324 



PLATYHELMINTHES. 



into the branchial cavity of the tadpole, lose their cilia, and are 

 transformed into young Palystomea by the formation of the two 

 median hooks and of the three pairs of suckers upon the posterior 

 disc. The young Polystomum, eight weeks after the migration into 

 the branchial cavity, at the time when the latter begins to abort, 

 passes through the stomach and intestine into the bladder, and there 



FIG. 260. Young Diplozoon (after B. Zeller). a, Two young Diporpa beginning to attach 

 themselves together, b, After both individuals have attached themselves. O, mouth ; 

 //, fixing apparatus ; Z, papilla? ; G, sucker. 



only becomes sexually mature after three and more years. In some 

 exceptional cases, and always when the larva has passed on to the 

 gills of a very young tadpole, it becomes sexually mature in the 

 branchial cavity of the latter. The forms then remain very small, 



are without the copulatory 

 canals and uterus, and die 

 after the production of a 

 single egg, without ever 

 getting to the bladder. 



a 



FIG. 261 .-Egg (a) and larva (b) of 

 E. Zeller). 



Fam. Polystomidse. With seve- 

 ral posterior suckers, which are 

 usually paired and arranged in 

 two lateral rows, and are rein- 

 forced by an armature of hooks. 

 The genital openings are fre- 

 (after quently surrounded by hooks. 

 Many species have a length of 

 only a few lines. 



Polystomum Zed., with four eyes ; with no lateral suckers at the anterior 

 end, but with oral sucker; with six suckers, two large median hooks and 

 sixteen small hooks at the posterior end. P. integerrimum Rud., in the 

 bladder of Jtana tniijjtn'ai-ia. P. occllittum in the pharyngcal cavity of 

 Einys. In the formation of the testis and the absence of the uterus it 

 resembles the adult form of P. integerrimum from the branchial cavity of 

 thetadpole. OctdbotTirium laneeolatum Duj. Onohocotyle agypendioulata Kuhn, 

 on the gills of Elasmobranchs. 



Diplozoon v. Nordm. The animal is double, two individuals being fused to 



