118 



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 Cercaria? (Fig. 98) 



Fig. 98. Metamorphosis of a Cercaria into a Distomum. A, parent nurse ; e, genna ; 

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

 Steenstmp. 



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 



