ZOOLOGY OF INVERTEBRATES IN THREE VOLUMES - BOOK 1 - H.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

BRANCH TRUE METAZOA (EUMETAZOA)

PHYLUM NEMERTINEANS (NEMERTINI)

CLASS NEMERTINEANS (NEMERTINI)

The body of nemertineans is typically elongated, and in some species, the HEAD may be set off by a slight bodily constriction. Its shape usually resembles a ribbon; filamentous or cylindrical forms occur less frequently. In nemertineans of the order Bdellonemertini, the posterior part of the heavily flattened body is widened, bearing a sucker on its ventral side, while Representatives of the subfamily Mucrurinae possess a caudal appendage (Fig. 172).

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Fig. 172. Nemertineans — Lineus genicumus (a), Hoplonemertini (б), Malacobdella grossa (б): 1 — lateral slits; 2 — sucker

The body length of nemertineans ranges from 3–10 mm (Agenomertes) to 10 m or more (Lineus longissimus), with a width of 1 mm. It should be added that due to the high elasticity of the body, determining the exact length of a given specimen can be quite difficult.

The coloration of nemertineans is highly variable; yellow, blue, red, green, or white species have been described. There are completely transparent species in which the color is determined by the Contents of the gut. Species with characteristic patterns are frequently encountered.

Externally, the body of nemertineans is covered by an epithelium composed of tall ciliated Cells, numerous unicellular glands, and sensory cells. Beneath the epithelium lie Muscles, which most commonly form three layers — an outer circular, a middle longitudinal, and an inner circular layer. In representatives of various orders, the number of these layers may be considerably greater. Both circular layers can be interconnected by Muscle fibers weaving through the mass of longitudinal muscles. The parenchyma in nemertineans is frequently permeated by dorsoventral muscles as well. Nemertineans belong to animals possessing a large muscle mass, which accounts for over 50 % of their total body weight.

Nemertineans are active predators feeding on polychaetes, Mollusks, and crustaceans. They can subdue their prey by coiling their proboscis and entire body around it, or by piercing it with stylets and paralyzing it with venom.

Fig. 173. Anatomy of nemertineans — general diagram (a), Cytology/practical/54.html">Longitudinal section of Cerebralis fuscus (б): 1 — main Blood vessel; 2 — cerebral ganglion; 3 — Mouth; 4 — Esophagus; 5 — lateral and 6 — dorsal Blood Vessels; 7 — Gonads; 8 — genital openings; 9 — anus; 10 — hindgut; 11 — intestinal diverticula; 12 — frontal organ; 13 — proboscis sheath; 14 — lateral nerve cord; 15 — retracted proboscis; 16 — Stomach

The Digestive System of nemertineans possesses features not found in Flatworms. The mouth opening is located at the anterior end of the body on the ventral side. It leads into the Pharynx, which passes into the esophagus and further into the midgut, the latter often bearing lateral outgrowths called diverticula. The digestive system terminates in a short hindgut that opens to the exterior via the anus. Thus, nemertineans possess a complete digestive tract. The feeding apparatus of nemertineans also includes an organ characteristic of this group — a proboscis in the form of a muscular tube of varying length in different species. The proboscis is housed within a special proboscis cavity (rhynchocoel), with which its base is fused.

The proboscis is everted to the outside through a special pore or via the mouth. It moves with the help of specialized muscles that turn it inside out like a glove finger. The proboscis is retracted back into the sheath by means of specialized retractor muscles. In some nemertineans, the proboscis bears specialized poison glands and sharp stylets that pierce the prey.

The excretory system consists of two protonephridia located in the anterior part of the body; each of them opens to the exterior through its own pore. The excretory system is closely linked to the Circulatory system — the lateral tubules, terminating in cyrtocytes, deeply penetrate the walls of the lateral blood vessels, which greatly facilitates the diffusion of Metabolic waste products from the blood into the excretory system.

An important feature of nemertineans compared to flatworms is the presence of a circulatory system. It is closed, consisting of three main longitudinal vessels — one dorsal and two lateral. The dorsal vessel is situated above the gut (between it and the proboscis sheath). All three vessels merge in the cephalic region and, furthermore, are interconnected throughout the body length by transverse commissures. The main vessels can branch into smaller ones leading to various Organs (esophagus, proboscis sheath, etc.). In lower nemertineans, blood flows irregularly — back and forth due to the contractions of body muscles. In more highly organized forms, the vessel walls possess their own musculature, and Blood Circulation becomes orderly: it flows from front to back in the lateral vessels, and from back to front in the dorsal vessel.

The circulatory system plays a major role in transporting nutrients from the gut to all organs and PARTS OF THE body. Feeding on large prey requires a higher metabolic rate for its rapid assimilation. The parenchyma filling the spaces between organs hinders the rapid transport of nutrients. The appearance of a circulatory system, whose vessels and capillaries grow through the parenchyma, eliminates this obstacle. The blood contains cellular elements (blood corpuscles) bearing various respiratory pigments, notably Hemoglobin. They impart a red, yellow, or green color to the blood. Thus, the circulatory system transports not only nutrients but also oxygen entering through the body wall.

The Nervous System in various groups of nemertineans has a different Structure. In some species, it is represented by an epithelial nerve plexus, in which the longitudinal cords are merely condensations of Nerve Cells, while in others, the cords are better defined and located deeper. The Brain of nemertineans consists of two dorsal and two ventral ganglia connected by commissures. It differs from the brain of other lower worms by a more complex structure, necessitated by the higher development of Sensory Organs. Two lateral longitudinal cords originate from the ventral ganglia. A large number of sensory cells are distributed throughout the body, especially on the ventral side. Nemertineans actively respond even to very weak stimuli. At the anterior end of the body, at the level of the brain ganglia, nemertineans possess organs of chemical sense. These are the lateral slits and cerebral organs.

The lateral slits appear as depressions located on the sides of the head region, lined with ciliated epithelium and sensory cells that perceive chemical stimuli. The cerebral organs are also slit-like, but they are more deeply invaginated into the body wall and reach directly to the brain ganglia; sometimes they push into the posterior part of the ganglia and even fuse with them. Thanks to chemoreceptors, nemertineans respond not only to The chemical composition of the Water but also to the taste of food.

Most nemertineans possess one or several pairs of primitive eyes consisting of a small number of photoreceptor cells sunken into pigmented cups. Like turbellarians, nemertineans perceive only light stimuli, lacking image-forming Vision.

Nemertineans are dioecious animals, and their reproductive apparatus has a simple structure. The Ovaries of females and Testes of males are epithelial sacs filled with Germ Cells. They are located in the parenchyma in pairs on the sides of the gut. If the gut has diverticula, the gonads are situated between adjacent diverticula. Genital ducts are formed at the time of gamete maturation and appear as short outgrowths of the gonad walls that open to the exterior independently of one another.

As a rule, external Fertilization occurs in nemertineans. In some species, up to 50 individuals gather into a cluster and secrete mucus into which the Gametes are released; fertilization and subsequent egg development take place right there.

The egg of a nemertinean undergoes spiral Cleavage. Gastrulation occurs via invagination or the ingression of a dense Cell mass.

Nemertines develop with metamorphosis. A free-swimming larva hatches from the egg, which varies in structure across different species. In the relatively small order Palaeonemertini, a gastrula-like larva emerges from the egg; it differs from the adult by a simpler structure and the presence of a sensory apical organ at one pole (the animal pole), consisting of small cells with a tuft of long cilia. The larva is covered with ciliated epithelium, and it possesses a mouth, a foregut, and a blind-ending midgut. Later, the hindgut and anus form at the posterior end. During the metamorphosis of such a larva, the ectodermal epithelium is shed, and the definitive epithelium of the adult nemertine forms beneath it. Along with it, the larval apical organ is also shed.

In other nemertines, the egg hatches into a more complex larva called a pilidium. Its external appearance bears no resemblance to the adult Organism (Fig. 174). Outwardly, the pilidium resembles a helmet with a tassel and side flaps. At the apex of the dome-shaped body lies a tuft of long cilia (the tassel) situated on a specialized ectodermal thickening, the apical plate. The entire larva is covered with ciliated epithelium, but a ring of particularly long cilia—the preoral ciliated band—runs along the lower margin of the body and flaps. In the middle of the lower half of the larval body is the mouth opening, which leads into the foregut, transitioning into a blind-ending midgut. The pilidium transforms into the adult in a rather unique way. Invasaginations appear in the ectoderm of the pilidium directed toward the gut; these subsequently detach from the ectoderm and form closed vesicles known as imaginal discs. Gradually expanding, they fuse together, enclosing the gut. A new ectoderm forms from these ectodermal invaginations beneath the pilidium's integument. The nemertine emerges by rupturing the pilidium's covering and consuming it, after which it settles to the bottom and transitions to a crawling mode of life.

The majority of nemertine species inhabit the coastal zone of seas, where they hide in gravel, rock crevices, thickets of marine Algae, and so forth. Some of them live in tubes formed by the secretion of dermal glands.

Fig. 174. Nemertine larvae — gastrula-like larva (a), pilidium (b): 1 — apical organ; 2 — eyes; 3 — gut; 4 — imaginal epithelium; 5 — parenchyma; 6 — larval epithelium; 7 — imaginal discs

Among nemertines, There are also pelagic forms, meaning inhabitants of the open ocean. Representatives of a few species have adapted to life on land (Geonemertes) in damp places and in freshwater bodies (Prostoma, Planodineus, Siolineus).

In addition to predators, commensal species are known, and parasites of mollusks (Malacobdella), crabs (Carcinonemertes), and certain other invertebrates are occasionally encountered.