THE MOLLUSCA 185 



sense organs are present (e.g. the eyes in pecten) they are 

 borne on some other part of the body. The body is en- 

 closed in a fleshy envelope, the " mantle." The gills are 

 large and flat. 



Some have a protrusible " foot," which serves for 

 locomotion e.g. cockle. In others the foot is suppressed 

 e.g. oyster. The shell is closed by adductor muscles 

 firmly attached to the inside of each valve. 



Just within the dorsal edge of the valves there is a set 

 of " teeth," which interlock when the shell is closed and 

 prevent lateral slipping of the valves. 



In some forms e.g. in Mactra there is a little projection 

 of the shell inwards, and upon this the ligament or hinge 

 is fixed. It is in the form of a little pad or cushion, of a 

 consistency like firm india-rubber. When the valves are 

 pulled close it is compressed, and when the tension on the 

 muscles is removed it reasserts itself, and forces the valves 

 apart. 



In others e.g. in the cockle the mechanism is reversed. 

 The ligament is strap-shaped, and on the outsider --of the 

 shell. The inner margins of the latter, near the hinge, 

 are rounded, so that when the valves are closed the liga- 

 ment is stretched. Its contraction when the pull of the 

 muscles is released reopens the shell ; so that the animal 

 only uses exertion in closing : the opening process is 

 mechanical. 



Within the body are two chambers, one lying on the 

 lower side and one on the upper. They join at the anterior 

 end of the animal, and present two openings, close to one 

 another, at the posterior one. 



These chambers are richly lined with ciliated cells, the 

 cilia of which, by their movement, cause a steady flow of 

 water into the lower chamber, round the anterior end, and 

 into the upper one, and then out by the upper opening. 



These openings are termed the lower one the " in- 



