LAMP-SHELLS AND THEIR ALLIES 23 



It is interesting to capture a healthy clam and put 

 him in a jar of sea-water with a thick bed of sand 

 at the bottom, and see him adjust himself to the 

 changed conditions. He digs with his foot, and he 

 pumps water through his siphons. His pumps, how- 

 ever, are invisible, for in reality they consist of in- 

 numerable little lashing hairs, or cilia, covering the 

 surface of the gills. A bit of gill may be snipped 

 off from a freshly-opened oyster or clam and placed 

 in a drop of sea-water under a microscope, and the 

 movement can be plainly observed. 



Although the clam has no head, the part which 

 goes down into the burrow first is called the front 

 end, and the siphons alwa3^s follow. In the dead 

 shell the pallial sinus is therefore always at the pos- 

 terior end. When a shell is in position for describ- 

 ing, as in the figure, the ligament is at the top. It 

 is easy to see that Figure 57 represents a left-hand 

 valve, for the position of the sinus is plain. 



In describing the shells of the pelecypods we shall 

 have occasion to call attention to all of these fea- 

 tures, also to the epidermis, or periostracum, which 

 is a kind of horny covering on the outside of the 

 shell that is found in some species. It is secreted 

 by the mantle, somewhat as the shell is formed, but 

 is composed of a horny material and contains very 

 little lime, while the shell proper contains a great 

 deal. With these simple explanations we will pass 

 on to a description of the species, leaving a fuller 

 account of the anatomy of the animal to the numer- 

 ous excellent books on zoology. 



The first shell of this great class that we are to 



