INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, or BILATERIA ANIMALS

SUBDIVISION ECDYSOZOA

PHYLUM CEPHALORHYNCHA

CLASS PRIAPULIDA

Marine benthic animals distributed in cold and temperate waters of the littoral and sublittoral zones, and in the equatorial zone of the World Ocean only at great depths (up to several thousand meters). Certain species (e.g., Priapulus caudatus) can penetrate into brackish waters. 16 species of these animals are known. Their name originates from the Greek god Priapus, who symbolizes male reproductive power.

Anatomy and Morphology. The body of priapulids is cylindrical, circular in cross-section, whitish in color, ranging from a few millimeters to 15 cm in length (genus Priapulus). The anterior part of the body is a large proboscis, the length of which can reach up to one-third of the total body length (Fig. 283). The proboscis is capable of changing shape: stretching into a thin wedge-shaped outgrowth, shortening and widening, or retracting into the body with the help of retractor Muscles. Scalids bent backwards are arranged in longitudinal rows on the proboscis. A neck region is located between the proboscis and the next section, the trunk. The surface of the trunk is covered with annular folds and grooves, irregularly scattered short outgrowths-papillae, and occasionally spines or scales. In some species, the posterior end of the body bears a caudal gill—a long grape-like appendage; sometimes There are two.

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Fig. 283. Priapulus caudatus (from Malakhov): a - general view; b - internal Structure:

1 - proboscis; 2 - scalids; 3 - trunk; 4 - gill appendage; 5 - nerve ring; 6 - Pharynx; 7 - proboscis retractor muscles;

8 - cuticle; 9 - midgut; 10 - urogenital organ; 11 - anus;

12 - glands; 13 - hindgut; 14 - circular musculature; 15 - longitudinal muscles

The Skin-muscular sac consists of a thin elastic cuticle, a single-layered hypodermis, and two layers of muscles—circular and longitudinal. On the caudal gill appendages, the cuticle is extremely thin. From time to time, the old cuticle is detached and shed, and a new one forms underneath, meaning the molting process takes place. Thanks to the contraction of the Muscles of the skin-muscular sac, the priapulid body can shorten and elongate, allowing them to move through dense sandy marine sediment.

The body cavity is a schizocoel that extends into the proboscis, communicating with the cavity of the caudal gill appendage. It is filled with fluid that performs supporting, transport, and protective Functions. Cells float in the coelomic fluid: some—erythrocytes—contain a respiratory pigment capable of storing oxygen; others—granulocytes—perform a protective function by phagocytizing small foreign particles and microorganisms, and also form clusters that plug wounds in case of mechanical damage to the integument. Long ribbon-like proboscis retractor muscles pass through the body cavity: with one end they attach to the oral cone, and with the other to the lateral body walls.

Digestive System. The Mouth opening is located at the apex of the proboscis and surrounded by a ring of cuticular Teeth. It leads into the intestine, which has the appearance of a straight tube and is divided into the pharynx, Esophagus, middle, and hind SECTIONS. The anus opens at the posterior end of the body, and when a caudal gill appendage is present, at its base. The muscular pharynx bears cuticular teeth, while the posterior part of the esophagus expands and features powerful musculature and cuticular teeth that serve to grind food. Priapulids are predators that hunt invertebrate animals: polychaetes, holothurians, ophiuroids, etc.

Excretory Organs are represented by protonephridia, which are linked with the Organs of the Reproductive System into a single urogenital complex. It consists of two elongated organs located on either side of the intestine in the posterior part of the body. The protonephridia look like small bundles consisting of numerous cyrtocytes. The protonephridia open into the urogenital duct.

Respiration occurs through the integument and caudal gill appendages. Priapulids that have lost their gill appendages do not regenerate them subsequently; therefore, the trunk and proboscis integument plays the leading role in gas exchange.

The Nervous system consists of a circumenteric nerve ring and a ventral nerve cord. Thin longitudinal peripheral nerves, which extend along the entire body and form branches to the muscles, depart from the nerve ring, which is formed by The Cell bodies of Neurons and their processes and functions as a Brain. Thin annular nerves also branch off from the ventral nerve cord. Another nerve center lies at the boundary between the esophagus and the midgut; it is connected to the circumenteric nerve ring and innerve the intestine.

Specialized Sense Organs are absent. Their functions are performed by scalids and papillae, which presumably serve as mechanoreceptors.

Reproductive system. Priapulids are dioecious animals, but Sexual Dimorphism is not expressed in them. The reproductive part of the urogenital complex consists of numerous genital pouches that are blindly closed at one end and enter the urogenital duct at the other. Two urogenital ducts open to the outside through apertures located at the posterior end of the body near the anus.

Reproduction. Fertilization is external. Development proceeds with metamorphosis: a larva emerges from the egg, possessing a proboscis and a body surrounded by a carapace consisting of dorsal, ventral, and several lateral plates. Postembryonic development can last for several years and is accompanied by molts.

Priapulids constitute one of the links in the food webs of marine ecosystems. Sometimes they can consume prey comparable in size to the animals themselves (e.g., Priapulus caudatus). In turn, they serve as food for fish, and are occasionally used as live bait for fishing.