188 GENERAL ZOOLOGY 



Most Gordiacea spend the greater part of their lives as 

 parasites in insects and often have very complicated life 

 cycles. Parachordodes tolosanus has two larval stages : the 

 first enters the aquatic larva of the orl-fly and lives through 

 the winter; the second develops in a beetle which has 

 eaten an adult orl-fly and passes another winter in the 

 beetle. The adult lives in the water, as a parasite in 

 man, or in some other animal. 



CLASS 3. ACANTHOCEPHALA 



The hook-headed worms (Fig. 78, B) are parasitic in 

 fishes and other vertebrates, sometimes even in man. They 



B 



A 



FIG. 78. A, a horsehair worm, Gordius; B, a hook-headed worm 

 ( Acanthocephala) . 



have no digestive system but live attached to the wall of 

 the intestine like a tapeworm. At the anterior end there is 

 a protrusible proboscis which is armed with hooks and 

 serves as an organ of attachment. 



The development of acanthocephalans requires an alter- 

 nation of -hosts; the larva usually living in some insect and 

 the adult in a vertebrate. The common June-bug, or June- 

 beetle, carries the larva of Gigantorhynchus suis. This 

 parasite lives as an adult worm in the intestines of pigs, 

 which become infected by eating beetles. In South 

 Russia, where beetles are eaten raw, the worm is said to 

 occur in man. 



PHYLUM ROTIFERA 



On account of the ciliated discs at the anterior end, 

 rotifers (Fig. 79), were called " wheel-animalcules' 7 by 

 the zoologists who first studied them. When the cilia are 



