INVERTEBRATE ZOOLOGY IN THREE VOLUMES — BOOK 1 — H.Y. Shcherbak — 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE METAZOA (EUMETAZOA)

PHYLUM CEPHALORHYNCHA

CLASS PRIAPULIDA

These are marine benthic animals distributed in temperate zones and at great depths in the equatorial zone of the World Ocean. Fifteen species of them are known.

Priapulids possess a whitish, cylindrical body ranging in length from a few millimeters to 10–15 cm. At the anterior end of the body is a large proboscis (up to 1/3 of the total body length) capable of changing its shape: it can extend into a slender wedge-shaped outgrowth, shorten and widen, or retract into the body cavity via specialized retractor Muscles. A distinct oral cone is absent in priapulids. The proboscis bears backward-curving spines, known as scalids, arranged in longitudinal rows whose number is a multiple of five. In Priapulus caudatus, the most widespread species in northern seas, there are 25 spines. The surface of the trunk is covered with numerous annular folds and grooves, irregularly scattered short papillae-like outgrowths, and occasionally small spines. In some species, the posterior end of the body bears a caudal gill—a long, branching appendage consisting of numerous elongated lobes; occasionally, two gills are present (Fig. 208).

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Fig. 208. Priapulids: Priapulus caudatus (a), Halicryptus spinulosus (b), its larva (c), Tubiluclius corallicola (d), Maccabeus tentaculatus (e): 1 — proboscis, 2 — scalids, 3 — trunk, 4 — gill appendage, 5 — lorica

The integumentary-muscular sac consists of a thin elastic cuticle, a single-layered epidermis, a basal membrane, and two Muscle layers—circular and longitudinal. The ULTRASTRUCTURE OF THE cuticle is simple, comprising a denser outer layer and a thin-fibrous inner layer. On the gill appendages, the cuticle is extremely thin. From time to time, the old cuticle detaches, a new one forms underneath, and the old one is shed, meaning ecdysis (molting) takes place. The epidermis has a standard cellular Structure. The circular and longitudinal muscles are well developed, enabling the priapulid body to elongate and contract.

Beneath the integumentary-muscular sac lies a large body cavity that extends into the proboscis and gill appendages (Fig. 209). It is filled with fluid containing numerous Cells of two types. Cells of one type contain a respiratory pigment (hemerythrin) and serve to store oxygen, while those of the other type phagocytize small particles (such as Bacteria) and also form aggregates that seal wounds in the event of mechanical tissue damage.

Fig. 209. Diagram of the anatomy of priapulids:

1 — nerve ring; 2 — Pharynx; 3 — proboscis retractor muscles; 4 — cuticle; 5 — midgut; 6 — urogenital organ; 7 — anus; 8 — glands; 9 — hindgut; 10 — circular musculature; 11 — longitudinal muscles

The Nature of the priapulid body cavity has long been a subject of debate. Some researchers considered it a primary body cavity (pseudocoel), while others regarded it as a secondary body cavity (coelom) on the grounds that it is ostensibly lined on the inside with mesodermal epithelium. However, Electron Cell/15.html">Microscopy studies have shown that there is no epithelial lining beneath the integumentary-muscular sac; the innermost muscle layer—the circular muscle layer—is in direct contact with the coelomic fluid, and the intestine is likewise surrounded by a layer of circular and longitudinal muscles that contact the coelomic fluid.

The body cavity contains long, ribbon-like proboscis retractor muscles attached at one end to the inner wall of the oral cone and at the other to the lateral body wall. Priapulids move in a very distinctive manner. They inhabit dense sandy sediments, burrowing through them using the proboscis and the entire body. Having expanded its body and thus anchored itself within the burrow, the priapulid forcefully thrusts forward the proboscis, which can narrow and acquire a wedge-like shape, thereby easily penetrating the dense sediment. Following this, by contracting the circular Muscles of the Trunk and pumping coelomic fluid into the proboscis, the worm expands it and anchors it in the sediment using the scalids; then, by contracting the longitudinal muscles, it draws the trunk toward the proboscis. As we can see, a major role in locomotion is played by the large continuous body cavity and the fluid that fills it, which is pumped back and forth between the trunk and the proboscis. The cavity of the caudal gill communicates with the general body cavity; hence, the pumping of coelomic fluid drives both movement and fluid exchange within the gill.

The Digestive System of these animals begins with the Mouth opening at the tip of the proboscis, surrounded by cuticular Teeth. The intestine is a straight tube divided into a pharynx, Esophagus (foregut), midgut, and hindgut, which opens at the posterior end of the body or, if a gill appendage is present, near its base. The muscular pharynx bears cuticular teeth, while the posterior part of the esophagus forms an expanded region with powerful musculature and cuticular spines for grinding food.

Priapulids are predators that feed on polychaetes, holothurians, ophiuroids, and other invertebrates.

They respire through their entire body surface or via the gill appendage, which possesses very thin integuments that facilitate the diffusion of oxygen directly into the coelomic fluid. Excretory Organs are represented by protonephridia connected with the reproductive organs into a unified urogenital complex. The Nervous system consists of a circumoral nerve ring and a ventral nerve cord. A true Brain (cerebral ganglion) is absent in priapulids, as in other Cephalorhyncha. Its function is performed by the circumoral nerve ring, which consists of Nerve Cells and their processes. Fine peripheral longitudinal nerves extend from the ring along the entire body and give off branches to the muscles. Minute circular nerves branch out along the entire length of the ventral nerve cord. Specialized Sense Organs are lacking in priapulids; their Functions are carried out by scalids and papillae, inside of which sensory nerve cells have been discovered. Presumably, these act as mechanoreceptors.

The excretory and reproductive organs of priapulids are combined into a common Urogenital System. It consists of two elongated organs located on either side of the intestine in the posterior part of the trunk and attached to the body walls by thin mesenteries. The excretory portion of each organ is represented by protonephridia, which appear as several small bundles consisting of numerous flame cells. The reproductive portion consists of numerous gonadal pouches that are blindly closed at one end and open into the urogenital duct at the other. Both ducts communicate with the external environment via a pair of openings at the posterior end of the body.

Priapulids are dioecious animals, though females do not differ morphologically from males. Cleavage in priapulids is radial, holoblastic, and indeterminate. The mouth opening breaks through at the anterior end of the embryo independently of the blastopore. The anus forms at the site of the blastopore. The egg hatches into a larva that already possesses a proboscis, but unlike the adult, its trunk is enclosed in a lorica consisting of continuous dorsal, ventral, and several lateral plates. The transformation of the larva into an adult occurs via molting and may take several years. Priapulids inhabit the coastal intertidal zone, or littoral (e.g., Priapulus), coral reefs (family Tubiluchidae), and great depths (order Seticoronaria). Seticoronaria construct protective tubes around their bodies using foreign particles.