PHYLUM NEMATHELMINTHES 185 



phylum. The class to which this worm belongs, known as the Gordiacea, 

 and of which the type genus is Gordius, includes several forms, some 

 occurring in fresh water and others being marine. The egg is laid in 

 the water and from it hatches a larva with a large proboscis and hooks 

 at the anterior end. Using these in boring, a larva has been known to 

 force its way into the body of an aquatic insect. Normally, however, 

 the larva encysts on submerged vegetation and is ingested by its host. 

 If the water level is lowered, these cysts will survive drying and may be 

 eaten by a terrestrial host, such as a grasshopper. Full-grown larvae 

 have been found in beetles, grasshoppers, and crickets, and larvae in 

 different stages of development in spiders, earthworms, snails, and even 

 fishes. The fully developed larva escapes from its insect host into water, 

 where it matures and where it may be found, often coiled about among 

 the leaves of aquatic vegetation (Fig. 94). 



^^^ Proboscis „ / ^ , 



^j^\y Bra//^ Testes 



W,t*\ ^__/^_-_ V\^ Pr-<?sfcfte gf/a/7c/s 



/fiver for musc/es Refractor musc/es ^-~-^=^ 



Fig. 95. — Echinorhynchus ranae (Schrank), showing general organization of a young 

 male. (Fro7n VanCleave, "Invertebrate Zoology," by the courtesy of McGraw-Hill Book 

 Company, Inc.) 



217. Spiny-headed Worms. — Another class of worms which may also 

 be placed in Nemathclminthes is that of the spiny-headed worms, or 

 Acanthocephala. Such a worm has a protrusible proboscis covered with 

 hooks at the anterior end of the body (Fig. 95). By means of the pro- 

 boscis it becomes attached to the intestinal wall of the host, which is 

 always a vertebrate. The worm lacks an alimentary canal and absorbs 

 digested food in the intestine of the animal in which it lives. All classes 

 of vertebrates are parasitized by these worms, but they occur more com- 

 monly in the fishes, in turtles, and in birds like the herons and bitterns. 

 The intermediate hosts are Crustacea or insects. A spiny-headed worm 

 which infects the pigs in the southern parts of this country has as its inter- 

 mediate host the larva of the June beetle, known as the white grub, 

 which is rooted up and eaten by the pig. 



218. Economic Importance. — It is clear that some of the parasitic 

 nematodes are of great economic importance on account of the produc- 

 tion of disease both in man and in domestic animals. Among the thread- 

 worms are also some which are injurious to cultivated plants. Others 

 are distinctly beneficial, feechng upon the injurious forms or destroying 

 various injurious microorganisms. 



