192 HALF AN HOUR WITH SHELL-FISH. 



gills, contained in a chamber improvised by a simple 

 folding or doubling of the mantle. To the latter 

 contrivance the water is usually admitted by a tube 

 or siphon, the effete, or exhausted water, being 

 expelled by another tube, carrying away the faeces, 

 or excrements, of the animal away with it. The 

 young of all the gasteropods, as a rule, are provided 

 with an embryonic shell, and swim freely about by 

 means of ciliated lobes placed near the head, which 

 give to them the appearance of the pteropods. As 

 the latter are among the oldest known shell-fish, 

 it is not improbable that the gasteropods were 

 derived through them, and still exhibit their descent 

 in their embryonic condition. 



We have already dwelt on the geometrically de- 

 signed structure of a univalve shell. The normal 

 plan is realized, perhaps, most simply in the Common 

 F ig> 102. Limpet, (Patella vul- 



gata,Fig. 102), where 

 it consists of a hol- 

 low cone, having the 

 apex a little on one 

 side. The spiral 

 form, however, is the 

 commonest ; and this 

 is best typified in 

 the common wentle- 



trap (Turritella communis). The last and largest of 

 the whorls, when there are several, is called the 

 " body-whorl." A simple means of identifying the 



