228 THE SEA SHORE 



giving descriptions of individual shells, and we shall use them 

 somewhat liberally in noting the distinguishing characteristics 

 of the families and genera; but before entering into this portion 

 of our work we must briefly note the general features of the bodies 

 of the Cephalophora. 



Sometimes these bodies are bilaterally symmetrical, as we have 

 observed is the case with the worms, but more commonly the 

 organs on one side are aborted, while the growth proceeds apace 

 on the opposite side. Thus the animal assumes a spiral form, 

 being coiled towards the aborted side, with the gills and other 

 organs developed on that side only. As a rule this curvature is 



FIG. 157. DIAGRAM OF THE ANATOMY OF THE WHELK, THE SHELL 

 BEING BEHOVED 



c, stomach ; e, end of intestine : g, gills ; h, ventricle of the heart : a, auricle ; 

 /, nerve ganglia ; 6, digestive gland ; ft, foot ; o, operculum ; </, liver 



such that the body takes the form of a right-handed or dextral 

 spiral, as we have already observed in the shells which cover them, 

 the mouth being thus thrown to the right, but sometimes it takes 

 the opposite direction. 



When one of these animals is extended and creeping, we observe 

 that it has a distinct head, furnished with a mouth below, and 

 tentacles and eyes above ; also, if an aquatic species, the gills are 

 more or less prominent. Further, the exposed portion of the body 

 is covered with a leathery mantle, and the animal creeps on a 

 broad, flattened surface which is called the foot. 



The tentacles or feelers are usually retractile, and, when retracted, 

 are turned outside-in. Each one is provided with a muscle that 



