PLATHELMINTHES ■ 189 



is a spherical mass of cells, which subsequently becomes 

 elongated, and thereby exhibits, even at this stage, the form 

 of the embryo. The fundaments of the eyes, the sixteen 

 hooks of the clasping disc [retinaculum], the cavity of the 

 intestine, and the pharynx soon make their appearance (Fig. 

 94). The newly hatched embryo possesses in addition five 

 rows of cilia, of which the three anterior belong to the 

 ventral surface, the two posterior to the dorsal surface. 

 Furthermore there is a fringe of cilia in front on the head 

 (Fig, 94). The embryo, leaving the egg at this stage, now 

 seeks the tadpole of the frog, to the gills of which it attaches 

 itself by means of the hooks and suckers. Here the ciliated 

 cells, which are no longer of any use to the animal, degene- 

 rate, and the Polystomum larva approaches more and more 

 the form of the parent. In extraordinary cases it can 

 attain this condition even in the branchial cavity, but as a 

 rule this is not the case ; on the contrary, the ^'ouns' 

 Polystomum, upon the degeneration of the gills of the tad- 

 pole, penetrates into its mouth-cavity, migrates through the 

 entire length of its intestine, and finally passes from the 

 cloaca into the urinary bladder, where it attains sexual 

 maturity. 



Biplozoon paradoxum, which is remarkable on account of 

 its subsequent habits, also leaves the egg as a ciliated larva 

 (Zeller, No. 18). The larva, known under the name of 

 Biporpa, bears suckers and hooks, by the aid of which it 

 attaches itself to the gills of fresh- water fishes {Fhoxinus 

 l(Bvis, for example). It can remain here for weeks and 

 months, gradually approaching the organization of the adult. 

 But before it arrives at this condition it is necessary for one 

 individual to unite with a second, and, in fact, for the rest of 

 their existence. This takes place by the larva seizing with 

 its ventral sucker a knob-like outgrowth situated on the 

 back of the other animal. Then the second individual turns 

 and twists its body, so that it too may grasp the dorsal 

 prominence of its mate with its ventral sucker. In this 

 position the two animals grow together firmly, and in this 

 condition reach sexual maturity. 



The course of development in Gyrodactylus elegaus, one of 



