3440 



THE WORM-LIKE ANIMALS 



itself partially out of the frog's body deposits them in the water. The eggs take 

 from six to eight weeks to hatch; and the young worm is an active little animal, 

 swimming by means of the cilia with which its body is bordered. It differs from 

 the adult by the presence of the fringe of cilia which extends along the sides of the 



body from head to tail, and also by the absence 

 of suckers at the posterior extremity of the 

 body. The latter, however, is furnished with 

 eight pairs of hooks, which are retained in the 

 adult. After leading a free life for a short 

 time, the larva attaches itself to the external 

 gills of a tadpole, and speedily loses its clothing 

 of cilia. When the gills shrivel with the con- 

 version of the tadpole into a frog, the larva 

 enters the mouth of its host, and, passing 

 thence into its intestine, succeeds in ultimately 

 making its way into the bladder, where it lives 

 some five or six years before reaching maturity. 



TwO-SUCKERED GROUP SUBORDER Distomeae 



The second division of trematodes is dis- 

 tinguished by the smaller number of suckers, 

 the absence of hooks, and the circumstance that 

 all the members of the group are internal para- 

 sites, laying an immense number of small eggs; 

 while in the course of their development the 

 young are inhabitants of more than one host. 

 It is evident that parasites living upon the skin 



or gills of fish, where they are constantly in danger of being washed away, have 



much greater need of sucking discs and clinging hooks than those living within 



shelter of some internal organ. On the other hand, it is equally clear that the large 



number of eggs laid by the internal forms, which pass 



through a complicated metamorphosis, is a means for 



providing against the remoteness of the chances of 



the larvae meeting with their appropriate hosts. Some 



of these worms are of importance, on account of the 



destruction they bring upon the hosts they infest. 



One of the best known is the liver fluke {Distomum 



hepaticum}, found in the mature stage in the livers of 



sheep. It is about an inch in length, and nearly 



half an inch broad. The hinder portion of its body 



is flattened and leaf-like, but the front is thick and 



conical, and the outer skin is furnished with many UVER FLUKE AND LARVA 



backwardly-directed spines. The eggs of which it (Enlarged.) 



Polystomum AND LARVA. 

 (Magnified.) 



