MOLLUSCA. 7 



and afterwards confirmed by Van Beneden in the A'ply- 

 siidie ; since then it has been extended to the Proso- 

 branchs by Loven and Mihie Edwards. The Bivalves 

 also undergo a metamorphosis, but the Cephalopods do 

 not appear liable to these changes. 



The larva is always enclosed in a little, transparent, 

 nautiloid, calcareous shell, provided with an operculum, and 

 it swims freely through the ^vater by means of two fin- 

 like lobes fringed with long cilia. Instead of tentacles 

 it has two veils shaped like ears, which disappear at a 

 later period ; the eyes, the ears, the liver, and the vent 

 are as in the adult. This is the first stage. In the 

 second stage it is still enclosed in a shell, but the mantle 

 has become detached and covers the viscera, the foot is 

 enlarged and projects beyond the operculum, and the 

 head has two short conical ciliated tentacles. In the 

 third stage the shell has fallen off, and the general shape 

 is that of the parent, but the veils still remain. In the 

 fourth stage the animal begins to crawl by means of its 

 foot, the gills appear, and the mouth is armed with jaws 

 and a spiny tongue. The last stage is marked by the 

 fall of the frontal veils, and the completion of the ten- 

 tacles and gills, thus concluding the metamorphosis. 



The young bivalves are hatched in the gills of their 

 parents, and have a swimming disc, fringed with long 

 cilia, and furnished with a slender tentacular filament. 

 The labial palps next become developed, and the disc 

 gradually disappears. They then acquire a foot, and are 

 provided with eyes near the labial tentacles, which, how- 

 ever, are afterwards lost. The young of the Tunicata^ 

 which are fixed in the adult state, swim freely about by 

 means of a caudal filament or tail, until they have selected 

 a proper spot to which they can attach themselves. 



