106 



ZOOLOGY. 



very transparent epidermis, under which is a tolerably thick 

 membrane furnished with transverse muscular fibres, while 

 between each pair of these transverse fibres is placed a globu- 

 lar vesicle which appears to be a mucous follicle or gland ; 

 the innermost tube is opaque and of firmer consistence ; it 

 contains the longitudinal muscular fibres, and is usually re- 

 ticulated on the surface. Through the centre of these tubes 

 there passes a slightly narrower canal, which becomes very 

 small toward the extremity of the tail. The existence of 

 the same layers in the body itself of the Cercaria can easily 

 be demonstrated ; but the transversely striated layer is here 

 not so much developed. 



Steenstrup states that these Echinate Cercariae (Fig. 68) 



Fig. 68. Metamorphosis of a Cercaria into a Distomum. A, parent nurse ; , germs ; 

 a, nurse. B. larva. C, encysted, pupal Cercaria. D, adult Distomum. After 

 Steenetrup. 



are found by thousands, and frequently by millions, in the 

 water in which two of the largest European fresh-water 

 snails, Planorbis cornea and Limnceus stagnalis, have been 

 kept. After swimming about in the water some time, they 

 fix themselves by means of their suckers (B, s) to the slimy 

 skin of the snails, in such numbers that the latter look as if 

 covered with bits of wool. 



The Cercaria, by contractions of its body and violent lash- 

 ing of the tail, forces its way into the body of its host, loses 

 its tail, and then resembles a mature Distoma. By turning 



