GASTEROPODA. 291 



amounting in some species to several thousands. 

 Before the period of exclusion, the young may be seen 

 revolving by means of vibratile cilia, and on escaping 

 from the egg, they swim about freely in the water by 

 the same means. The embryo is extremely minute, 



FlG. 233. YOUNG OF EOLIS. 



and looks more like a Rotifer than a Mollusk ; more- 

 over, to add to its extraordinary appearance, it is en- 

 closed in a transparent Nautilus-like shell, provided 

 with an operculum. 



ORDER HETEROPODA.* 



In these Gasteropods, the foot, instead of forming a 

 flat horizontal sole, has a vertical direction, and 

 assumes the figure of a compressed fin, which being 

 moved by its own muscles from right to left, propels 

 the animal forward, like a sculler who works his 

 boat with a single oar. In the Carinaria (beautiful 

 creatures, clear as crystal and painted with the 

 liveliest colours) this ventral fin is aided in its office 

 by some subsidiary membranes situated upon the 

 neck, or near the tail. Combined, they give these 

 animals a velocity superior to what has been noticed 

 in any other tribe of Mollusks. They are, indeed, 

 quite remarkable for their quickness, propelling them- 



* eVepos, heteros, of another kind ; ^ovs, iro$6s, pons, podos, a foci. 



o 2 



