64 Invertebrata. 



tapeworms, but each consists of a single segment bear- 

 ing one or two suckers (a, b}. In many respects they 

 resemble the Turbellarian worms, but are not ciliated 

 and often present formidable armatures of recurved 

 hooks. They are like tapeworms in the development 

 and complexity of their ovaries, and many of them 

 show in their history alternations of generations as 

 curious as those of their relatives the tapeworms ; for 

 example, the larvae of some liver flukes live for a time 

 free, in water, and develop within their bodies little 

 cylindrical worms, which are set free on the bursting 

 of the wall of the parent, and in turn enjoy an inde- 

 pendent life. Within these worms again there may form 

 another brood of internal buds, which also grow, burst 

 their envelope, and become for a time free, but soon 

 attach themselves to some soft aquatic animal in whose 

 body they become encysted, to develop finally into the 

 mature forms when their first host is eaten by some 

 larger animal. Thus the flukes found in water-fowl 

 have their larvae in water molluscs, &c. To these flat 

 sucker-bearing parasites the name Trematoda is given. 



CLASS TIL Nematelmia. These, the commonest 

 forms of parasitic worms, are cylindrical, tapering to 

 each end, and possessing a body cavity (fig. 38.) They 

 are never divided into successive joints, although their 

 surface may be finely ringed, and there is almost always 

 a digestive canal with an outlet, as well as a mouth. 



The round worm, often found in the small intes- 

 tines of children, is a good example of the order. It is 

 about seven or eight inches long, ringed on its surface, 

 with the mouth at its anterior end, surrounded by three 

 little lobes ; from this, a tube, the osstiphagus, passes 



