INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION DEUTEROSTOMIA

PHYLUM HEMICHORDATA

CLASS ENTEROPNEUSTA

Benthic worm-like animals that predominantly lead a burrowing lifestyle, ranging in length from a few centimeters to 2.5 m. About 70 species are known.

The body coloration is most commonly yellowish-brown or brown, although grayish-white, black-violet, or bright red species are also found.

Anatomy. The body is clearly divided into three regions: the proboscis, collar, and trunk. The proboscis has a characteristic acorn-like or elongated oval shape. It is a muscular Structure well-adapted for burrowing. The Base of the proboscis narrows into a slender stalk embraced by the second, also muscular, body region—the collar—which rises above the body surface as a ridge. Next is the trunk region, which accounts for 9/10 of the body length. The anterior part of the trunk is perforated on the sides by two rows of numerous narrow gill slits (Fig. 421).

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Fig. 421. External appearance of Saccoglossus kowalevskii (from Dogiel): 1 — proboscis; 2 — collar; 3 — trunk; 4 — gill slits

Integument. The body is covered with a delicate ciliated epithelium containing numerous unicellular glands and sensory Nerve Cells. Abundant glandular cells produce mucus that envelops the body in a thick layer, with glands being especially numerous on the proboscis and collar. The epithelium rests on a thin, structureless basal membrane.

Musculature. Two layers of Muscle fibers lie beneath the basal membrane: an outer circular layer and an inner longitudinal layer. In addition, each body region, apart from the cutaneous Muscles, features well-developed specialized muscles, such as pharyngeal muscles, muscles that flex and extend the proboscis, etc. From the inside, the longitudinal cutaneous muscles are lined with the ciliated peritoneal epithelium of the coelomic sacs.

Body cavity. The coelom is represented by three regions: an unpaired proboscis coelom and paired collar and trunk coeloms. The proboscis cavity communicates with the external environment via a short canal with a pore on the DORSAL SIDE OF the proboscis (occasionally There are two canals). The two collar coelomic sacs have similar canals with pores, opening into the first pair of gill slits. Excess fluid is excreted through the proboscis pore, while the collar pores serve to fill its coeloms with Water, regulating the volume and turgor of the proboscis necessary for burrowing. The trunk coeloms do not communicate with the external environment.

The Digestive System begins with the Mouth located on the ventral side at the boundary between the First and Second body regions. The mouth leads into a short endodermal Pharynx, from the dorsal side of which, at its very beginning, a blind diverticulum—the stomochord (notochord)—extends into the proboscis (Fig. 422). In most species, it has a narrow lumen, and its walls consist of a single layer of large vacuolated cells. In the proboscis, the stomochord reaches almost to its middle. Between the ventral wall of the proboscis and the stomochord, a Cartilage-like plate develops with two projections embracing the sides of the pharynx base. Together with the stomochord, it forms the supporting STRUCTURE OF THE proboscis. It was initially believed that the stomochord was homologous to the chordate notochord (hence the phylum name, from Greek hemi - half), but many scientists consider the stomochord to be merely a pre-oral outgrowth of the gut, bearing no relation to the notochord of Chordates. At the beginning of the trunk region, the pharynx transitions into the Esophagus, the lateral walls of which are pierced by two rows of U-shaped gill slits (hence the class name). A longitudinal groove, or endostyle, runs along the ventral side of the esophagus. Some endostylar cells are glandular and secrete mucus, while others bear cilia. Small food and soil particles entering the esophagus are entangled in mucus and transported by ciliated cells to the midgut, where Digestion AND ABSORPTION take place. The anterior part of this region, often called the hepatic region, features numerous lateral pockets where food is digested by Enzymes produced by the cells of these outgrowths. The posterior part of the midgut appears as a simple tube that transitions into a short hindgut near the posterior end of the body, opening to the outside via the anus. It should be noted that all PARTS OF THE digestive system, except the Oral Cavity, are of endodermal origin.

Fig. 422. Internal anatomy of Saccoglossus kowalevskii (lateral section through the anterior end of the body) (from Vinogradova):

1 — longitudinal Muscles of the proboscis; 2 — proboscis coelom; 3 — glomerulus; 4 — Pericardium; 5 — circular nerve ring of the collar; 6 — strands connecting the nerve tube to the dorsal Skin; 7 — canal of the nerve tube; 8 — dorsal Blood vessel;

9 — dorsal mesentery of the trunk; 10 — dorsal branchial region of the pharynx; 11 — ventral intestinal region of the pharynx; 12 — ventral mesentery of the trunk; 13 — ventral nerve cord; 14 — ventral blood vessel; 15 — collar coelom; 16 — mouth; 17 — skeletal plate;

18 — stomochord; 19 — circular muscles of the proboscis

Enteropneusts are geophagous animals; they ingest large amounts of soil from which they extract microscopic animals, Algae, plant debris, and other organic matter for food.

The Circulatory system OF enteropneusts is fairly well developed (Fig. 423).

Fig. 423. Diagram of the circulatory system structure in Balanoglossus (from Beklemishev)

1 - ventral proboscis artery; 2 - glomerulus; 3 - notochord; 4 - circumoesophageal vessel; 5 - body wall capillaries; 6 - ventral longitudinal vessel; 7 - capillaries of the pharyngeal ventral wall; 8 - lateral longitudinal vessel; 9 - intestinal capillaries;

10 - hepatic cecae; 11 - vessel connecting the lateral and dorsal longitudinal vessels;

12 - branchial vessels; 13 - dorsal longitudinal vessel; 14 - its anterior end; 15 - cardiac lacuna; 16 - pericardium;

17 - proboscis vein; 18 - proboscis artery

The dorsal vessel lies in the dorsal mesentery, with blood flowing through it from back to front. At the esophageal level, part of the blood enters the lacunae in the walls of the gill slits via lateral afferent vessels, where it undergoes oxygenation. Then, the oxygenated blood returns via efferent vessels to the dorsal vessel, which empties into the cardiac lacuna in the proboscis, situated between the notochord and the pericardium. The pericardium is a closed sac formed of coelomic epithelium and muscles. Blood does not pass through it, but its rhythmic contractions propel the blood from the cardiac lacuna through two circumoesophageal vessels located in the collar, leading to the ventral vessel, through which blood flows toward the posterior end of the body. Anterior to the cardiac lacuna, a network of fine vessels (the glomerulus) is formed. From the ventral vessel, via numerous loop-like vessels passing through the body wall around the gut and other Internal Organs, the blood first returns to the lateral Blood Vessels and then to the dorsal vessel.

Excretion. The removal of excretory products from the blood occurs within the Vessels of the glomerulus, the walls of which contain phagocytic cells. These cells become engorged with breakdown products and are discharged outward along with water through the proboscis pore.

Respiration is carried out by means of the branchial apparatus, which is closely associated with the gut. The gill slits piercing the esophagus (Fig. 422) are horseshoe-shaped, and their walls are supported by a complex Skeleton composed of fork-like rods—local thickenings of the basal membrane that prevent the walls from collapsing. Each gill slit is formed by an outpocketing of the esophageal wall that extends to the body wall, where it opens to the outside via an aperture. Water entering the mouth flows out through the gill slits, delivering oxygen to the blood lacunae that permeate the partitions between the slits. Such a structure of the branchial apparatus is unknown in other invertebrates and has become widespread only among chordates.

The Nervous system is represented by a continuous subepithelial nerve plexus with two main condensations forming the dorsal and ventral longitudinal cords; at the beginning of the collar, both cords are joined by a ring cord. Further into the collar and proboscis, only the dorsal cord continues. The trunk Regions of the cords, as well as the collar ring, are located superficially in the epithelium; the dorsal trunk of the collar occupies this position only in early developmental stages, later sinking beneath the skin—initially as a groove and subsequently separating from the skin to form a thick-walled neural tube. In some species, a lumen

persists, but in most of them it is absent. Upon entering the proboscis, the dorsal trunk branches into two branches that curve around the base of the proboscis.

Sense Organs are absent; external stimuli are perceived by numerous sensory Cells of the cutaneous nerve plexus, among which light-sensitive cells have also been detected. It is known that all body parts of enteropneusts respond to mechanical stimuli and light. In doing so, the animal begins to flee, burrow into the substrate, etc., with the proboscis reacting most strongly to such stimuli.

Reproductive System. Enteropneusts are dioecious, and Sexual Dimorphism is absent. The Gonads in females and males are located in the middle part of the trunk along the gut (there are more than 30 of them). Each of them opens to the outside via a duct.

Reproduction. Fertilization is most commonly external. As a result of embryonic development, a typical dipleurula is formed. The developed

larva possesses an apical organ in the form of a plate with a tuft of cilia and a pair of simple ocelli, as well as two rings of large cilia—the circumoral and telotroch rings, the latter forming numerous convolutions. The larva swims in the water with its tuft pointing upward, constantly rotating around its own axis (Fig. 424), which is why it received the name tornaria (from Lat. torno - to bore). It actively feeds by driving food into the mouth using cilia. After a certain time, the tornaria settles to the bottom, where it gradually transforms into an adult specimen. In the process, the larva's body is divided by constrictions into three regions. The two anterior ones grow comparatively little, while the posterior one increases manifold, transforming into the trunk.

Fig. 424. Enteropneust larva — tornaria (from Dogel): 1 - apical plate; 2 - ciliated bands

Among enteropneusts, there is one known species, *Balanoglossus proliferans*, which is capable of reproducing not only sexually but also asexually. As a result of transverse fission of the adult body in individuals lacking gonads, new individuals are formed that are already capable of sexual reproduction. The alternation of sexual and asexual generations occurs continuously, so in this case one can speak of true metagenesis. Individuals of both generations differ so much externally that they were previously considered two distinct species.

Enteropneusts are characterized by a high level of regenerative processes, which is important given their burrowing lifestyle and the ingestion of vast amounts of sediment containing sharp skeletal fragments of various invertebrates. Even a small piece of the posterior body part can regenerate the entire body, whereas a severed anterior part perishes.

Enteropneusts are distributed in all seas of the globe with normal water salinity; the majority of them are found in warm seas within the intertidal zone or in shallow waters elsewhere. Some burrow temporary passages or permanent U-shaped burrows in the substrate with walls reinforced by mucus, others live On the surface of the sediment, hiding under stones, in empty mollusk shells filled with sand, among algal beds, or under the roots of higher plants in the coastal zone. Some species crawl over The surface of the seabed. Much less frequently, enteropneusts are found at great depths, including deep-sea trenches.