TKEMATODA. 



cases (Tristomui)i coccineuin) 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 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, 2.59). The production of eggs begins W 

 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 Poli/sto- 



mea in the 



process of 



reciprocal 



copulation. 



When the 

 are 



being laid, 



Ov 



eggs 



FIG. 259. Egg with embryo (a, and hatched 

 larva (6) of PoJystomim/ integerrimum ; ZU-, ,1 

 operculum (after E. Zeller)! 



forces the 



E 



Me- 



anterior end of the body with the -enital FlG - '^s. 



qerrimum (after E. Zeller). 



opening through the mouth of the bladder 'o, mouth; Go, genital 

 nearly as far as the anus. The development P en ' n s ; D , intestine ; 



C , -I -i . "", copulatory opening 



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

 pies a period of many weeks, so that the sland rtuct ; s> sucker ; 



Or, ovary ; If, hooks. 



young larvae are only hatched when the tad- 



poles have already acquired internal gills. The larvae resemble 

 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 larva? now migrate 



