NUDIBRANCHIATA. 21 



period of exclusion, the young may be seen revolving 

 on their own axis by means of vibratile cilia ; and on 

 escaping from the egg, they swim about freely in the 

 water by the same means. The larva is extremely 

 minute, and has more the appearance of a rotiferous 

 animalcule than a mollusk. It is enclosed in a trans- 

 parent, calcareous, nautiloid shell, with an operculum. 

 Its structure is very simple, showing no signs of the 

 external organs that distinguish the future adult. The 

 principal portion idsible outside the shell is composed 

 of two flat disks or lobes, fringed with long cilia, by 

 the motion of which it swims freely through the water. 

 These are often withdrawn into the shell ; and the oper- 

 culum is closed upon them when the animal is at rest. 

 "We have not been able to trace the animal further than 

 the first stage of its development, and are therefore 

 unable to say by what process it assumes the very diffe- 

 rent form of the adult state. We have succeeded in 

 bringing out the larvae of Doris, Tritonia, Melibcea, and 

 Eolis, between all of which there is a very great 

 resemblance.^^ Nordmann gave, in the '^Annales des 

 Sciences Naturelles ' for 1846, some particulars of their 

 development, and showed how the metamorphosis is 

 effected. 



In their adult state the senses are very imperfect. 

 For instance we are told that "the sole object of vision 

 appears to be that of ascertaining the presence of light, 

 and thus directing the animal in its search for shelter 

 in dark and concealed places." The dorsal tentacles 

 are the organs of smell ; but " olfaction in these animals 

 probably is not so much to assist in the discovery of 

 alimentary matters, as to give warning of the unhealthy 

 state of the surrounding medium, arising from putres- 

 cence or other causes." The skin of the Doris family 



