190 ZOOLOGY 
phylum are pelagic, and consist of the most transparent and 
delicate tissues, others are sessile, being fixed either by cords 
secreted by a gland in the foot (Mytilus) or by the surface of 
the shell (Ostvea), whilst, again, others bore long funnel-shaped 
passages in the rocks or in submerged pieces of wood, ete. 
The very various animals which compose this phylum 
may be separated into two main divisions, according as to 
whether they retain a well-marked prostomium or not. 
Those which have lost a definite cephalic region have probably 
done so in correlation with a sessile, inactive hfe. They form 
the division Lipocephala. The other division comprises those 
Mollusea which possess a well-developed head, associated with 
a toothed lingual ribbon, capable of a biting or rasping action, 
borne on a cushion and moved by certain muscles, the whole 
apparatus constituting the odontophore. This organ has given 
a name to the division, the Glossophora. 
Division I. LIPOCEPHALA. 
CHARACTERISTICS.—Mollusca with rudimentary prostomium, 
no odontophore, and no eyes. Hither sessile, or with very 
feeble powers of locomotion. 
This division contains but one class, the Lamellibranchiata. 
CuLass Lamellibranchiata. 
Lipocephala which have retained the primitive molluscan 
bilateral symmetry. The body is laterally compressed, and 
the mantle is bilobed, each lobe secreting one valve of the 
bivalved shell. The two valves, right and left, are united by 
a dorsal elastic ligament. The ctenidia or gills are largely de- 
veloped, and by the currents their cilia create, assist in bringing 
food to the mouth. The foot is usually plough-shaped, and 
contains part of the viscera. It may be used for boring in 
sand or rock, more rarely for crawling. ‘The pericardium, part 
of the coelom, is in communication with the exterior by means 
of a pair of nephridia. The generative glands are simple, and 
have no accessory organs connected with them. 
In the common freshwater mussel, Anodonta cygnea, the 
shells are equivalve. Each valve is composed of three layers: 
