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 Cercarie (Fig. 68) 
Fig. 68.—Metamorphosis of a Cercariaintoa Distomum. A, parent nurse ; ¢, germs ;, 
- nuree. B. larva. ©, encysted, pupal Cercaria. D, adult Distomum.—After 
teenstrup. 
are found by thousands, and frequently by millions, in the 
water in which two of the largest European fresh-water 
snails, Planorbis cornea and Limneus 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 
