Chap. 26 ROUNDWORMS THE TUBULAR PLAN 531 



observations in the natural habitats. The life cycle of hairworms is another evi- 

 dence of the precision with which an individual parasite must follow a fixed 

 schedule of life or perish. Production of great numbers is the safeguard of the 

 species. 



After mating, the females lay their eggs in strings usually twined about twigs 

 submerged in the water. These are from 15 to 20 cm. long and contain an 

 enormous number of minute eggs. Gordius lays more than half a million eggs 

 and Paragordius about six million. Toward fall the adults die, the males before 

 the females. The microscopic larva pierces the egg shell at a point that it 

 softens with its own secretion. Within 24 hours after hatching it surrounds 

 itself with a cyst wall and becomes inactive. If it is prevented from doing this 

 on time, it loses its power to do so. Larvae may live for two months within 

 cysts submerged in water, and for a month when they are in damp air. 



The cysts are swallowed by aquatic insects or by land insects, such as grass- 

 hoppers and crickets, that forage on the grasses at the water's edge. As soon 

 as the cyst walls are digested off, the larvae pierce the wall of the gut and bur- 

 row into fatty tissue from which they absorb abundant nourishment. There the 

 young Gordius grows and changes to the mature form. If the host is an aquatic 

 insect, the parasite escapes directly into the water. If it is a land insect, its 

 successful escape must await the host's visit to the waterside. Most of these 

 facts have been learned from experimental infections of insects. 



It is noticeable that Gordius does not strictly specify its host. Well-grown 

 worms have been found in various species of insects; larvae are probably 

 swallowed and mature in several different aquatic invertebrates. 



Phylum Acanthocephala 



Spiny-headed Worms. Spiny-headed worms constitute a peculiar group of 

 about 300 species ranging in length from six to 460 mm. (IVi ft.). All are 

 parasites of vertebrates, from fishes to mammals. The name refers to their dis- 

 tinctive feature, a relatively short retractile proboscis armed with rows of stout 

 recurved hooks (Fig. 26.8). The worm projects this proboscis in among the 

 folds of the lining of the intestine of its host and holds its place with the hooks 

 while it absorbs nourishment through the delicate porous cuticle that covers 

 its body. Neither larva nor adult has a digestive tract, and no circulatory or 

 respiratory organs. There are two primitive kidneys, and a roomy body cavity 

 but, lacking a peritoneal lining, it is not a true coelom. The sexes are separate. 

 The eggs are fertilized internally and the embryos well developed before they 

 are extruded into the intestine of the host. 



Life Cycle. The life cycle includes an intermediate host, usually an ar- 

 thropod: small crustaceans for those that are parasites of fishes and other 

 aquatic vertebrates; cockroaches, larvae of June beetles and other terrestrial 

 arthropods for those that are parasites of pigs, rats, and other land vertebrates. 



