INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION DEUTEROSTOMES (DEUTEROSTOMIA)

PHYLUM ECHINODERMS (ECHINODERMATA)

CLASS HOLOTHURIANS, OR SEA CUCUMBERS (HOLOTHUROIDEA)

Unlike other Echinoderms, holothurians are less sensitive to Water salinity; some apodous sea cucumbers can even live in considerably brackish waters of mangrove swamps. About 1.1 thousand species are known worldwide, eight of which have been recorded in the Black Sea, with Stereoderma kirschbergi being the most common.

Holothurians are fairly large animals, with an average size of 10-40 cm, although some species exceed 2 m in length. Most of them are colored brown, dirty white, or gray, though brightly colored species also occur.

Anatomy. The body of holothurians is elongated along the axis from the oral to the aboral pole. Unlike other echinoderms, the oral-aboral

body axis is positioned parallel rather than perpendicular to the substrate, causing the animals to rest as if on their side. Most of them resemble thick worms with tentacles surrounding the Mouth. However, nearly cylindrical, spindle-shaped, spherical, or other body forms also exist. Particularly curious in appearance are holothurians of the family Pelagothuriidae (order Elasipoda), which lead a pelagic lifestyle. Externally, they resemble small jellyfish, whereas Representatives of the order Molpadonia feature a spindle- or barrel-shaped body whose posterior end is extended into a tail of variable length (Fig. 436).

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Fig. 436. Holothurians:

a - Cucumaria frondosa (after Baranova, modified); b - Pelagothura ludwigii; c - Ypsilo-thura bitentaculata; d - Molpadia musculus:

1 - cloacal opening; 2 - ambulacral podia of the bivium; 3 - tentacle ampullae; 4 - tentacles; 5 - ambulacral podia of the trivium; 6 - mouth

Anatomy. Bilateral Symmetry predominates in holothurians, especially in their external Structure. The side facing the substrate is conventionally called the ventral side, and the opposite one - the dorsal side. In many species, the ventral side is flattened to some extent, while the dorsal side is convex. Three radii lie on the ventral side, which is therefore referred to as the trivium, while two radii lie on the dorsal side, known as the bivium. The anterior end of the body bears a ring of tentacles around the mouth, and the posterior end bears the anus.

Integument. The body of holothurians is covered by a non-ciliated epithelium. The integument of most species is soft due to a significant reduction of the Skeleton, which is represented only by microscopic calcareous ossicles of various shapes scattered throughout the dermis (Fig. 437). In addition, holothurians possess an internal skeleton - a circumoral calcareous ring composed of skeletal plates. It serves as an attachment site for various Muscles and protects the circumoral nerve ring.

Fig. 437. Skeletal elements of the holothurian Skin (after Baranova)

Musculature is well developed in correlation with the reduction of the skeleton. A continuous layer of circular muscles lies beneath the skin, followed underneath by longitudinal musculature consisting of five bands. Special retractor muscles pull the anterior part of the body inward, and in some species, the tentacles as well.

The water-Vascular System is characterized by the fact that ambulacral podia with suckers are developed only on the trivium; the podia of the bivium lose their suckers, become thinner, and perform a purely sensory function. In apodous (order Apoda) and barrel-shaped (order Molpadonia) holothurians, the radial canals of the water-vascular system are completely absent, and locomotion is achieved solely through the action of body wall muscles. The circumoral tentacles are modified ambulacral podia. The madreporite is located near the anterior end of the body; in most species, it does not reach the body surface and opens into the coelom. Typically, a thin-walled, often large sac with easily distensible walls arises from the ring canal. This is the so-called Polian vesicle, which serves as a reservoir for the coelomic fluid. Occasionally, Polian vesicles may be numerous - up to 20 or more.

Digestive System. The intestine in holothurians is long, forms several loops, and is suspended from the body walls by mesenterial strands; it opens into the cloaca at the posterior end of the body (Fig. 438).

Fig. 438. Diagram of the internal anatomy of a holothurian (after Kilias):

1 - retracted tentacles; 2 - ring canal of the water-vascular system; 3 - gonad tubules; 4 - intestine; 5 - respiratory trees;

6 - ampullae of tube feet; 7 - radial canal of the water-vascular system; 8 - Blood system channels;

9 - longitudinal muscles; 10 - cloaca; 11 - Polian vesicle

Most holothurians feed primarily on detritus: organic remains and small organisms found in the substrate; species with dendritic tentacles feed on organic particles or small organisms settling on their tentacles, while pelagic species feed on plankton.

Tentacles play a significant role in food acquisition, burrowing, and locomotion, serve as Organs of Touch, and sometimes assist in Respiration.

In some holothurians of the order Aspidochirota, the ducts of the so-called Cuvierian tubules empty into the cloaca. These are glandular, tubular structures that, when the holothurian is irritated, are expelled through the cloaca and transform into long, white, sticky threads that entangle the stimulus-causing object or creature. When subjected to severe stress, holothurians are capable of autotomy of certain organs: the cloacal wall ruptures, through which the animal expels either just the intestine, the left respiratory tree, or the Gonads, or even all of its viscera. Despite this, the animal does not die and regenerates all lost organs within a short period.

The Circulatory system OF holothurians is more developed than in other echinoderms. In addition to lacunae typical of all echinoderms, holothurians possess A large number of lacunae in the Connective Tissue, as well as a system of well-developed vessels in the intestinal walls. These branch and intertwine to form a complex plexus known as the "rete mirabile" (Fig. 439). Sometimes the rete mirabile envelops the left respiratory tree, allowing oxygen from it to enter the blood directly rather than the coelomic fluid.

Fig. 439. Part of the circulatory system associated with the intestine in Cucumaria frondosa (after Strelkov et al.):

1 - anterior descending intestinal loop; 2, 3 - ventral and dorsal Blood Vessels, respectively; 4 - crop; 5 - mesentery; 6 - esophageal blood vessel; 7 - Esophagus; 8 - ascending intestinal loop; 9 - anastomoses between the ascending and descending PARTS OF THE ventral vessel; 10 - posterior descending intestinal loop; 11 - "rete mirabile"

Respiration in many holothurians occurs through the skin and ambulacral tentacles, while representatives of three orders (Dendrochirota, Aspidochirota, and Molpadonia) possess specialized respiratory organs known as water Lungs or respiratory trees (Fig. 438). These are two long, often highly branched, sometimes brightly colored trunks situated on either side of the intestine within the body cavity, opening into the cloaca. Through rhythmic muscular contractions and relaxations, water is alternately drawn through the cloaca into the respiratory trees, filling their smallest branches, and expelled. During this process, oxygen dissolved in the water passes through the thin walls of the respiratory trees into the coelomic fluid and is distributed throughout the body.

Excretory organs. The respiratory trees also function as excretory organs: amoeboid Cells (coelomocytes) laden with metabolic wastes pass from the coelomic fluid through their walls and are subsequently expelled outside via the cloaca. Only apodous holothurians feature specialized excretory devices:

ciliated funnels located along the intestinal mesentery, into which waste-laden coelomocytes migrate, agglomerating into distinct corpuscles before being pushed back into the coelomic fluid.

Reproductive System. Most holothurians are dioecious; unlike other echinoderms, they lack a genital cord and possess only a single gonad consisting of a large number of long, blindly ending tubules upon whose outer walls Germ Cells are formed. All tubules merge into a single genital duct that opens in the dorsal interradius near the anterior end of the body, sometimes at the base or even at the tip of one of the tentacles.

Reproduction. Eggs are shed into the water, where Fertilization takes place. The Development of holothurians can proceed via two pathways. In species with yolk-poor eggs, the egg develops into a ciliated blastula or gastrula, which swims for several weeks during which its structure undergoes profound changes. Initially, it transforms into a bilaterally symmetrical dipleurula, after which a winding ciliated band appears; at this stage, the larva is called an auricularia. It swims freely in the water, actively feeding on small planktonic organisms, grows rapidly, and transforms into a doliolaria, which features a barrel-shaped body encircled by ciliary bands. The doliolaria swims initially, then settles to the bottom and, without feeding, metamorphoses into a pentactula that already resembles a juvenile individual (Fig. 440).

Fig. 440. Holothurian larvae (from Ivanova-Kazas): a - auricularia of Synapta vittata; b - doliolaria; c - pentactula of Cucumaria planci:

1 - ciliated band; 2 - mouth; 3 - Stomach; 4 - anus; 5 - preoral lobe; 6 - circumoral tentacles; 7 - tube feet

In holothurians with yolk-rich eggs, a later developmental stage known as a pseudodoliolaria hatches directly from the egg; it differs from the true doliolaria in THE POSITION OF the mouth and soon transforms into a pentactula. Species in which a pentactula hatches directly from the egg are also known.

In some species, particularly inhabitants of Arctic waters, parental care is observed. In the simplest cases, eggs and larvae develop On the surface of the maternal body, protected by various dermal projections; in others, specialized body invaginations appear, such as brood chambers projecting into the coelomic cavity, or development occurs within the Ovary or body cavity. The exact mechanism of fertilization in such cases remains unclear, but development is abbreviated.

Holothurians are characterized by a high capacity for regeneration. Some species can divide transversely into anterior and posterior parts, after which each part regenerates the missing portion, thus exhibiting asexual reproduction.

As noted previously, holothurians are benthic animals; typically, they crawl slowly along the bottom using tube feet, tentacles, or muscular body contractions, less commonly burrowing into the sediment, and even more rarely swimming throughout their lives. When disturbed, they retract the anterior part of the body with its tentacles, expel water from the cloaca, and contract into a dense lump.

Holothurians provide shelter for numerous invertebrates (Protozoans, worms, Mollusks, crustaceans) and Fishes that inhabit their body surface, intestine, Polian vesicles, respiratory trees, circulatory system, etc.

These animals play a vital role in marine food webs. They consume organic detritus and small organisms while serving as prey for many larger marine animals. About 40 species (most belonging to the order Aspidochirota), collectively known as trepangs or sea cucumbers, are consumed by humans. Trepang fisheries are particularly well-developed off the coasts of Japan, China, Indonesia, and the Philippines, and harvesting also occurs along the shores of Africa, America, Australia, etc. Trepangs are marketed dried, boiled, salted, or canned. In Eastern medicine, sea cucumbers are referred to as "marine ginseng," and their meat is rich in Proteins and valuable mineral salts.

Among true sea cucumbers (family Holothuriidae), poisonous species exist; for instance, the toxin of the black sea cucumber (Ludwigothuria atra) is used by Pacific islanders to stun fish. However, this toxin is not harmful to humans, and the species is edible.