2 18 GASTEROPODA. 



the shape of a gelatinous band, always arranged in a more or less 

 spiral form, and fastened by one of its edges to corallines or the 

 under surface of a stone. The eggs are minute and very numerous, 

 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, and looks more like a Rotifer than a Mollusk ; moreover, 

 to add to its extraordinary appearance, it is enclosed in a tran- 

 sparent nautilus-like shell, provided with an operculum. 



ORDER HETEROPODA.* 



In these Gasteropods the foot, instead of forming a flat hori- 

 zontal 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 



FIG. 234. CARIXAHIA. 



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 themselves forward or backward, in a straight 

 line or a curved one, with equal facility. But the Heteropods 

 need occasional repose and a cessation from activity; and ad- 

 mirably is the foreseen want provided against. Where are they 

 to rest ? where fix their anchor in the world of unstable water 



'' rcpos, heteros, of another kind ; TTOUJ, ~o5js, pou.s, podos, a foo.'. 



