160 REPRODUCTION. 



the mouth, (m,) however, being free in each, (Fig. 135.) 



Fig. 135. 



Fig. 136. 



The individuals thus joined in floating colonies produce eggs ; 

 but in each animal there is generally but one egg formed, 

 which is developed in the body of the parent, and from 

 which is hatched a little mollusk, (Fig. 136,) which remains 

 solitary, and differs in many respects from the parent. This 

 little animal, on the other hand, does not produce eggs, but 

 propagates by a kind of budding, which gives rise to chains 

 already seen within the body of their parent, (a,) and these 

 again bring forth solitary individuals, &c. 



338. In some parasitic worms, alternate generation is 

 accompanied by still more extraordinary phenomena, as is 

 shown by the late discoveries of the Danish naturalist, Steen- 

 strup. Among the numerous animals which inhabit stagnant 

 pools, in which fresh-water shells, particularly Lymnea and 



Paludina, are found, there is a small worm, 

 know to naturalists under the name of Cer- 

 caria, (Fig. 137.) When examined with 

 a lens, it looks much like a tadpole, with a 

 long tail, a triangular head, and a large 

 sucker (a) in the middle of the body. Va- 

 rious viscera appear within, and, among 

 others, a very distinct forked cord, (c,) 

 which embraces the sucker, and which is 

 thought to be the liver. 



339. If we watch these worms, which 

 always abound in company with the shells 

 mentioned, we find them after a while attaching themselves, 

 by means of their sucker, to the bodies of the mollusks. When 



Fig. 137. 



