342 



HEPRODTJCTION. 



The phenomena attending it have been of late the object of 

 numerous scientific researches, which are the more deserving 

 of our attention, as they furnish a solution of several problems 

 alike interesting in a zoological and philosophical point of 

 view. 



519. Alternate generation was first observed among the 

 SalptB, marine mollusca, without shells, belonging to the 

 family tunicata. They are distinguished by the curious pe- 

 culiarity of being united together in considerable numbers, 

 so as to form long chains, which float in the sea (fig. 359), 

 the mouth (m), however, being free in each. The indivi- 

 duals 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, arid from which is hatched 

 a little mollusk (fig. 360), 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 the parent (a), and these again bring forth solitary 

 individuals, &c. 



Fi ? . 359. Fig. 360. 



520. 

 Fig. 



In some parasitic worms, alternate generation is 

 361 accompanied by still more extraordinary phe- 

 nomena, as shown by the late discoveries of 

 Steenstrup, a Danish naturalist. Among the 

 numerous animals inhabiting stagnant pools, 

 in which fresh-water-mollusca (particularly 

 Lymncea and Paludina) are found, there is a 

 small worm, known to naturalists under the 

 name of Cercaria (fig. 361). 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. Various viscera 

 appear within, and among others a very dis- 

 tinctly forked cord (c), embracing the sucker 

 and which is thought to be the liver. 



