INVERTEBRATE ZOOLOGY IN THREE VOLUMES — BOOK 1 — H.Y. Shcherbak — 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE METAZOANS (EUMETAZOA)

PHYLUM CEPHALORHYNCHA

CLASS HORSEHAIR WORMS (GORDIACEA OR NEMATOMORPHA)

Horsehair worms (Gordiacea) are a relatively small group of parasitic worms characterized by larval parasitism. Over 300 species have been described. The fauna of Ukraine includes two known species: the common freshwater Gordius aquaticus and Nectonema agile, which parasitizes Black Sea crustaceans. Only newly hatched larvae and adult worms lead a free-living lifestyle. Gordiacea inhabit both marine and freshwater environments. The larvae parasitize insects, while marine species parasitize crustaceans.

The body of horsehair worms is long and thread-like, measuring 40–50 cm, and in some species up to 1.5 m in length (with a width of 1–3 mm). This shape, combined with a dark brown coloration, earned Gordiacea the common names "living Hair" or "horsehair worm". Adult horsehair worms lack a proboscis; it is present only in the larval stage. The anterior end of the body is rounded, while the posterior end in males is bifurcated (Fig. 212).

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Fig. 212. Horsehair worms (Gordiacea):

a — the marine horsehair worm Nectonema agile inside a crab; b — Chordodes emerging from a cockroach; c — anterior, d — posterior ends of the body of Gordius aquaticus

The integument of Gordiacea consists of a dense multilayered cuticle overlying a thin layer of hypodermis, which forms a single longitudinal thickening known as the ventral cord. Beneath the hypodermis lies a continuous layer of longitudinal Muscles. A large space between the gut and the body wall is filled with parenchyma containing prominent lacunar spaces. The parenchyma is composed of vesicular Cells interspersed with a well-developed fibrous intercellular matrix (see Fig. 212). Adult horsehair worms exhibit typical undulating locomotion based on the interaction between the cuticle and the longitudinal musculature; however, unlike nematodes, they bend the body in a dorsoventral direction. The tension of the cuticle is maintained by the high turgor pressure of the parenchymal cells.

The Digestive System of horsehair worms is rudimentary. In most species, the Mouth is absent, and the foregut lacks a lumen and does not connect with the midgut. The hindgut Functions as a cloaca. At no stage of development do their digestive systems participate in nutrient absorption. Young larvae that have not yet begun a parasitic life rely on energy reserves obtained from the egg. Parasitic larvae do feed, but nutrients are absorbed directly through the cuticle, which exhibits high permeability during this period. Horsehair worms that have left their host do not feed in the external environment, and the permeability of their cuticle drops significantly. They survive on energy reserves accumulated during the parasitic phase of development.

Fig. 213. Diagram of the cross-sectional anatomy of a female horsehair worm:

1 — cuticle; 2 — hypodermis; 3 — longitudinal muscles; 4 — parenchyma; 5 — gut; 6 — ventral nerve cord; 7 — Ovary

Excretory Organs are absent in horsehair worms; the gut likely performs this function.

The Nervous system consists of a circumoesophageal nerve ring connected to a ventral nerve cord situated within the ventral epidermal cord. Sense Organs are poorly developed. Males possess sensory genital papillae, and in some species, clusters of dark pigment resembling ocelli are located at the anterior end.

Horsehair worms are dioecious animals. Males differ from females in their smaller size and the Morphology of the posterior end of the body. The FEMALE Reproductive System consists of two elongated Ovaries with numerous lateral outgrowths. The posterior ends of the ovaries transition into short oviducts that empty into a single Uterus equipped with a small tubular seminal receptacle. The uterus opens into the hindgut, which thus functions as a cloaca. The Male Reproductive System consists of two long, sausage-shaped Testes extending along the body and continuing into vas deferens, which open into the cloaca. During copulation, the male grasps the posterior end of the female's body with his bifurcated posterior tip.

KEY FEATURES OF embryonic development include deuterostomy, as seen in priapulids, and indeterminate Cleavage of the ovum. Following copulation, the adult female lays about 1 million eggs. The egg masses appear as long, thin gelatinous strings. The hatching larvae differ significantly in morphology from adult worms. They are short, 0.1–0.4 mm in length, and feature an anterior proboscis consisting of a mouth cone armed with three pointed cuticular plates (stylets), a middle section bearing two to three tiers of scalids, and a posterior region transitioning into the trunk (Fig. 214).

Fig. 214. Embryonic development of Gordius — blastula (a), parenchymula (b), Formation of the secondary mouth (c), newly hatched larva (d): 1 — primordium of the proboscis region; 2 — secondary mouth; 3 — blastopore; 4 — gut rudiment

The Development of horsehair worms involves one or, more rarely, two hosts. The larva enters its host—most commonly an insect—by piercing the integument in thinner areas using its proboscis. In other cases, the larva is ingested by the host, subsequently migrating from the gut into the body cavity, and from there into the musculature or other Tissues. When development involves two hosts, the second host is typically a predatory arthropod that preys upon the first host. The larva develops over several months, growing to the size of an adult stage, but never reaches sexual maturity inside the host. Eventually, the larva emerges into the Water from the host's body, which typically dies in the process. In the water, horsehair worms reach sexual maturity, copulate, lay eggs, and soon die.