TKEMATODA. 



323 



cases (Tristomum coccineum) transverse rows of bristles are found. 

 Paired eyes are frequently present. In some species the elongated 

 body presents a kind of external segmentation. They are for the 

 most part ectoparasitic, to a certain extent like the ffirudinea, and 

 they develop directly without alternation of generations from eggs 

 which are usually hatched in the locality inhabited by the mother. 

 Sometimes the development is a metamorphosis (Polystomum), and 

 the young larva? live in another place. 



The development of Polystomum integerri- 

 mum from the bladder of the frog is the best 

 known, owing to the researches of E. Zeller 

 (figs. 258, 259). The production of eggs begins 

 in the spring, when the frog awakes from 

 hibernation and proceeds to pair. It lasts 



from three 



to four 



weeks. It 



is easy then 



to observe 



the Polysto- 



mea in the 



process of 



reciprocal 



copulation. 



When the 



FIG. 259. Egg with embryo{),and hatched 

 larva (b) of Polystomum integerrimum ; Dk, 

 operculum (after E. Zeller). 



eggs 

 being 

 the 

 forces 



are 

 laid, 



the 



anterior end of the body with the genital FlG - 2o8 - 



..,/;,. gerrimum (after E. Zeller). 



opening through the mouth of the bladder o, mouth; GO, genital 

 nearly as far as the anus. The development P enin g; A intestine; 



. ., ' , . 7F copulatory opening 



or the embryo takes place in water and occu- (lateral pads) ; Zty, yolk 

 pies a period of many weeks, so that the 

 young larva? are only hatched when the tad- 

 poles have already acquired internal gills. 

 GyrodactyluSj and possess four eyes, a pharynx and alimentary canal, 

 as well as a posterior disc (for attachment), which is surrounded by 

 sixteen hooks. They possess five transverse rows of cilia ; three are 

 ventral and anterior, two dorsal and posterior. There is also a 

 ciliated cell upon the anterior extremity. The larva? now migrate 



The larvae resemble 



