SUPPORTING LECTURE NOTES IN ZOOLOGY

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

Content Module 2.4. Nemathelminthes and Cephalochordata

Topic 2.4.1. Phylum Nemathelminthes

General characteristics of the phylum. Origin, evolution, paleontological records, contemporary diversity, geographic distribution, and their role in nature and human life. A review of major taxonomic groups important for human life and the functioning of natural ecosystems. Class Gastrotricha. Class Nematoda.

Free-living, less frequently parasitic forms inhabiting marine and fresh waters, soil, as well as PLANT AND ANIMAL organisms. Comprising over 20,000 species, this group remains insufficiently studied. The body is covered with a cuticle. Ciliated epithelium is restricted to the ventral side or completely reduced; the Skin-muscular sac varies in development, with the musculature being predominantly longitudinal. The primary body cavity (schizocoel) appears as fluid-filled spaces between Internal Organs, lacking a true epithelial lining. The intestine forms a straight, complete tube divided into three sections: an ectodermal foregut—the Pharynx featuring a triangular lumen—an endodermal midgut, and an ectodermal hindgut terminating in the anus. The excretory system is of the protonephridial type or represented by modified dermal glands. The Nervous system consists of a circumoesophageal nerve ring connected to a variable number of longitudinal trunks. True neural ganglia are absent. Sense Organs are poorly developed. Predominantly dioecious, with a relatively simple reproductive apparatus. A distinctive feature is bilateral, strictly determinate Cleavage. Development is direct or proceeds via a larva that generally resembles the adult animal. Larval growth is accompanied by molting; adult individuals do not molt.

Previously, the phylum Nemathelminthes included the following classes: Gastrotricha, Nematoda, Rotatoria, Kinorhyncha, Nematomorpha, and Priapulida. All of them share a cuticle, a primary body cavity, and a complete gut. Today, however, the phylum Nemathelminthes is divided into two classes: Gastrotricha and Nematoda (Roundworms).

Class Gastrotricha. Very small animals (0.5 – 1.5 mm) with both marine and freshwater representatives. Approximately 300 species are known. The fauna of Ukrainian freshwater bodies includes about 40 species, while the Black Sea hosts around 20. The body is bilaterally symmetrical, elongated, flattened ventrally, and convex dorsally, terminating in a bifurcated, fork-like tail. There are two distinct ecological life forms differing in habitat, body shape, and structural features. Most marine gastrotrichs (order Macrodasyoidea) inhabit capillary spaces between sediment particles. Their bodies are narrow and elongated. Species of the order Chaetonotoidea are predominantly freshwater (with only a few marine representatives). They live On the surface of substrates and aquatic vegetation, possessing a short, flask-shaped body. The ventral side bears numerous cilia, and the body is enclosed in a cuticle that covers both the dorsal and ventral surfaces as well as the bases of the cilia without hindering their beat. In macrodasyoids, the cuticle is thin, whereas in chaetonotids, it becomes quite thick on the dorsal side, serving a protective function. In many species, it bears small spines, scales, and other structures.

Class Nematoda. One of the most numerous and widespread animal groups. They inhabit seas, freshwater bodies, and soils, and include numerous plant and animal parasites. About 20,000 species have been described; however, according to various estimates, the total number of species ranges from 80,000–100,000 to as high as 1 million. The majority are free-living organisms inhabiting narrow capillary spaces between particles of soil, sand, or bottom silt in aquatic environments or on land, while only about 7,000 species (one-third of known species) are plant and animal parasites. Ukrainian freshwaters harbor over 300 species, and the Black and Azov seas contain about 200. Furthermore, 561 species of parasitic nematodes have been recorded in vertebrates. The body is typically spindle-shaped and round in cross-section; in parasitic species, it is more elongated and thread-like. Females of certain plant-parasitic species (such as Meloidogyne) are pear-shaped due to the extensive development of their Reproductive System. Free-living forms measure 0.3–1 mm, phytonematodes reach 8–10 mm, and the largest vertebrate parasites (such as the human ascariid) grow to 30–40 cm, while *Placentonema gigantissima*, a parasite inhabiting the Placenta of the sperm whale, can reach 6–8 meters. The Mouth is located at the anterior end, and the anus is situated on the ventral side near the posterior end. Bilateral Symmetry is poorly expressed, whereas radial symmetry is more pronounced. The body is spindle-shaped and circular in cross-section. The anterior end of free-living species bears sensory bristles arranged in accordance with triradiate symmetry, a pattern also reflected in the arrangement of the three or six Lips surrounding the mouth. In internal Organization, however, bilateral symmetry is distinct. The skin-muscular sac consists of the cuticle, hypodermis, and a layer of longitudinal Muscles. The cuticle is a complex multi-layered Structure comprising four zones (epicuticle, exocuticle, mesocuticle, and endocuticle), each built from multiple sublayers. The cuticle features alternating layers of parallel rods oriented perpendicularly to the surface, oblique, annular, and longitudinal fibrils, and an amorphous dense matrix (which imparts strength and elasticity). The outer layers are permeated by a system of microscopic canals. At the anterior end, the cuticle becomes thickened due to the expansion of the loose mesocuticular layer.