INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA (SPIRALIA)

PHYLUM ANNELIDS (ANNELIDA)

SUBPHYLUM ACLITELLATA (ACLITEIJATA)

CLASS DINOPHILIDA (DINOPHILIDA)

This Class includes a small group of marine benthic worms (several dozen species) measuring just a few millimeters in size.

Anatomy and Morphology. The body of dinophilids is elongated and cylindrical, consisting—as in other Annelids—of a HEAD lobe (prostomium), a trunk with five to six poorly defined segments, and a posterior region (pygidium) (Fig. 178). Instead of parapodia, the segments bear ciliary bands, while a wide ciliated tract runs along the ventral side of the body from the circumoral region to the tail.

The Skin-Muscle sac has a specific Structure: instead of a traditional cuticle, the body is externally covered by a procuticle composed of epithelial Cell outgrowths and their secretory products, interspersed with ciliated areas. The musculature is represented by isolated circular and longitudinal muscle fibers rather than a continuous layer.

Body Cavity. Dinophilids possess a well-developed, unsegmented coelom consisting of three elongated sacs that extend along the body on either side of and beneath the gut.

Parenchyma is distributed between the skin-muscle sac, the coelomic sacs, and the gut; its Cells store nutrient reserves. Once these reserves are consumed during overwintering, the parenchyma cells degenerate, leaving behind a fluid-filled schizocoelomic space.

Digestive System. A characteristic feature of the digestive system is the presence of a postoral organ situated just behind the Mouth, containing a rigid Tongue covered with

a folded cuticle. Dinophilids use this tongue to scrape diatoms and other micro-organisms off substrates for food.

Fig. 178. Diagram of the Organization of Dinophilus vorticoides (after Beklemishev): a - general body plan of a male; b - cross-section of a male through the middle body region: 1 - supraesophageal ganglion; 2 - ciliary rings; 3 - ocellus; 4 - mouth; 5 - Pharynx; 6 - ventral nerve ladder; 7 - Testis; 8 - anus; 9 - gonopore; 10 - intestine; 11 - nephridia; 12 - commissure; 13 - lateral coelomic sac;

14 - ventral coelomic sac; 15 - ganglion; 16 - ventral ciliary tract; 17 - longitudinal Muscles

Excretory Organs. These are represented by several pairs of segmentally arranged protonephridia that project into the coelom via thin-walled vesicles equipped with flame bulbs (modified cyrtocytes).

The Circulatory system is absent.

Respiratory organs are also lacking; gas exchange occurs directly across the body surface.

The Nervous system takes the form of a "ladder-type" nerve cord with widely spaced longitudinal trunks. The structure of the supraesophageal ganglion is relatively simple.

Sense Organs. These include a pair of dorsal ocelli on the prostomium structurally resembling flatworm eyes, a pair of nuchal pits on the first trunk segment, and tactile receptors scattered across the integument.

Reproductive System. Dinophilids are dioecious. The Gonads are paired and appear as long sacs extending along the sides of the body. In males, they form a pair of sausagelike Testes located on either side of the ventral coelomic sac. Paired ducts extend from them, forming expanded Seminal Vesicles before merging into a single median duct that opens externally and features a copulatory organ. The exact mechanism of Fertilization remains unclear. Females possess two pairs of Ovaries situated analogously to the testes in males, though lacking efferent ducts. In sexually mature females, male sperm is stored within the ventral coelomic sac; later, eggs rupture the ovarian walls to enter this cavity, where fertilization takes place.

Development proceeds without metamorphosis. Dinophilids lack regenerative capabilities, and asexual reproduction has not been observed in this group.

Dinophilids inhabit the littoral zone among algal mats. Dinophilus vorticoides, found along the coasts of the White and Barents Seas, is the most thoroughly studied species. It exhibits a unique life cycle. Juveniles hatch in the spring, grow to 0.5–0.8 mm, and encyst at the end of summer to enter a dormant state for the winter. In February or March, they emerge from the cysts, resume growth (reaching 1.2–1.5 mm), and, upon reaching sexual maturity, reproduce.

There is no consensus regarding the taxonomic affinity of these animals. While dinophilids display certain metameric features (ciliary bands, protonephridia, and a scalariform nervous system), true segments are absent, and the coelom is non-metameric. Some researchers consider them neotenic, reproductively capable annelid larvae (metatrochophores), whereas others (more plausibly) view them as a distinct Lineage that branched off early from other annelids and retained features of a more primitive organization. At the same time, they have acquired specialized traits, such as a simplified life cycle and a loss of regenerative capacity.