ALTERNATE AND EQUIVOCAL REPRODUCTION. 131 



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

 Cercarise leave the body of the nurse, and move freely in the 

 abdominal 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 Lymnea, worms 

 somewhat like the nurses of the Cercaria in 

 shape (Fig. 141), but rather longer, more slen- 

 der, and having a much more elongated stomach 

 (s). These worms contain, in the hinder part 

 of the body, little embryos (a), which are the 

 Fig. MI. y un g nurses of Figures 139, 140. This gen- 

 eration has received the name of 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 arrives at the 

 great-grandson. 



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

 generations is still greater. The first generation, which is 

 produced from eggs, soon undergoes metamorphoses, and 

 then gives birth to a second generation, w T hich is followed 

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

 ninth generation before the perfect animal appears as male 

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

 and the male provided with wings. The female lays eggs 

 which are hatched the following year, to repeat the same 

 succession. Each generation is an additional step to- 

 wards the perfect state ; and as each member of the sue- 



