568 DEVELOPMENT OF ENTOZOA. 
(fig. 309) having a sac-like body clothed with cilia and capable 
of motion ; within the anterior part of which is to be observed 
an elongated stomach s, whilst the posterior portion of the 
cavity is occupied by a number of bodies a, having a general | 
resemblance to itself, which are produced from it by a process 
of internal gemmation. These, when mature, burst forth 
from the containing cyst, and develope themselves into 
worms (fig. 310) not very unlike the preceding in general 
form and organization, though differing from them in some 
unportant particulars; and in their turn they produce, by 
internal gemmation, a fresh brood of bodies much more 
dissimilar to themselves. These, when set free in due time, 
Fig. 309, Fig. 310. Fig. 311. 
Granp-NuRsE or CERCARIA. Nurse or CEKCARIA. CERCARIA. 
develope themselves into a form which has long been known 
as the Cercaria (fig. 311), and which has a tadpole-like body 
with a large sucker a in its middle, a triangular head, a long 
tail by the motion of which it swims, and various viscera ¢ 
in its interior. The Cercarie attach themselves by means of 
their sucker to the bodies of the Lymnzeus, and then begin to 
undergo an important metamorphosis. The tail, being now 
useless, falls off; the animal becomes invested with a mucous 
substance, within which it lies encysted like a chrysalis 
within its cocoon ; and, on its emergence from this it presents 
itself as a Distoma, which is ready by the performanee of the 
true generative process, to set in renewed operation the whole 
