TKEMATODA. 



323 



cases (Tristomum coccineuni) 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 Hirudinea, 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 larvae live in another place. 



The development of Polystomum inteyerri- 

 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 W- 

 in the spring, when the frog awakes from 

 hibernation and proceeds to pair. It lasts 



from three 



fio. 



D 



m 



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

 larva (b) of Polystomum integerrimum ; Dk, 

 operculum (after E. Zeller). 



to four 

 weeks. It 

 is easy then 

 to observe 

 the Polysto- 

 mea in the 

 process of 

 reciprocal 

 copulation. 

 When the 

 eggs are 

 being laid, 

 the parasite 

 forces the 

 the 



258 - Polystomum inte- 

 gerrimum (after E. Zeller). 

 0, mouth ; Go, genital 

 opening ; D, intestine ; 

 IK, copulatory opening 

 (lateral pads) ; Dg, yolk 

 gland duct ; S, sucker ; 

 Ov, ovary ; If, hooks. 



anterior end of the body with 

 opening through the mouth of the bladder 

 nearly as far as the anus. The development 

 of the embryo takes place in water and occu- 

 pies a period of many weeks, so that the 

 young larvae are only hatched when the tad- 

 poles have already acquired internal gills. 

 Gyrodactylus, 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 larvae now migrate 



The larvae resemble 



