INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 1 - H.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

PHYLUM TRUE METAZOA (EUMETAZOA)

PHYLUM CEPHALORHYNCHA

CLASS KINORHYNCHA

These are marine benthic animals inhabiting marine mud, sand, and Algae. They occur in all seas and oceans, with the majority of species living in the sublittoral and littoral zones. Approximately 100 species of kinorhynchs are known, with 14 species found in the Black Sea, three of which are endemic, for example, Centroderes spinosus and Semnoderes ponticus.

Kinorhynchs possess an elongated body ranging from 0.18 to 1 mm in length, consisting of a short proboscis and an elongated trunk (Figs. 210, 211). The proboscis region can be subdivided into the oral cone, bearing anteriorly directed cuticular projections; the middle section, armed with recurved backward-directed spine-like scalids; and the smooth cervical region. The trunk is covered by a cuticular carapace consisting of separate rings, or zonites, most commonly numbering 11. The cuticular covering of each zonite is formed by three plates of hard cuticle: a dorsal plate and two ventral plates. The hard plates of the cuticular carapace are connected by ligaments of flexible fibrous cuticle. The trunk, particularly at its posterior end, bears numerous cuticular spines and bristles. The cuticular carapace of each zonite contains numerous pores associated with glands that secrete mucus. A cellular hypodermis lies beneath the cuticle. The musculature does not form a continuous muscular sac; instead, it is divided into separate bundles. Circular Muscles persist only in the proboscis, while longitudinal muscles form two dorsal and two ventral bands. These consist of individual Muscle fibers connecting the anterior margins of adjacent zonites. Additionally, in each zonite, the dorsal and ventral plates are connected by a pair of dorsoventral muscles. A complex system of retractor muscles ensures the invaginating of the proboscis (see Fig. 210).

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Fig. 210. Diagram of kinorhynch anatomy — longitudinal (d) and transverse (b) sections:

1 —; 2 — oral cone; 3 — Pharynx; 4 — cuticular zonite plates; 5 — sensory bristles; 6 — gonad; 7 — ventral nerve cord; 8 — midgut; 9 — adhesive 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

Fig. 211. Kinorhynchs: a — Semnoderes armiger with partially retracted proboscis; b — with everted proboscis; c — Centroderes spinosus; d, e — two stages of larval development

Between the body wall and Internal Organs lies a body cavity filled with a dark granular mass. No cellular elements have been detected within the body cavity. The organs lie quite closely against one another, so the body cavity is small and appears as narrow clefts between the organs.

Locomotion in kinorhynchs is accomplished by means of the proboscis and trunk. The name of these animals reflects their movable proboscis. Body muscles and body cavity fluid participate in locomotion, and The Structure of the carapace is also of great importance. Through the contraction of the dorsoventral muscles, the body is flattened and elongated, which increases the pressure of the body cavity fluid. Concurrently, the proboscis extends forward and anchors into the substrate using the scalids, which are recurved backwards. During the contraction of the longitudinal muscles, the trunk shortens and is pulled toward the proboscis. The locomotory activity of kinorhynchs is facilitated by The flexibility of the short carapace, which is due to its division into zonites. The muscles are also divided into separate bundles connecting adjacent zonites. This structure of kinorhynchs resembles the metamerism (segmentation of the body into similar parts) of Arthropods. Metamerism of the ventral nerve cord also occurs. However, this does not indicate an affinity between kinorhynchs and arthropods, which occupy a significantly higher level of Organization.

The Digestive System of kinorhynchs begins with the Mouth opening at the tip of the proboscis, which leads into a muscular pharynx that can be retracted deep into the oral cone by specialized muscles. The pharynx transitions into the midgut, followed by the hindgut, which opens via the anus at the posterior end of the body. Kinorhynchs feed on small detritus particles or unicellular algae. The animal is surrounded by a mucous sheath to which food particles adhere. During the contraction of the longitudinal muscles, the mucus along with the food particles moves toward the posterior end of the body; the kinorhynch then bends and passes the caudal spines through the proboscis scalids, consuming the food particles.

The excretory system of animals is represented by a pair of unbranched protonephridial canals, each containing a single flame bulb.

The Nervous system consists of a circumoesophageal nerve ring located at the boundary between the oral cone and the middle section of the proboscis, and a ventral nerve cord bearing clusters of Nerve Cells (ganglia), one in each zonite. The formation of metameric ganglia on the nerve cord is associated with the metamerism of the zonites and muscles.

Sense Organs are represented by scalids and groups of sensory bristles on the scalids, which are innervated by sensory cells, as well as a pair of small ocelli lying above the circumoesophageal nerve ring.

Kinorhynchs are dioecious animals. The Gonads appear as a pair of elongated sacs lying on either side of the body and opening externally via two separate pores on the last zonite. In the Ovaries, In addition to egg cells, yolk cells are formed, and near the genital pores in their walls are small seminal receptacles where sperm is stored until egg Fertilization. Males possess a pair of Testes with sperm ducts; a copulatory organ is absent. During mating, the male attaches a sperm sac, or spermatophore, to the posterior end of the female's body. The embryonic development of kinorhynchs remains unstudied. A larva with an incomplete number of zonites hatches from the fertilized egg; it grows and molts, successively adding zonites one by one (see Fig. 211).