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. 
Crass 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 
