FLAT-WORMS. 



467 



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 conversion 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 Distomese. 



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 

 parasites, 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 backwardly-directed 

 •spines. The eggs — of which it has been computed half a million may be laid at a 

 time — pass into the intestine of the sheep by way of the bile-ducts, and thence make 

 their way to the exterior. Many of these eggs fall upon dry ground, where they 



Polystornum and larva (magnified). 



fr 



ijver-fluke and larva (enlarged). 



