162 BEPRODULTION. 



341. When they liave reached a certain size, the young 

 Cercv*iria3 lea 'e the body of the nurse, and move freely in 

 the abJoniina cavity of the mollusks, or escape from it into 

 the water, to fix themselves, in their turn, to the body of 

 another mollusk, and begin their transformations anew. 



342. But this is not the end of the series. The nurses of 

 the Cercaria are themselves the offspring of little 

 worms of yet another kind. At certain seasons, 

 we find in the viscera of the Limnea, worms 

 somewhat like the nurses of Cercaria in shape, 

 (Fig. 141,) but rather longer, more slender, and 

 having a much more elongated stomach, (s.) 

 These worms contain, in the hinder part of the 

 body, little embryos, («,) which are the young 

 nurses, like Figures 139, 140. This generation 



'^' * has received the name o^ grand-nurses. 



343. Supposing these grand-nurses to be the immediate 

 offspring of the Distoma, (Fig. 138,) as is probable, we have 

 thus a quadruple series of generation. Four generations 

 and one metamorphosis are required to evolve the perfect 

 animal ; in other words, the parent finds no resemblance to 

 himself in any of his progeny, until he comes down to the 

 great-grandson. 



344. Among the Aphides, or plant-lice, the number of 

 generations is still greater. The first generation, which i3 

 produced from eggs, soon undergoes metamorphoses, and 

 then gives birth to a second generation, which is followed by 

 a third, and so on ; so that it is sometimes the eighth or 

 ninth generation before the perfect animals appear as males 

 and females, the sexes being then for the first time distinct, 

 and the males provided with wings. The females lay eggs, 

 which are hatched the following year, to repeat the same 

 succession. Each generation is an additional step towards 

 l.ie perfq^it state ; and, as each member of the succession is 



