LECTURE OUTLINE IN ZOOLOGY

Module 2. SUWKIND MULTICELLULAR ANIMALS (METAZOA). DIVISION PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA). LOWER METAZOA (EUMETAZOA)

Content Module 2.3. Flatworms, Nemerteans, Rotifers, Spiny-headed worms

Topic 2.3.2. Phylum Nemertea (Nemertini)

General characteristics of the phylum. Origin, evolution, paleontological data, modern diversity, geographical distribution, and Significance in nature and human life. Overview of major taxonomic groups important for human life and the functioning of natural ecosystems. Class Nemertea (Nemertini).

Nemerteans are free-living marine predators. There are about 1,000 species, with 33 species found in the Black Sea. The anterior end of the body features a long proboscis that retracts into a special rhynchocoel (proboscis sheath). The integumentary-muscular sac with ciliated epithelium is well developed; there is no body cavity, and the spaces between Internal Organs are filled with parenchyma. The Digestive System is complete, consisting of foregut and midgut, with the hindgut opening via an anus. The excretory system is of the protonephridial type. The Circulatory system is well developed and closed. The Nervous system consists of two pairs of cerebral ganglia, with two lateral nerve cords extending from the ventral pair and connected by transverse commissures.

Dioecious animals with numerous simply structured Gonads. The phylum includes 1 class.

Class Nemertea (Nemertini). The body is elongated, ribbon-like (rarely thread-like or cylindrical); in some species, the HEAD may be separated by a slight constriction. In nemerteans of the order Bdellonemertini, the posterior part of the body is widened, with a sucker on the ventral side, while Representatives of the subfamily Mururinae possess a caudal appendage. Body length ranges from 3–10 mm (Aenopmertes) up to 10 m or more (Lineus longissimus), and width ranges from 0.3 to 10 mm. They exhibit extreme elasticity. Coloration varies: there are yellow, blue, red, green, or white species. Transparent species also occur, in which case the color is determined by the Contents of the gut. Patterned species can be found. Externally, the body is covered by an epithelium composed of tall ciliated Cells, numerous unicellular glands, and sensory cells. Beneath the epithelium lie Muscles forming three layers: an outer circular layer, a middle longitudinal layer, and an inner circular layer. Both circular layers may be interconnected by Muscle fibers interwoven through the mass of longitudinal muscles. The parenchyma is also penetrated by dorsoventral muscles. Muscle mass accounts for over 50% of the total body weight. Active predators, they feed on polychaetes, Mollusks, and crustaceans. They can subdue prey using their proboscis and body coils, or pierce it with stylets and paralyze it with venom.