232 ^^^ ANIMAL KINGDOM 



Kinorhynchs have a body musculature reduced to separate strands as in 

 the rotilers. Ihe cuticle is segmented into l'^ or 14 joints, and internal 

 structures such as the muscles and nerve cells are segmentally arranged. 



95. Class Gordiacea 



The Gordiacea are the hairworms (Fig. 12.1) that often appear in 

 spring water. The body is extremely long and slender and tapers little 

 if at all at either end. Hairworms are parasitic as juveniles, free-living 

 as adults. The adults live near or in water, in which they lay long strings 

 of eggs. These hatch into short fat larvae that infect grasshoppers, 

 crickets and other insects. They bore through the digestive tract into the 

 body cavity where they grow to adult size, following a single molt. After 

 the adult leaves the host it apparently does not feed and its digestive 

 tract may become closed and degenerate. The adult is often much 

 tangled with itself, suggesting a Gordian knot. 



96. Class Acanthocephala 



Adult spiny-headed worms (Fig. 12.1) live in the digestive tracts of 

 vertebrates. The head is retractile, and may be withdrawn as the worm 

 crawls about, or everted and thrust into the intestinal wall as an anchor. 

 The head bears rows of recurved spines and the wounds produced by 

 them may become serious if infected. Large numbers of eggs, usually well 

 advanced in development, pass out in the host feces, and hatch only if 

 they are eaten by an arthropod. The young larva bores through the 

 digestive tract of this first host into the body cavity, where it develops 

 into a miniature adult. If the arthropod host is eaten by the vertebrate 

 host, the worm matures in the intestine of the latter. Most species are 

 small, not more than an inch long. The spiny-headed worm of the pig, 

 however, which parasitizes beetle grubs as the arthropod host, grows to a 

 length of 25 inches. 



The pseudocoelom of this group is not well developed, and biol- 

 ogists are not agreed that the Acanthocephala belong in the Aschel- 

 minthes. The total absence of a digestive tract in both larva and adult 

 and the many other specializations for parasitism make comparisons 

 difficult. Unlike the nematoda, the Acanthocephala have circular mus- 

 cles in the body wall and ciliated excretory organs. 



97. Phylum Nemertea 



The nemerteans are a small group numbering 550 species, most of 

 which are marine. They are predacious but sluggish, creeping slowly or 

 burrowing deep into mud in search of prey by the contraction of muscles 

 and by the beating of the cilia on the surface. 



In several respects nemerteans resemble the turbellarians: they lack 

 a body cavity, they tend to be flattened, the epidermis is ciliated, the 

 excretory system includes flame cells, and the nervous system and sense 



