AND THE LOWER ANIMALS. 199 



soon completely envelops it. As it grows,, this disk 

 assumes a radiate form ; gradually the ambulacra and 

 spines present themselves; then the mouth opens 

 externally, always on the lateral surface of the larva. 

 The latter is partly resorbed, and partly laid aside, 

 when the new animal is fully formed. In most 

 Asterise these things occur in a similar manner, but in 

 others the larva [tornaria) is completely absorbed by 

 the Echinoderm developed within. In the Holothurige 

 the tentacular crown is developed upon the larva's 

 stomach, but most of the organs immediately employed 

 are completed and acquire their final characters by a 

 simple transformation. 



We cannot dwell here upon all the remarkable 

 features of this mode of development. We shall, 

 therefore, confine ourselves to those matters directly 

 related to the subject.* From the sea-urchin's egg 

 there is produced a sort of infusorian, which is meta- 

 morphosed into a gluteus. In the interior of the 

 latter there is developed an animal of quite another 

 nature. Here we have two very distinct generations 

 produced by different processes, yet both owing their 

 existence to a single primitive germ. This, then, is a 

 case of geneagenesis. 



But what here characterizes this phenomenon are 

 the loans, so to speak, which the second generation 

 gives to the first. In all the species which we have 

 hitherto examined, the bud merely borrows the 



* We desire, however, to call attention to the exceptional fact 

 of an animal which is destined to become radiated, beginning its 

 career with a bilateral symmetry like that of the Annulosa. It is 

 the only exception to the embryogenical rule which we have always 

 maintained. — Souvenirs d'un Naturaliste. 



