AND THE LOWER ANIMALS. 179 



frequently — it is necessary to institute many investi- 

 gations before generalizing, in regard to the lower 

 forms of animal life. It has been shewn by Krohn 

 and Lacazo du Thiers, that certain medusae and 

 polyps are reproduced directly, and by a simple 

 metamorphosis. The first proved that the larva of 

 Pelagia noctiluca becomes an acaleph without assum- 

 ing the polyp form ft and it was demonstrated by the 

 second, that neither the sea-anemones, nor corals and 

 their kindred, give rise to medusae. f When science 

 is concerned, well-proven exceptions are as valuable 

 as the discovery of a new series of geneagenetic 

 phenomena. 



Leaving the Medusas and Polyps, we come to a 

 group of beings whose real nature is still somewhat 

 involved in obscurity — the Infusoria and Sponges, 

 united under the name of Protozoa. Here again we 

 meet with geneagenesis. Unfortunately, the obser- 

 vations which it is necessary to make in following 

 an animal through the progressively complex phases 

 of its existence, present in this instance many 

 difficulties, which, in consequence of the extreme 

 minuteness of the creatures to be investigated, are too 

 frequently insurmountable. Nevertheless, we shall 



* "On the earliest stages in the Development of Pelagia 

 noctiluca." — Annals of Natural History, 1856. 



t " Comptes Rendus de l'Academie des Sciences," 1859 and 

 1861. In the sea-anemones there is but a metamorphosis properly 

 so called. The ciliated larvae are transformed into perfect animals, 

 and each of them proceeds from a single egg and produces but one 

 individual. In the coral, the ciliated larva gives birth to an 

 entire colony. In this case then, there is geneagenesis, but the 

 phenomenon is reduced to its simplest condition, since the colony 

 produces directly both egg^, and larvae. 



N 2 



