Lecture Notes on Invertebrate Zoology

Semantic Module 2. Origin of Metazoans and Their Phyla: Flatworms, Roundworms, Annelids

Lecture Topic 8. Characteristics of the Phylum Nematoda (Roundworms or Pseudocoelomates)

Biological progress and biological regression. Formation of the primary body cavity (pseudocoelom) and its Functions. Evolution of The Nervous system, straight-through intestine. Structural Features of the cuticular-muscular sac. Dioecious forms, Sexual Dimorphism. Origin of Roundworms, major classes: Gastrotricha, Rotifera, Nematoda. Geohelminths and biohelminths. Developmental cycles of Ascaris, Trichinella, Ancylostoma. Parasitism as a form of interorganismic relationship. Nature of parasitism. V.A. Dogiel's theory on THE ORIGIN OF parasitism. Influence of lifestyle on the external and internal Morphology of parasites. Principal features of parasitic worms.

Roundworms are predominantly free-living, less frequently parasitic organisms adapted to diverse living conditions (marine and fresh waters, soil, PLANT AND ANIMAL Tissues). Over 20,000 extant species have been described; however, their actual number is significantly higher, as this group remains insufficiently studied. The body is covered by a cuticle. Ciliated epithelium is retained only on the ventral side or is entirely reduced; the cuticular-muscular sac varies in development, and the musculature is predominantly longitudinal. Nemathelminths possess a primary body cavity (schizocoel), which appears as fluid-filled clefts between Internal Organs without a proprietary epithelial lining. The intestine forms a straight, through tube divided into three regions: the ectodermal foregut—the Pharynx (usually with a triangular lumen), the entodermal midgut, and the ectodermal hindgut terminating in the anus. The excretory system is of the protonephridial type or represented by modified dermal glands. The nervous system is represented by a circumesophageal nerve ring from which a varying number of longitudinal trunks extend. True neural ganglia are absent. Sense Organs in pseudocoelomates are poorly developed. Pseudocoelomates are primarily dioecious animals with a simple reproductive apparatus. A characteristic feature of embryonic development is direct development or the hatching of a larva generally similar to the adult animal. Larval growth is accompanied by molts; adults do not molt. The phylum Pseudocoelomates is divided into two classes: Gastrotricha and Nematoda (Roundworms). Nematodes are one of the most abundant and widespread animal groups. They inhabit seas, freshwater bodies, soils, and act as plant and animal parasites. Approximately 20,000 nematode species have been described. The majority are free-living, inhabiting narrow capillary spaces between soil particles, sand, bottom silt, or terrestrial substrates, while only about 7,000 species (roughly one-third of known species) are plant and animal parasites. Over 300 nematode species have been found in the freshwaters of Ukraine, and about 200 in the Black and Azov Seas. Among vertebrate animals, 561 species of parasitic nematodes have been recorded.

The body shape in nematodes is predominantly fusiform, circular in cross-section; in parasitic species, it is more elongated and thread-like. Females of certain plant-parasitic species (e.g., Meloidogyne) are pyriform, which is attributed to

the hyperdevelopment of their Reproductive System. Free-living nematodes are typically microscopic in size (0.3–1 mm), whereas phytonematodes reach 8–10 mm. The largest vertebrate parasites, such as the human roundworm (*Ascaris lumbricoides*), reach lengths of 30–40 cm, while *Placentonema gigantissima*, a parasite inhabiting the Placenta of the sperm whale, attains 6–8 m. The Mouth opening is located at the anterior end of the body, and the anus is situated on the ventral side near the posterior end. Nematodes are characterized by poorly pronounced bilateral Symmetry, with radial symmetry being more apparent in their external appearance. The DORSAL SIDE OF the body differs little from the ventral side. Their body is fusiform and circular in cross-section, which explains the common name of nematodes—roundworms. The anterior end of free-living species bears sensory bristles whose arrangement follows a triradiate symmetry, as does the arrangement of three or six Lips surrounding the mouth. Internal morphology exhibits more pronounced bilateral symmetry. The cuticular-muscular sac of nematodes consists of a cuticle, hypodermis, and a layer of longitudinal Muscles. The nematode cuticle is a multilayered Structure comprising four layers (epi-, exo-, meso-, and endocuticle), each built from multiple sublayers. At the cephalic end in many nematodes, the cuticle thickens due to the expansion of the loose mesocuticular layer. The cuticle not only covers the exterior of the nematode body but also lines all ectodermal Regions of the Digestive System—the Oral Cavity, pharynx, and hindgut. In these regions, its structure is considerably simpler than that of the outer cuticle. Nematode growth is accompanied by molting. Beneath the cuticle lies the hypodermis, a modified syncytial epidermal epithelium of the embedded type.

The nematode hypodermis contains a limited number of nuclei arranged in regular longitudinal rows. The hypodermis serves as an important barrier tissue, regulating the selective permeability of substances, and additionally functions as a primary energy-storage tissue accumulating Lipids and Glycogen. Nematode musculature consists of a single layer of longitudinal Muscle Cells forming four quadrants (two dorsal and two ventral bands). Nematodes are characterized by a primary body cavity—the schizocoel. The mouth of nematodes is surrounded by three to six lips, though some species lack lips. The mouth leads into the foregut, or pharynx, lined with a thin cuticle. The pharynx is subdivided into an anterior region (othom) and a posterior region (Esophagus). The othom frequently bears immovable cuticular thickenings known as onchia, as well as movable Teeth. Certain nematodes even possess jaws, while others feature a sharp stylet or spear. The pharynx functions as a suction pump for ingesting liquid food. In the posterior region of the pharynx of some nematodes (e.g., *Enterobius vermicularis*), There is a muscular bulb that acts like a pipette, increasing suction power. Digestion in nematodes initiates within the oral cavity under the action of Enzymes secreted by pharyngeal glands. In some species, primarily plant-parasitic nematodes, extraintestinal digestion occurs: saliva is secreted externally, liquefying plant tissues, after which the liquid food is ingested and fully digested in the midgut. Besides DIGESTION AND ABSORPTION, the midgut functions as a nutrient storage organ, accumulating glycogen, lipids, and protein granules within its cells. The midgut also acts as an excretory organ by accumulating various Inorganic Components that are eliminated from the body along with sloughed cells. The hindgut is a short tube internally lined with cuticle. A muscular valve is located at the junction of the Midgut and Hindgut. Protonephridia are absent in nematodes. The primary excretory organ is the cervical gland. Phagocytic cells mentioned previously, along with the intestine, also participate in The excretion of Metabolic waste products.

The nervous system of nematodes consists of a circumesophageal nerve ring. The highest concentration of sense organs is located at the anterior (cephalic) end of nematodes, forming two or three concentric rings. These include labial and cephalic papillae—conical cuticular elevations surrounded at the base by a cuticular collar—and setae, which are longer than the papillae. Frequently, a pair of pocket-like or spiral cuticular depressions known as amphids is situated at the anterior end. The sense organs of nematodes, particularly free-living forms, exhibit diverse structural variations. The nematode cuticle is insensitive to mechanical stimuli, hence free sensory nerve endings are absent in the integument.

Nematodes are dioecious animals with internal Fertilization. Their reproductive system has a comparatively simple tubular structure. In females, the slender free ends of both tubes are Ovaries, where gametogenesis and egg Cell production occur. The ovaries directly transition into tubular oviducts of a somewhat larger diameter. Oocytes entering the oviducts gradually advance into the uteri, which are formed by tubes of an even greater diameter. The walls of the Uterus contain muscular and glandular cells, where fertilization of the egg cells and the Formation of protective egg membranes take place. Two uteri fuse into a single unpaired duct—the Vagina—which opens on the ventral side of the body via the female genital pore. In some species, one of the reproductive tubes is reduced, and

the reproductive system becomes unpaired. The Male Reproductive System is similarly built from paired reproductive tubes, though often one of them is reduced. It consists of a long, slender Testis where spermatozoa are formed. The testis continues into a vas deferens, followed by a slightly expanded Seminal Vesicle and a long, cylindrical ejaculatory duct with muscular walls, which opens along with the hindgut into the cloaca. The cloaca contains a specialized copulatory apparatus consisting of a pair of cuticular spicules operated by specialized muscles. The spicules aid in fixing the male to the female's body, as well as dilating the female's vagina and transferring spermatozoa during copulation.

In free-living nematodes, a first-stage larva hatches from the egg, and all four molts occur externally in the environment. Parasitic nematodes of the subclass Rhabditia are characterized by partial embryonization of development, wherein one or two molts occur under the egg shell. In certain zooparasites (order Strongylida), a third-stage larva hatches from the egg. Larvae of free-living nematodes share the same lifestyle as adults. In many parasitic forms, infective larvae inhabit intermediate hosts, soil, or Water, and differ significantly in morphology from the adult stage.