INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, or BILATERIA ANIMALS

SUBDIVISION ECDYSOZOA

PHYLUM CEPHALORHYNCHA

CLASS KINORHYNCHA

Kinorhynchs are marine benthic animals that inhabit sand, mud, and Algae. They are distributed in all seas and oceans, mostly

occupying the sublittoral and littoral zones, though some species can be found at depths of up to 6,000 m. They may enter brackish waters, but do not occur in freshwater bodies. About 150 species are known, with 14 in the Black Sea, three of which are endemic (e.g., Semnoderes ponticus).

Anatomy. The body of these microscopic animals (ranging from 0.18 to 1 mm in length) is divided into a proboscis and an elongated trunk (Fig. 285). The proboscis bears a Mouth cone with forward-directed cuticular projections called stylets. Scalids directed backwards are located on the middle part of the proboscis, while its posterior part — the neck — is smooth.

The trunk is covered with a cuticular carapace consisting mostly of separate rings called zonites. Each zonite has three cuticular plates: one dorsal and two ventral, connected by areas of flexible fibrous cuticle. The trunk, especially its posterior part, bears numerous cuticular spines and bristles. The last zonite features a pair of long spines, and sometimes an unpaired terminal spine. The ducts of numerous glands that secrete mucus open onto The surface of the cuticle.

Integument. Kinorhynchs lack a cutculo-muscular sac. Hypodermis lies beneath the cuticle, forming protrusions — the middorsal and lateral cords — that extend into the body cavity.

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Fig. 285. Kinorhynchan Semnoderes armiger (from Ioffe): a, b - with partially retracted and everted proboscis, respectively

Musculature. Striated longitudinal Muscles are differentiated into separate short bundles that connect the anterior margins of adjacent zonites, as well as the dorsal and ventral cuticular plates of each zonite (Fig. 286). Retractor muscles ensure the withdrawal of the proboscis. In some species, circular muscles are present in the middle part of the proboscis.

Fig. 286. Diagram of The Structure of kinorhynchs (from Malakhov):

a, b - longitudinal and cross sections: 1 - proboscis with scalids; 2 - mouth cone; 3 - Pharynx; 4 - cuticular plates of zonites;

5 - dorsal bristles; 6 - gonad; 7 - ventral nerve cord; 8 - midgut; 9 - attachment tube;

10 - circumoesophageal nerve ring; 11 - hypodermis; 12 - scalids of the retracted proboscis; 13 - longitudinal muscles; 14 - cuticle;

15 - fibrous ligaments between individual zonite plates; 16 - gland; 17 - dorsoventral muscles

Locomotion involves the Muscles of the proboscis and trunk, as well as the coelomic fluid; hence the name of the class (from Greek kinema - movement and rhynchos - proboscis). Due to the contraction of dorsoventral muscles, the body of these animals, covered with a flexible carapace divided into zonites, becomes flattened and elongated. Simultaneously, due to the pressure increase, the proboscis extends forward and anchors to the substrate using backward-curved scalids. The contraction of longitudinal muscles shortens the trunk and pulls it toward the proboscis.

The body cavity - a schizocoel - appears as slits between Internal Organs. The coelomic fluid contains amoeboid Cells capable of phagocytosis.

Digestive System. The mouth opening, located at the apex of the mouth cone, leads into a muscular pharynx. By means of specialized muscles, it can be retracted into the mouth cone. The middle section of the intestine transitions into the posterior section, which opens with an anus at the posterior end of the body.

Kinorhynchs exhibit a peculiar feeding mechanism. They consume small detritus particles or unicellular algae (particularly diatoms) that adhere to the mucus enveloping their body. Through the contraction of longitudinal muscles, the mucus along with food particles moves toward the posterior end of the body, after which the animal bends and passes the caudal spines through the proboscis scalids, directing the food toward the mouth.

Excretory system. A pair of unbranched protonephridial canals is located in the posterior part of the body, each beginning with a single cyrtocyte.

Respiration occurs through the body surface (tegument).

The Nervous System consists of a circumpharyngeal nerve ring and a ventral nerve cord with small ganglia, one located in each zonite.

Sense Organs include scalids bearing groups of sensory bristles, as well as a pair of ocelli situated above the circumpharyngeal nerve ring (in the genus Echinoderes). Sensory spots—groups of microscopic pores surrounded by cuticular projections—are also considered Sensory Organs.

Reproductive System. Kinorhynchs are dioecious. The Gonads appear as two elongated sacs situated along the sides of the body, opening via separate pores on the last zonite. In the Ovaries, yolk cells (vitellocytes) are produced alongside oocytes. The FEMALE REPRODUCTIVE SYSTEM also includes seminal receptacles, where sperm is stored until Fertilization. Males lack a copulatory organ.

Reproduction. During mating, the male attaches a spermatophore (a packet of spermatozoa enclosed in a protective envelope) to the posterior end of the female's body. Peranal spines are believed to play a role in this process. Development proceeds through metamorphosis. The larva hatching from the egg lacks distinct zonites; boundaries between individual zonites subsequently appear and their number increases during development. At later stages, larval growth is accompanied by periodic molting.