INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION ECDYSOZOA

PHYLUM NEMATOMORPHA (HORSEHAIR WORMS)

A small group of worms characterized by larval parasitism. Adult horsehair worms are free-living, marine or freshwater aquatic organisms, whereas the larvae are parasites of insects, or crustaceans in the case of marine species. Approximately 300 species have been described, with only two species known in the fauna of Ukraine: the freshwater Gordius aquaticus and the marine Nectonema agile.

Morphology. The body is elongated, thread-like, ranging from 10 cm to 1.5 m in length and 1–3 mm in diameter. This dark brown, slender body form, reminiscent of a peculiar "living Hair," gave rise to their common name. The anterior end of the body is rounded, while the posterior end in males features two lobes (Fig. 281).

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Fig. 281. The freshwater horsehair worm Chordodes:

a - emerging from a container (after Dogel); b, c - anterior and posterior ends of the body, respectively (after Paramonov)

Skin-muscular sac.

The body wall is covered by a dense, multi-layered cuticle, beneath which lies a thin layer of epidermis (hypodermis) featuring a single longitudinal thickening known as the ventral cord. A continuous layer of longitudinal Muscles underlies the hypodermis. Serpentine locomotion is achieved through the interaction of the longitudinal musculature and the elastic cuticle; unlike nematodes, horsehair worms bend their bodies in the dorsoventral plane.

The body cavity is a schizocoel, more or less filled with parenchyma containing large lacunae.

The Digestive System is vestigial. The Mouth is absent in most species, and the foregut lacks a lumen and is not connected to the midgut. The hindgut Functions as a cloaca, into which the ducts of the Reproductive System of both females and males empty. At no stage of development does the digestive system participate in Nutrition. Newly hatched larvae survive on nutrient reserves obtained from the maternal Organism. Parasitic larvae absorb nutrients from the host's body through the integument; it is during this phase that the cuticle of horsehair worms reaches its highest permeability. Adult horsehair worms subsist entirely on nutrients accumulated during the larval stage.

The excretory system is absent in horsehair worms.

The Nervous system consists of a circumoesophageal nerve ring and an associated longitudinal ventral nerve cord embedded in the ventral epidermal cord. In marine horsehair worms (class Nectonematoidea), a dorsal nerve cord is also present.

Sense Organs are poorly developed. Males possess sensory genital papillae, while some species feature clusters of dark pigment at the anterior end of the body resembling primitive eyes.

Reproductive system. Horsehair worms are dioecious. Sexual Dimorphism is manifested in body size (males are smaller than females) and the Morphology of the posterior body end. Females have two Ovaries with numerous lateral outgrowths, from which short oviducts extend. These unite to form an unpaired Uterus with a small tubular seminal receptacle, opening into the posterior region of the digestive tract (the cloaca). Males possess two elongated, sausage-shaped Testes stretching along the body. Vasa deferentia extend from them and open into the posterior part of the gut. A copulatory organ is present.

Reproduction. The sexes encounter one another in Water, where females release biologically active compounds that attract males. Fertilization is internal; during copulation, the male grasps the posterior end of the female's body with his bifurcated posterior extremity. Development proceeds through metamorphosis. Following copulation, the female lays about a million eggs, which hatch into tiny larvae (0.1–0.4 mm long). The anterior end of the larva bears a proboscis armed with hooks (Fig. 282, b). The larva enters the host's body either by piercing the integument using its proboscis or by being swallowed. It then migrates into the host's body cavity, and subsequently into the musculature or other Tissues. The life cycle may involve a change of hosts if a predator consumes the primary host. In such cases, the primary hosts are small aquatic insects (larvae of mayflies, dragonflies, mosquitoes, etc.), while the secondary (paratenic) hosts are larger predatory insects. Horsehair worms can parasitize not only aquatic Arthropods but also terrestrial ones (e.g., centipedes, mantises, cockroaches, grasshoppers), provided the latter come into contact with water or consume amphibiotic insects; they are only rarely found in leeches. Marine horsehair worms parasitize decapod crustaceans (crabs, shrimps, hermit crabs, etc.).

Fig. 282. Horsehair worms (after Malakhov): a - schematic cross-section of a female; b - larva:

1 - cuticle; 2 - hypodermis; 3 - longitudinal muscles; 4 - parenchyma; 5 - intestine;

6 - ventral nerve cord; 7 - Ovary

Within the host's body, the larva can grow to the size of an adult, but it reaches sexual maturity only after leaving the host and entering the water, following which the host typically dies.

The phylum Nematomorpha is divided into two classes: Nectonematoidea, which are marine horsehair worms, and Gordioidea, which inhabit freshwater environments.