ALTERNATE AND EQUIVOCAL REPRODUCTION. 



129 



form long chains which float in the sea (Fig. 135). The 



Fig. 136. 



individuals thus joined in floating colonies produce eggs ; 

 but in each individual 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 seen within the body of the parent (a), and these 

 again bring forth solitary individuals, &c. 



338. In some parasitic worms, the alternate reproduction 

 is accompanied by still more extraordinary phenomena, as is 

 shown by the late discoveries of the Danish naturalist, 

 Steenstrup. It is well known that the stagnant pools in 

 which fresh- water shells (particularly the Lymnea and the 

 Paludina) are found, contain an innumerable variety of 

 minute animals of various kinds. Among these is a small 

 worm, known to naturalists under the 

 name of Cercaria (Fig. 137). When ex- 

 amined with a lens, it looks much like 

 a tadpole, with a long tail, a triangular 

 head, and a large sucker (a) in the mid- 

 dle of the body. Various viscera appear 

 within, and among others a very distinct 

 forked cord (c), which embraces the 

 sucker, and which is thought to be the 

 liver. 



Fig. 137. 339. If we watch these worms, which 



always abound in the neighborhood of the shells mentioned , 



