I90 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 (Ostrea), whilst, again, others bore long funnel-shaped 

 passages in the rocks or in submerged pieces of wood, etc. 



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 life. They form 

 the division Lipocephala. The other division comprises those 

 Mollusca 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 Crlossophora. 



Division I. LIPOCEPHALA. 



Chakactbristics. — Mollusca with rudimentary ^prostomium, 

 no odontopJiore, and no eyes. Hither sessile, or with very 

 feeble powers of locomotion. 

 This division contains but one class, the LameUibranchiata. 



Class LameUibranchiata. 



Lipocephala which have retained the primitive moUuscan 

 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 eqtiivalve. Each valve is composed of three layers : 



