section):
1 - ciliated cells; 2 - glandular cells; 3 - skeleton of the gill apparatus; 4 - overall

Rice. 2. Lancelet nephridia:
A - whole tubule with nephrostomes and solenocytes 1 - upper end of the gill slit; 2 - opening of the renal tubule into the peribranchial cavity; 3 - nephrostomy; B - part of the wall of the excretory tubule with solenocytes sitting on it

glandular cells of the endostyle and ciliated epithelium are driven forward to the oral opening. Here, lumps of food rise along the perioral grooves to the epibranchial groove and are transported along it to the intestine

The lancelet's blood is colorless and there is no heart.

The excretory organs are represented by metamerically located nephridia - short tubes, which in the number of 90 pairs are located above the pharynx (Fig. 2). Each tube at one end opens with several openings - nephrostomes - in general, and at the other - with one opening into the atrial cavity. Nephrostomes are seated with special club-shaped cells - solenocytes, inside each of them there is a tubule with a ciliated hair included in it. Excretion products are excreted through the nephridia into the atrial cavity directly from the coelom. The reproductive organs - testes and ovaries - are similar in external structure and are round bodies. They are located in the gill section of the coelom. Reproductive products are excreted into the atrial cavity through temporary genital ducts.

Skullless are exclusively marine animals. They spend most of their time buried in the sandy soil of the bottom. They feed passively, extracting food particles from the water, which is driven by the movement of the tentacles through the animal’s throat.

A similar lifestyle is ensured in modern skullless animals by a number of morphological adaptations. The epithelium contains special unicellular glands that secrete mucus, which protects the lancelet's integument from mechanical damage when buried in the ground. Successful burrowing in the ground is facilitated by relatively powerful muscles and the lanceolate shape of the tail. The atrial cavity mentioned above protects the respiratory apparatus from clogging with soil particles. Numerous gill slits ensure passage through them large quantity water, which is necessary for breathing of an animal half-buried in the sand. Presence in the throat

The endostyle and the initial movement of mucus along it towards the flow of water help to more quickly and completely remove food from it, which is very important with a passive method of nutrition.

TASKS

Study the main features of the external and internal structure lancelet

Consider:

External structure

Body size and shape; fins - dorsal, subcaudal and caudal; metapleural folds; preoral opening with tactile tentacles; anal hole.

Internal structure

Total preparation of lancelet: chord; connective tissue membrane; neural tube; myomeres; myosepta; sail with velar tentacles; pharynx with gill slits; intestine; hepatic outgrowth; gonads.

Preparation of a transverse incision in the pharynx: covers; myomeres; metapleural folds; chord; neural tube with neurocoel; pharynx; pierced by gill slits; endostyle; epibranchial groove; coelomic sacs; gonads; hepatic outgrowth; aortic roots.

Preparation of a transverse incision in the intestinal area. Compare the location of the internal organs with the previous preparation.

Circulatory system: abdominal aorta; branchial arteries; dorsal aorta; anterior and posterior cardinal veins. Using the preparation and drawing, trace the blood circulation pattern.

Sketch:

1) general location of internal organs; 2) a transverse incision in the pharynx; 3) transverse incision in the intestinal area; 4) diagram of the circulatory system (homework).

External structure

Examine the external structure of the lancelet using a magnifying glass on whole adult fixed specimens.

The translucent body of the animal has a length of 5 - 8 cm. It is compressed laterally and pointed at both ends (Fig. 3). A low dorsal fin runs along the back, turning into a lanceolate caudal fin at the rear end of the body. The latter continues on the ventral side in the form of a small subcaudal fin, reaching the opening of the atrial cavity - atriopore. Fin

Rice. 3. The structure of the lancelet:
1 - tactile tentacles; 2 - preoral funnel; 3 - velar tentacles; 4 - chord; 5 - neural tube; 6 - pharynx with gill slits; 7 - hepatic outgrowth; 8 - intestine; 9 - atriopor; 10 - subcaudal fin; 11 - metapleural fold; 12 - gonads; 13 - muscles; 14 - myomer; 75 - myosepta; 16 - caudal fin; 17 - eyes of Hesse; 18 - anal hole

the rim is supported by columnar fin chambers filled with a gelatinous substance. From the atriopore, on the sides of the body, two metapleural folds stretch forward to the preoral opening (visible in the form of a small depression).

During an external examination of the lancelet, the segmentation of the muscles is clearly visible: through the thin integument connective tissue partitions - myosepts - are visible, separating the muscle segments - myomeres. The myoseptae and, accordingly, the myomeres are curved at acute angles, the apices of which are directed towards the anterior end of the body. The myomeres of the right and left sides of the body are located asymmetrically in relation to each other: against the whole myomere of one side there are two halves of adjacent myomeres of the opposite side, which contributes to greater mobility of the body in the horizontal plane.

Internal structure

The structure and relative position of the main organs of the animal are clearly visible on the stained total preparation of the lancelet (see Fig. 3).

A chord (axial skeleton) stretches along the entire body of the lancelet (approximately along the midline) from the head to the tail. The specimen clearly shows its transverse striations, since the notochord is a complex system of transverse cell plates. The notochord, together with the neural tube, is surrounded by a thick sheath of gelatinous connective tissue. The processes of this membrane in the form of myosepta separate the muscle segments. The cephalic end of the notochord protrudes far beyond the anterior edge of the neural tube, which is a specific feature of skullless animals.

Rice. 4. Head section of the lancelet:
1 - chord; 2 - neural tube; 3 - olfactory fossa; 4 - sail; 5 - velar tentacles; 6 - preoral tentacles; 7 - eyes of Hesse

In addition to the notochord and connective tissue case, the skeleton of the lancelet is also formed by supporting structures in the interbranchial septa, dorsal fin and preoral funnel.

Above the notochord is the central nervous system, represented by the already mentioned neural tube. Its internal cavity is called the neurocoel. By placing the specimen under a low magnification microscope, you can see numerous dark spots - Hessian eyes (Fig. 4), which are located in the walls of the neural tube closer to its inner surface. The eyes of Hesse are formations that perform a light-sensitive function.

At the anterior end of the neural tube, you can see an unpaired olfactory fossa, which is a rudiment of a neuropore - an opening through which the neurocoel communicates with the external environment in the early stages of animal development. A cephalic extension of the neural tube can be seen, sometimes called the cerebral ventricle, although the anterior part of the lancelet neural tube, corresponding to the vertebrate brain, is undifferentiated.

Peripheral nervous system lancelet, represented by nerves extending from the tube, cannot be seen in the preparation.

The digestive tube is located under the notochord. It begins with a preoral funnel, surrounded by numerous tactile tentacles. The oral cavity is separated from the pharynx by a ring-shaped fold - a sail, on the inner side of which there are special velar tentacles. The walls of the pharynx are penetrated by numerous (up to 50 - 60 pairs) obliquely located gill slits. The pharynx gradually passes into an undifferentiated intestinal tube, ending in the posterior part of the body with the anus, shifted to the left side. From this tube, immediately behind the pharynx, a hepatic process extends forward, the main part of which is located on the right side of the pharynx. Therefore, on a preparation of a whole specimen, the hepatic outgrowth is noticeable in the form of a yellowish body visible through the gill section.

Rice. 5. Cross-section of a lancelet in the pharynx area:
I - covers; 2 - dorsal fin; 3 - metapleural folds; 4 - trunk muscles; 5 - abdominal muscle; 6 - chord; 7 - neural tube; 8 - neurocoel; 9 - neural tube slit; 10 - connective tissue membrane; II - myosepta; 12 - interbranchial septa; 13 - endostyle; 14 - epibranchial groove; 15 - coelomic channels; 16 - atrial cavity; 17 - hepatic outgrowth; 18 - gonads; 19 - aortic roots

In sexually mature individuals of lancelets, gonads are noticeable (usually 26 pairs of them), which in the form of dark round spots are visible through the abdominal wall of the body

On a cross-section of a lancelet in the pharynx area, examine the relative position of the organs and the structural details of the animal under low magnification under a microscope (Fig. 5)

The integument is represented by a single-layer epidermis and a thin layer of cutis. On the dorsal side, a section of the low dorsal fin is clearly visible. On the sides of the body below there are metapleural folds fused under the pharynx.

The main part of the upper half of the preparation is occupied by the trunk muscles, represented on the section by several myomeres. This is due to the inclined position of the upper and lower half of each myomere. On the ventral side of the body you can see

also a paired abdominal muscle that contributes to the contraction and expansion of the atrial cavity of the lancelet during breathing.

Approximately in the center of the preparation there is a chord, which has an oval shape when cut. Above the notochord lies the neural tube with a clearly visible neurocoel. From the latter, a gap stretches to the surface of the body - a rudiment of the embryonic canal. The notochord and the neural tube are surrounded by a connective tissue membrane, from which the myosepta extend (several of them are visible in the section).

The lower part of the preparation is a section of the pharyngeal (branch) section of the digestive tube and the surrounding atrial cavity. In section, the walls of the pharynx are penetrated by a large number of gill slits (the result of their inclined position). At the bottom of the pharyngeal region, an endostyle is visible, the structure and functions of which are discussed above. On the dorsal side of the pharynx there is an epibranchial groove, on both sides of which sections of two blood vessels - the roots of the aorta - are clearly visible.

The coelomic cavity is found on the specimen in the form of two canals located directly under the notochord and a canal lying under the endostyle. The hepatic outgrowth is visible only on those sections that are made closer to the posterior end of the pharynx. In sexually mature individuals, the gonads are located on the inner walls of the metapleural folds in the remnants of the coelom.

Using a cross-section of a lancelet in the intestinal area (Fig. 6), we consider the structural features of the notochord, neural tube, connective tissue membrane, intestine, coelom and compare

Rice. 7. Diagram of the circulatory system of the lancelet:
1 - abdominal aorta; 2 - gill arteries; 3 - aortic roots; 4 - carotid arteries; 5 - dorsal aorta; 6 - anterior cardinal veins; 7 - posterior cardinal veins; 8 - Cuvier's ducts; 9 - venous sinus; 10 - subintestinal vein; 11 - portal system of the hepatic outgrowth; 12 - hepatic vein; 13 - tail vein. The arrows show the direction of blood flow; veins and abdominal aorta are painted black

the relative position of these organs with what was visible on the previous preparation.

Study the circulatory system of the lancelet using tables and drawings (Fig. 7), since blood vessels are not visible in the preparations.

As already mentioned, the lancelet does not have a heart and is replaced by an unpaired abdominal aorta. Its walls are formed by striated muscles, which ensures the pulsation of the aorta. Blood moves from the abdominal aorta to the branchial arteries, which are located in the interbranchial septa. The gill arteries of the lancelet do not form capillary branches. Venous blood, passing through the gill arteries, is saturated with oxygen directly through the thin walls of these vessels located at the surface of the gill slits. Oxygen-enriched blood is collected in the paired roots of the aorta. Some of the blood from them goes forward through the small carotid arteries, and the bulk goes to the tail. Approximately in the middle of the body, the roots of the aorta merge into the main trunk canal - the dorsal aorta, through which blood is carried throughout the body.

Venous blood from the head part of the body moves backward through the paired anterior cardinal veins, and from the tail part through the posterior cardinal veins forward. The anterior and posterior cardinal veins on each side of the body above the posterior end of the pharynx merge into the thin-walled duct of Cuvier. Both ducts empty into the venous sinus.

From the internal organs, blood collects in the intestinal vein, which in the hepatic outgrowth breaks up into a network of capillaries, forming the portal system. The short hepatic vein drains the blood into the venous sinus.

Number of species: about 30 species.

Habitat: Lives in warm seas, at the bottom of sandbanks. Buries itself in the sand, exposing the front part of its body. Biofilter - absorbs organic matter, single-celled animals and algae, filtering water.

Body lancelet is translucent, fusiform, covered with mucus to facilitate movement. A skin fin fold runs along the body, forming a circumbranchial cavity. The cavity opens outwards with a hole - atriopore. Available

fins: dorsal, caudal and anal.

Coverings of the body consist of a single layer of epithelium and a thin layer of connective tissue.
The chord extends from the head to the caudal end. At the anterior end of the body, the notochord extends further than the neural tube. Adjacent to the notochord are muscle segments made of striated muscle tissue - myomeres.
The digestive system consists of several sections. The preoral funnel formed by the tentacles leads into the mouth. The pharynx is penetrated by gill slits, so it combines two functions - a section of the digestive tube and a respiratory organ. The intestine has a blind outgrowth - the liver, which secretes enzymes.
Water with food enters the pharynx, settles on the mucous membrane lined with ciliated epithelium, and moves into the intestine with the help of the movement of cilia. Digestion and absorption of food occurs both in the intestines and in the liver. Undigested food remains are excreted through the anus.

Internal structure of the lancelet

Respiratory system. Gas exchange occurs in the interbranchial septa covered with ciliated epithelium. It is possible for oxygen to penetrate the capillary network through the skin in a diffuse manner.
Circulatory system closed, consists of one circle of blood circulation. There is no heart, it is replaced by the abdominal aorta, located under the pharynx. Vessels depart from it, carrying blood to the interbranchial septa. There the blood is saturated with oxygen and then goes into the dorsal aorta, which lies under the notochord. Smaller vessels supply blood to all organs of the body. After this, the blood is collected again in the abdominal aorta. Diagram of the structure of the circulatory system of the lancelet.

Excretory organs: about 100 pairs of nephridia in the form of short tubes lying above the pharynx in the interbranchial septa. One end of the tube opens generally, and the other into the peribranchial cavity.

Nervous system has the appearance of a tube, located above the chord. Nerves extend from it to the muscles and internal organs bodies. There is no brain.

Sense organs poorly developed. In the cavity of the neural tube there are light-sensitive ocelli. The tentacles and cells scattered in the skin act as organs of touch and chemical sense. There is an olfactory fossa at the anterior end of the body.

Reproductive system. Dioecious animals. The testes and ovaries are located in the branchial section of the coelom. Reproductive products are excreted through breaks in the body walls into the circumbranchial cavity.

Fertilization externally, development with metamorphosis (a larval stage is present). The larva is similar to the adult.

Meaning

Lancelets purify water and are also food for many animals. Some species of lancelets are consumed by humans.
New terms: biofilter, notochord, neural tube, peribranchial cavity, myomere, nephridium.

Questions for consolidation:

Where does the lancelet live?
Name the most significant differences between the lancelet and its ancestor the annelid.
What features in the structure of the lancelet allow it to be classified as a chordate?
List the lancelet's adaptations to an aquatic lifestyle.
Explain the term "biofilter".

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