26 THE LIFE OF THE MOLLUSCA 



Elephant's-Tusk Shell (Dentalium) is the type, form 

 a single small group by themselves. 



The animal is symmetrical, with a rudimentary 

 head, and a long cylindrical foot used for burrowing 

 in the mud in which these creatures live. 



The borders of the mantle are united beneath, 

 forming a tube, open at both ends, enclosing the 

 rest of the body, and encased in the tubular shell, 

 which is likewise open at both ends, there being no 

 approximation of the two ends of the alimentary 

 canal. There is generally a notch in the margin of 

 the shell at the smaller end (or posterior opening) 

 on the ventral or convex side. In one genus, Schizo- 

 dentalium, there are, besides, in a line with and close 

 to this notch, a series of openings. Sometimes the 

 shell is finely striated lengthwise. 



Class IV. : The PELECYPODA, or LAMELLI- 

 BRANCHIA (Plate XV., Figs. 3-9), commonly 

 known as Bivalves, of which the Oyster, Mussel, etc., 

 are familiar examples. In these the head is rudimen- 

 tary, hence they were called Acephala by Cuvier; 

 while, because a head is present in the embryo 

 but does not develop, Lankester proposed to term 

 them Lipocephala. The characteristic radula is, 

 of course, wanting in this group. The stone-axe- 

 shaped foot is usually well developed, and serves as 

 a burrowing and, rarely, as a creeping organ. 



The symmetrically disposed organs of the body 



