INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOANS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERALLY SYMMETRICAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA (SPIRALIA)

PHYLUM ECHIURA (ECHIURIDA)

Echiurans are marine bottom-dwelling worms, with only representatives of a single genus inhabiting the pelagic zone. They are found at various depths ranging from the shallows down to 10 km. A total of about 160 species are known.

The body of echiurans is unsegmented and consists of a trunk and a proboscis. A cutaneous-muscular sac is present. The body cavity is an unsegmented coelom. The excretory system is represented by anal sacs and nephromixia. The Circulatory system is closed. The general plan of The Nervous system resembles that of polychaetes, but it lacks ganglia. Echiurans are dioecious, and Germ Cells originate from the coelomic epithelium. Development proceeds with metamorphosis. The larva is a trochophore.

Echiurans are often colored yellow or green, as well as in various shades of red.

Anatomy. The body consists of a thickened trunk that extends anteriorly into a narrower region known as the proboscis (Fig. 198). The size (with the proboscis extended) ranges from 3 cm to 2 m in different species. The trunk length varies from a few millimeters to 20–50 cm. In some species, the length of the proboscis can exceed that of the trunk by tens of times; it is capable of strong contraction and elongation, which gave rise to the phylum name (from the Greek echis — serpent, ura — tail). The anterior end of the proboscis sometimes expands into a large blade, either continuous or divided into two parts (Bonellia), and its ventral surface is covered with cilia. Frequently, the proboscis is rolled longitudinally into a gutter, at the base of which the Mouth is located (Echiurus echiurus). In most echiurans, the trunk is elongated and cylindrical, whereas in some species it is short, sac-like, or even spherical. The surface of the trunk is smooth or covered with sensory papillae. It bears several groups of setae composed of a Chitin-like substance.

Cutaneous-muscular sac. The body of echiurans is covered by a cuticle, beneath which lies a single-layered epidermis (hypodermis). Cilia are retained only on the proboscis. In deep-sea forms, the cuticle is thin and transparent, whereas in shallow-Water species it is denser and covered with thickenings called papillae, which presumably perform a sensory function. Underlying the hypodermis are three layers of Muscles: circular, longitudinal, and oblique.

Body cavity. The inner surface of the cutaneous-muscular sac is lined with a layer of peritoneal epithelium enclosing a continuous coelom. The same epithelium surrounds the gut, forming numerous mesenterial strands and folds that connect the body wall with the peritoneal epithelium and support the loops of the intestine. The coelom occupies the entire trunk and is separated from the cavity of the proboscis by a septum containing an opening through which the two cavities communicate. The proboscis cavity is represented by narrow lacunae. The coelomic fluid participates in the hydraulic Movements of the proboscis and trunk. When trunk muscles contract, the fluid is forced under pressure into the channels of the proboscis, causing it to elongate and stiffen; upon Muscle relaxation, internal pressure drops, and the proboscis deflates.

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Fig. 198. Echiurus echiurus (after Dogiel):

a — general view; b — diagram of internal anatomy; c — trochophore: 1 — proboscis; 2 — mouth; 3 — genital setae; 4 — genital apertures; 5 — anal setae; 6 — anus; 7 — midgut loops; 8 — ventral nerve cord; 9 — dorsal Blood vessel;

10 — Pharynx; 11 — Esophagus; 12 — siphon; 13 — anal sac; 14 — gonad; 15 — ventral blood vessel; 16 — nephromixium; 17 — sacs of ventral setae; 18 — apical plate; 19 — circumoesophageal connective; 20 — prototroch;

21 — metatroch; 22 — protonephridium; 23 — anal sac rudiment;

24 — external epidermal folding; 25 — intestine

The Digestive System is adapted for feeding on small detritus particles, microorganisms, and the like. Food particles are driven by the cilia of the proboscis toward the mouth, which is situated on the ventral side of the body near the Base of the proboscis. The intestine is very long and convoluted, exceeding the body length tenfold, and consists of a pharynx, esophagus, crop, midgut, and hindgut. The anus is located at the posterior end of the body.

The excretory system comprises several Organs. Larvae possess a pair of protonephridia that degenerate during metamorphosis. In adults, two elongated anal sacs open into the hindgut. Each sac is covered with numerous (from 10 to 300) funnels. The funnel opens into the body cavity via a ciliated expanded end, and into the anal sac via a narrowed end. In the anterior region of the trunk, there is one or more (up to 200 pairs) nephromixia, which open to the exterior through a common pore on the ventral side, posterior to the mouth. The nephromixia discharge Gametes rather than metabolic wastes.

The circulatory system is closed. It consists of a long ventral vessel running along the entire body above the ventral nerve cord. Posterior to the mouth, it bifurcates, with both branches running inside the proboscis. At its anterior tip, they reunite into an unpaired dorsal vessel that exits the proboscis, passes over the esophagus, loops around it, and connects with the ventral vessel. The vascular ring encircling the esophagus has muscular walls and pulsates, thus functioning as a Heart. The blood is colorless and contains amoeboid cells.

Respiratory organs are absent; gas exchange occurs through the body wall. In species of the order Xenopneusta, the anal sacs function as respiratory organs.

The nervous system resembles that of Annelids, but ganglia are lacking. A highly elongated nerve ring extends the length of the proboscis and joins the ventral nerve cord posterior to the mouth; the nerve cord lies within the coelom, connected to the body wall by a narrow mesentery.

Sense Organs are poorly developed. Besides Skin papillae, chemoreceptors and specialized mechanoreceptors sensitive to water currents are located at the tip of the proboscis; when the latter are stimulated, the proboscis contracts.

Reproductive System. Echiurans are dioecious; males and females generally share the same Morphology, and pronounced Sexual Dimorphism is observed in only a few species. For instance, in members of the family Bonellidae, dwarf males (1–3 mm in length) have a simplified anatomy. Initially, they crawl on the female's proboscis and later enter the nephromixia, where they take up residence. For this reason, they were long considered parasites of the females. The Gonads in echiurans appear as groups of cells lying within the peritoneal epithelium along the posterior part of the ventral blood vessel. Germ cells mature in the coelomic fluid and are expelled to the exterior through the nephromixia.

Reproduction. Fertilization is typically external (though internal in members of the family Bonellidae). Cleavage is spiral and determinate. The egg develops into a trochophore larva (Fig. 198, c). It swims in the water Column for some time, feeds, and gradually undergoes metamorphosis. The upper hemisphere of the trochophore develops into a proboscis, while the lower hemisphere grows significantly and transforms into the trunk of the adult animal. Unlike annelids, no typical growth zone is formed, and the body increases in size more or less uniformly. After metamorphosis, the larva settles to the bottom.

Of particular interest is the metamorphosis of echiurans of the family Bonellidae, which, as mentioned earlier, possess dwarf males (Fig. 199).

Fig. 199. Metamorphosis and phenotypic Sex Determination in Bonellia viridis (after Dogiel)

a - swimming larva; b - older female larva; c - adult female; d - male:

1 - circumoesophageal nerve ring; 2 - eyes; 3 - prototroch; 4 - foregut; 5 - ventral nerve cords; 6 - midgut;

7 - anal ciliary ring; 8 - proboscis; 9 - setae; 10 - protonephridium; 11 - trunk; 12 - metanephridium;

13 - anal sac; 14 - female genital opening; 15 - larvae on the female's proboscis; 16 - coelom with developing germ cells; 17 - male genital funnel; 18 - sperm duct; 19 - male genital opening

The Transformation of a bonellid larva, such as Bonellia viridis, into a male or a female depends on the conditions under which it occurs. When larvae are kept in an aquarium without adult females, they slowly develop into females over the course of several weeks. If adult females are present in the aquarium, The pathway of metamorphosis depends on where the larva settles. If the larva settles on the trunk of a female, it develops into a female; if it settles on the proboscis, it develops into a dwarf male, which reaches sexual maturity within four to five days and enters the female's nephridiopores. Residing within these nephridiopores, the males fertilize the eggs passing by. Larvae also develop into males if only the female's proboscis or even an extract of proboscis tissue is placed in the aquarium. This is triggered by a specific biologically active compound, bonellin, which is synthesized by the glandular Cells of the female's proboscis. Thus, The Development of bonellids represents one of the rare cases where animal sex is determined after egg fertilization.

Most species of echiurans inhabit tropical seas, with only a small number extending into polar latitudes, such as Echiurus echiurus, which is widespread along the entire Arctic coast. Some species, notably Urechis caupo, are found in marine estuaries.

The overwhelming majority of echiurans are benthic animals. Many of them burrow into the substrate, creating burrows of various shapes, while others inhabit mollusk shells, echinoderm skeletons, rock crevices, or coral reefs. They conceal their trunks in the substrate while using a long, flexible proboscis to search for food in their surroundings. The proboscis frequently falls prey to various animals, but it possesses a high capacity for regeneration. Commensals such as polychaetes, Mollusks, crabs, and fish often inhabit the burrows of echiurans.

Echiurans are integral components of marine food webs. Certain species are utilized in laboratory research for embryological, biochemical, and physiological studies. Some species serve as bait for fishing. The biological compound bonellin, produced by the Skin glands of bonellids, exhibits antibiotic properties and has a lethal effect on various microorganisms.