INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 2 - H.Y. Shcherbak - 1996

PHYLUM ANNELIDA

CLASS DINOPHILIDA

This Class comprises a small group of marine benthic worms (several dozen species) measuring only a fraction of a millimeter.

The body of dinophilids is elongated and cylindrical, consisting—as in other Annelids—of a HEAD lobe (prostomium), a trunk, and a tail region (pygidium). The trunk includes a small number of indistinctly separated segments with transverse ciliated bands, for instance, six segments in Dinophilus vorticoides (Fig. 17); two additional bands of cilia are located on the head lobe.

Fig. 17. Diagram of the Organization of Dinophilus vorticoides:

a - general structural diagram of the male (coelom not shown); b - cross-sectional diagram of the male through the middle region of the body; 1 - parietal plate; 2 - eye; 3 - rings of cilia; 4 - copulatory organ; 5 - anus; 6 - genital aperture; 7 - hindgut; 8 - midgut; 9 - Testis; 10 - nephridia; 11 - ganglion of the ventral nerve ladder; 12 - Mouth; 13 - Brain; 14 - lateral coelomic pouch; 15 - ventral coelomic pouch; 16 - ventral ciliated strip; 17 - ventral nerve cord; 18 - longitudinal Muscles

The Structure OF THE Skin-muscular sac differs from that of other annelids. A significant portion of the epidermis is represented by ciliated epithelium. Most notably, this includes a rather broad ventral ciliated band extending from the circumoral region to the tail, which is also covered with ciliated epithelium. The ciliated bands connect with the ventral strip. Dinophilids lack a true cuticle; the envelope surrounding their body is better termed a "procuticle". It consists of epithelial Cell outgrowths and their secretion products (Fig. 18). This envelope covers the entire body, including areas with ciliated epithelium. The musculature is represented by separate circular and longitudinal Muscle fibers that do not form a continuous layer.

Fig. 18. Structure of the integument of Dinophilus vorticoides based on Electron Microscopy data:

1, 2 - respective processes and their terminal expansions of the epidermal cell; 3 - fibrous layer; 4 - epidermis

Dinophilids possess a well-developed coelom. It consists of three elongated pouches: a pair of lateral pouches lying on either side of the gut and connecting with each other in the posterior third of the body, and an unpaired ventral pouch located beneath the gut and joining the lateral pouches in the middle of the body (see Fig. 17, b). The coelom is non-metameric; septa and mesenteries are absent.

Between the skin-muscular sac, the coelomic pouches, and the gut lies a parenchyma formed of large Cells that store nutrient reserves. Once these reserves are depleted during overwintering in the encysted state, the parenchymal cells break down, and a schizocoelomic space forms in their place, filled with fluid containing remnants of parenchymal cells and cell membranes, as well as degraded organoids. Thus, depending on their physiological state, the same individuals initially feature a parenchyma alongside the coelom, and later—a primary body cavity (schizocoel).

The Digestive System opens with a mouth on the ventral side at the border between the prostomium and the trunk. The mouth is surrounded by a horseshoe-shaped ridge. Behind the mouth lies the opening of the postoral organ, which houses a dense Tongue covered with a folded cuticle. Dinophilids use this tongue to scrape diatoms and other small organisms from the substrate for food. The mouth leads to a narrow Esophagus (foregut), along which groups of glandular salivary cells are distributed. The midgut is sac-like and occupies the greater part of the body; its epithelium is rich in yellow and orange lipid inclusions, which give dinophilids their color. The hindgut is very short and opens on the DORSAL SIDE OF the body just before the pygidium. The epithelium of all gut regions contains ciliated cells, while the midgut also possesses secretory cells.

Excretory Organs are represented by several pairs of metamerically arranged protonephridia of specialized structure (D. vorticoides males have four pairs, while females have six). Each protonephridial tubule begins with a thin-walled vesicle containing a flame bulb (a modified cyrtocyte) projecting into the coelom, followed by an intermediate region from which an excretory duct extends, featuring numerous cilia along its walls. The cavity of the vesicle communicates with the excretory duct through vacuoles of the intermediate region.

Dinophilids lack a Circulatory system. Gas exchange occurs through the body surface.

The Nervous system consists of a relatively simple supraesophageal ganglion, from which circumesophageal connectives extend around the esophagus to join two ventral nerve cords on the ventral side of the body. These cords bear five or six pairs of ganglia connected in pairs by transverse commissures. Between the thick main cords lie three very thin additional ones. Consequently, the nervous system of dinophilids has the appearance of a ladder with widely spaced cords (see Fig. 17, a).

Sense Organs are represented by eyes, individual sensory cells scattered across the skin, and organs of smell. The eyes are located on the dorsal side of the prostomium and structurally resemble the eyes of turbellarians. The anterior margin of the prostomium bears long sensory hairs belonging to sensory cells; these function as Touch receptors. Similar solitary receptors occur across the entire body surface. The organs of smell appear as a pair of small pits lined with ciliated epithelium and are located on the first trunk segment.

Dinophilids are gonochoric (dioecious). The Gonads are paired and appear as long pouches lying along the sides of the body. In males, they are a pair of sausage-shaped Testes lying on either side of the ventral coelomic pouch. In sexually mature specimens, these connect to one another via broad bridges at the anterior and posterior ends. Paired ducts extend from the testes, forming enlargements—Seminal Vesicles—before merging into an unpaired duct that opens exteriorly via a copulatory organ. The Fertilization mechanism remains unclear. Females possess two pairs of Ovaries situated, like the testes in males, on either side of the ventral coelomic pouch. These ovaries lack ducts. In a sexually mature female, the ventral coelomic pouch contains male sperm, into which eggs subsequently enter through ruptured ovarian walls to be fertilized. Thus, unlike other annelids, dinophilids have gonads located outside the coelom, with Germ Cells entering the coelomic cavity only at a later stage. Development proceeds without metamorphosis. Dinophilids are incapable of regeneration. Asexual reproduction is unknown in them.

Dinophilids inhabit the littoral zone among clumps of filamentous Algae. Dinophilus vorticoides, which populates the coasts of the White and Barents Seas, is the most thoroughly studied species. It exhibits a peculiar life cycle. Young individuals hatching from eggs in spring grow to reach 0.5–0.8 mm and encyst at the end of summer, entering a dormant state. They secrete a protective envelope around themselves; their body becomes rounded, the cilia disappear, and the mouth and anus seal shut. The animals overwinter in this state, emerge from the cyst in February or March, resume growth (reaching 1.2–1.5 mm), and attain sexual maturity. After spawning, the females die. While encysted, dinophilids—as noted previously—survive by metabolizing parenchymal nutrient reserves.

There is no universal consensus regarding the evolutionary affinities of these animals. Although dinophilids exhibit certain features of metameric organization (ciliated rings, protonephridia, a nerve ladder), true segments are absent, and the coelom is non-metameric. Some researchers consider them neotenic larvae of annelids (metatrochophores) that have acquired The ability to reproduce, whereas others—more plausibly—regard them as a separate Lineage that diverged early from other annelids while retaining A number of primitive organizational traits. At the same time, they have also acquired specialized features, such as a simplified life cycle and a loss of regenerative capacity.

Until recently, dinophilids were placed within the Class Polychaeta and assigned to the order Eunicemorpha. Some scientists separate them into an independent order and, together with certain other polychaetes (such as Polygordiidae), group them into a class (or even a subphylum) Archiannelida. This issue remains ultimately unresolved.