INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA), or BILATERALLY SYMMETRICAL (BILATERIA) ANIMALS

SUBSECTION SPIRALIA (SPIRALIA)

PHYLUM NEMERTEA (NEMERTINI)

Predominantly free-living marine predatory animals, with only a few species inhabiting freshwater bodies. Mostly benthic. Marine species are found at various depths, ranging from the littoral zone to several hundred meters. Distributed primarily in Arctic and temperate latitudes, and less commonly in the tropics. Over 900 species have been described (with 33 species known in the Black Sea). The name of the phylum originates from Greek mythology: Nemertes was one of the Nereids, daughters of the sea god Nereus.

Nemerteans combine features characteristic of Flatworms (a well-developed cutaneous-muscular sac with ciliated epithelium, absence of a body cavity, parenchyma, protonephridial excretory system, and orthagonal Nervous system) with traits of a significantly higher level of Organization (a complete Digestive System consisting of a foregut, midgut, and an ectodermal hindgut with an anus, as well as a well-developed closed Circulatory system). A specific feature of nemerteans is a long proboscis located at the anterior end of the body.

The body length in most species ranges from 2-20 cm with a width of 1-5 mm; however, a giant nemertean (Lineus longissimus) measuring 30 m in length and 1 cm in width has been found off the coast of Great Britain.

The coloration of nemerteans is highly diverse, with green, blue, yellow, or white species described. Some are completely transparent, in which case their color is determined by the Contents of the intestine.

Anatomy. The body is typically slender, elongated, and ribbon-like, although some species resemble leeches (Fig. 262). In certain species, the HEAD may be demarcated by a slight constriction. The anterior end features a proboscis that invaginates into a specialized sac known as the rhynchocoel (proboscis sheath). The proboscis is everted outward through a specialized pore or through the Mouth. In many species, the proboscis is armed with one or more stylets. The length of the proboscis can sometimes exceed the length of the body itself, in which case it lies folded in loops within the rhynchocoel.

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Fig. 262. Nemerteans (from Livanov): a - Lineus geniculatus; b - Hoplonemertini; c - Malacobdellina grossa: 1 - lateral slits; 2 - sucker

Cutaneous-muscular sac. The exterior of the body is covered by a ciliated epithelium, among the tall Cells of which are numerous unicellular glands and sensory cells. Beneath the epithelium lie three layers of Muscles: circular, longitudinal, and another circular layer. Some species possess additional Muscle layers as well as dorsoventral muscles. The total muscle mass can exceed half of the body's total weight. This powerful musculature enables active locomotion in nemerteans.

Nemerteans move in various ways. Small species swim using cilia; larger ones crawl through muscular contractions; pelagic forms swim in the Water Column by undulatorily flexing their bodies. Terrestrial species use their proboscis for locomotion: they anchor it to the substrate ahead of them and then pull themselves forward.

A true body cavity is absent in nemerteans, with the spaces between Internal Organs filled with parenchyma. Recent studies have shown that the proboscis sheath is lined with mesodermal coelomic epithelium, and therefore it should be considered a coelomic Structure.

The digestive system is represented by a complete intestine. Located on the ventral side of the anterior end is the mouth opening, which leads into the Pharynx, followed by the Esophagus and then the midgut, the latter frequently forming lateral diverticula (pouches). A short hindgut terminates with the anus (Fig. 263).

Fig. 263. Anatomy of a nemertean:

a - general diagram (from Dogiel); b - Cytology/practical/54.html">Longitudinal section of Cerebralis fuscus (from Beklemishev):

1 - main Blood vessel; 2 - cerebral ganglion; 3 - mouth; 4 - esophagus; 5, 6 - lateral and dorsal Blood Vessels, respectively;

7 - Gonads; 8 - genital pores; 9 - anus; 10 - hindgut; 11 - intestinal diverticula; 12 - frontal organ; 13 - proboscis sheath (rhynchocoel);

14 - lateral nerve cord; 15 - retracted proboscis; 16 - Stomach

Nemerteans are predators that feed on Annelids, crustaceans, Mollusks, and fish. They can subdue their prey by wrapping their proboscis and entire body around it, or by piercing it with stylets and paralyzing it with venom secreted by specialized glands, as seen in Paranemertes peregrina. In unarmed nemerteans (class Anopla), toxic compounds are secreted by glandular Cells of the integument and become part of the mucus.

The excretory system is represented by two protonephridial canals that open to the outside. Spatially and functionally, it is linked to the circulatory system: the lateral ductules of the protonephridia, which terminate in cyrtocytes (flame cells), deeply invaginate into the walls of the lateral blood vessels, facilitating the diffusion of Metabolic waste products from the blood into the excretory system.

The circulatory system is closed; blood flows through vessels and capillaries that are continuous throughout. It consists of three main longitudinal vessels: a dorsal vessel (situated above the intestine) and two lateral ones. All of them unite in the anterior (head) region and are additionally interconnected by ring vessels at both the anterior and posterior ends of the body. The main vessels branch into finer ones and, ultimately, into capillaries. In most nemerteans, the vessel walls contain Muscle tissue. Blood flow is directional: it moves from anterior to posterior in the lateral vessels and from posterior to anterior in the dorsal vessel. The primary function of the circulatory system is to transport nutrients from the intestine to all PARTS OF THE body, which is especially vital for predatory species that move rapidly and require an intensive METABOLISM. As noted earlier, the circulatory system is closely linked to the excretory system to ensure more efficient Elimination of Metabolic waste. Blood also transports oxygen absorbed through the body surface, containing respiratory pigments, including Hemoglobin, which bind oxygen.

Nemerteans respire through their entire body surface.

The Nervous System is an orthogone. The Brain consists of two dorsal and two ventral ganglia connected by a commissure. Two lateral nerve cords extend from the brain, linked by numerous commissures.

Sense Organs are well developed. A large number of sensory cells are scattered across the entire body. The anterior end of the body bears organs of chemical sense: lateral slits—recesses lined with ciliated epithelium and sensory cells—and cerebral organs, which are even larger body-wall invaginations that extend directly into the cerebral ganglia. These organs not only monitor The chemical composition of the water but also aid in prey detection. Most nemerteans possess one or several pairs of eyes consisting of a small number of photoreceptor cells sunken into pigment cups. Nemerteans are only capable of distinguishing light intensity and cannot perceive the shapes of objects.

Reproductive System. Nemerteans are dioecious. The reproductive apparatus has a simple structure. The gonads (Ovaries and Testes) appear as sac-like pouches filled with Gametes. In both females and males, they are arranged in pairs on either side of the intestine. The genital ducts form during the maturation of Germ Cells and appear as short tubules opening externally via independent pores.

Reproduction is predominantly sexual, though some species are known to reproduce vegetatively through fragmentation. Vegetative Reproduction is associated with the remarkable regenerative capacity of nemerteans. In sexual reproduction, Fertilization is external. In some species, dozens of individuals of different sexes form a mucous-covered cluster into which male and female gametes are released; fertilization and the Early stages of embryonic development take place within this mass. Cleavage is spiral; Gastrulation occurs via invagination or the inward migration of a dense Cell mass.

Development proceeds with metamorphosis. In most nemerteans, the egg hatches into a complex, free-swimming larva known as a pilidium (from the Greek pylidion, meaning felt cap). Structurally, the larva indeed resembles a cap equipped with an apical tuft and lateral lobes (Fig. 264). Its upper pole bears a sensory organ—the apical plate with a tuft of long cilia. The entire body of the pilidium is covered with ciliated epithelium, while its lower margin and lobes bear a ring of exceptionally long cilia. A mouth and a blind-ending gut are present. The metamorphosis of the pilidium is quite unique. Invaginations appear within its ectoderm and eventually pinch off to form closed vesicles known as imaginal discs. Beneath the layer of larval epithelium, these discs subsequently develop into the adult structures characteristic of mature individuals. The fully formed nemertean emerges by rupturing the larval envelope of the pilidium and consuming it, after which it settles to the bottom, transitioning from a planktonic to a benthic lifestyle.

Fig. 264. Nemertean larvae (from Ivanov): a — gastrula-like larva; b — pilidium:

1 — apical organ; 2 — eyes; 3 — gut; 4 — imaginal epithelium; 5 — parenchyma;

6 — larval epithelium; 7 — imaginal disc

The phylum Nemertea is divided into two classes: Anopla, which lack stylets on the proboscis, and Enopla, in which the proboscis is armed with stylets. There are other distinguishing features as well: in anoplans, the nerve cords are located within the body wall, whereas in enoplans, they lie deeper, beneath the Skin-muscle tube. Among armed nemerteans, alongside benthic forms, there are pelagic species (genera Nectonemertes, Pelagonemertes) that swim in the water column, as well as isolated species inhabiting freshwaters (genus Stichostemma) or damp soil (genus Geonemertes). In addition to predators, this group includes commensals (e.g., species of the order Bdellonemertini) that take up residence in the mantle cavity of bivalve mollusks and feed by filter feeding, as well as true parasites (such as the crab parasite Carcinonemertes).