CONSEQUENCES OF ALTERNATE GENERATION. 169 



separate individuals, the grand-nurse, the nurse, the larva or 

 Cercaria, and the Distoma, in which the sexes are not sepa- 

 rate. Among the Aphides, the number is much greater 

 still. 



356. The study of alternate generation, besides making 

 us better acquainted with the organization of the lower ani- 

 mals, greatly simplifies our nomenclature. Thus, in future, 

 instead of enumerating the Distoma and the Cercaria, or the 

 Strobila, the Ephyra, and the Medusa, as distinct animals, 

 belonging to different classes and families, only the name 

 first given to one of these forms will be retained, and the 

 rest be struck from the pages of Zoology, as representing 

 only the transitory phases of the same species. 



357. Alternate generation always presupposes several 

 modes of reproduction, of which the primary is invariably 

 by ovulation. Thus, we have seen that the Polyps, the 

 Medusa, the Salpa, &c., produce eggs, which are generally 

 hatched within the mother. The subsequent generation, on 

 the contrary, is produced in a different manner, as we have 

 shown in the preceding paragraphs ; as among the Medusas, 

 by transverse division ; among the Polyps and Salpee, by 

 buds, &c. 



358. The subsequent generations are, moreover, not to be 

 regarded in the same light as those which first spring directly 

 from eggs. In fact, they are rather phases of development, 

 than generations properly so called ; they are either without 

 sex, or females whose sex is imperfectly developed. The 

 nurses of the Distoma, the Medusa, and the Campanularia, 

 are barren, and have none of the attributes of maternity, 

 except that of watching over the development of the species, 

 being themselves incapable of producing young. 



359. Another important result follows from the above ob- 

 servations, namely, that the differences between animals 

 which are produced by alternate generation are less, the 



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