LAMELLIBRANCHIA. 29 
have neither eyes nor tentacles, or a distinct head like Gastropods ; 
their organs of circulation and respiration are of a rudimentary 
kind, and they have no heart. The sexes are separate. Their foot 
is adapted for burrowing in sand, in which they live and obtain 
their food, which consists of Foraminifera and minute Bivalves. 
One species, Dentalium pretiosum, found on the shores of North- 
West America, was until recently used as money by the Indians. 
Class IV.— LAMELLIBRANCHIA.* 
The Molluscs belonging to this Class have neither head, nor Cases 
cephalic eyes, nor jaws or tongue like those of the other Classes, and 137-204. 
are enclosed in a shell which consists of two plates or valves held 
together on one side of the margin by a horny, elastic substance, 
called the " ligament." Bivalves do not creep about in search of 
food, but find their means of existence in the shape of minute 
particles, both animal and vegetable, which happen to be contained 
in the water which they breathe. Some, however, are capable of 
locomotion by means of a well-developed foot, and a few swim 
through the water by alternately opening and shutting their valves. 
The body is enclosed within two lobes of the mantle which line the 
interior of the valves, and which at their base are firmly attached to 
the shell, producing on the shell a scar or impression called the 
" pallial line." The gills are lamellar or leaf -like, and placed on each 
side of the body. Each gill is called a ctenidium, and consists of an 
axis which is partly attached to the body of the Mollusc. This axis 
generally gives off two plates consisting of hollow filaments which 
are parallel with one another, directed downwards towards the 
ventral side, and in most cases long and refolded upon themselves, so 
that each plate becomes in reality a double lamella. In a few 
instances, however, the filaments are simple and not reflected. They 
are connected with one another by microscopic cilia, sometimes by 
vascular junctions, and the dependent and reflected portions 
(lamellae) of each filament may be connected by " interlamellar 
vascular junctions." The mouth is merely an oval aperture at the 
anterior end of the body, and generally furnished on each side with 
* The term Lamellibranchia is used instead of Pelecypoda in deference to 
the wish of Professor Lankester. — E. A. S. 
