GASTEROPODS. 167 



at sea some have been encountered that weighed as much 

 as two tons. In Greece and in Italy the flesh of squids 

 is eaten, but it is very tough and dry. 



Cuttle-fish are common in all seas ; they are very 

 voracious, living on fish and crustaceans. Like many 

 other cephalopods, they have the power of throwing 

 out an inky liquid that darkens the water and allows 

 them to escape from pursuers. The cuttle-fish bones of 

 the drug- store are the calcareous remains of the bodies 

 of the cuttle-fish ; they are used as polishing stones, 

 and are given to cage-birds, both to furnish calcareous 

 matter and to enable them to sharpen their beaks. 



GASTEROPODS. 



The gasteropods climb about by the aid of a fleshy 

 disk with which the inferior surface of the body is pro- 

 vided, and which sometimes has the form of a fin. The 

 head is always visible outside of the mantle, and has 

 above the mouth tentacles that appear to be organs of 

 sensation ; these tentacles are sometimes furnished with 

 eyes. 



Some gasteropods are naked, but most of them have 

 a univalve shell in which the body may be more or less 

 completely enclosed. The form of this shell presents 

 infinite variation : sometimes it is a straight cone, but 

 more often it is curved and rolled up on itself several 

 times, forming a spiral. It is said to be discoid when 

 the spiral is in one plane, and turbinated when the coils 

 are piled on one another. In some cases the coils are 

 independent and not contiguous ; but in most cases they 

 are exactly fitted one on another, so that the central 

 axis is occupied by a sort of twisted column. 



The species of gasteropods are very numerous. Nearly 



