282 LOWER INVERTEBRATES. 



drawn entirely within the valves. Like the last, it is boreal in its distiibution, but it 

 is not uncommon in Massachusetts Bay. 



The Anatinid^ is an almost extinct family. To-day it is represented by a few 

 species, but in geological time they played a much more important part than now, 

 their remains being found in the oldest palaeozoic rocks. The internal surface of the 

 shell is pearly, the external granular, and the hinge is usually toothless. About half a 

 dozen species are found on our coasts, representing the genera Pandora, Thracia, 

 Periplonia and Z/yonsia. 



The SoLENiD^, the family of the razor-clams, is next in order, and the typical 

 forms, like Solen, well deserve the common name which has been given them. In all 

 the family the shell is long, and in some of the genera immensely so ; at each end it 

 gapes, and the hinge teeth are usually two in one valve and three in the other. The 

 foot is very large, and more or less cylindrical, the siphons are short or moderate. 

 The old genus Solen has recently been broken up, those with straight shells and one 

 tooth in each ^■alve being retained in that genus, while those «ith the typical number 

 of teeth, and usually slightly curved shells, belong to Ensis. 



In American waters PJ/isis atnericana is the razor-fish, or razor-clam. The shell 

 is long and sub-cylindrical, and bears no slight resemblance to the familiar tonsorial in- 

 strument. It is a common inhabitant of the sandy shores. These clams excavate large 

 elliptical holes, which penetrate downward, usually in a nearly vertical direction, to a 

 depth of two or three feet. Up and down this hole they go. When the tide is in, and 

 no danger is near, one end of the shell usually projects above the surrounding surface 

 of the sand for an inch or so, but a sudden jar startles them, and down they go with 



great rapidity. It is useless to at- 

 tempt to dig them out, for they 

 can burrow as fast as a man can 

 shovel out the sand, and besides 



FiG.308.-^«.,>a»>«-ica,m,ra.or-oiam. ^hey have two or three feet the 



start. The process of burrowing 

 is interesting. The foot is bevelled off to a point, and this is readily j)uslied down 

 into the sand. Then the animal inflates the foot with water so that it becomes 

 bulbous at the extremity ; this at the same time crowds aside the sand and gives 

 the animal a hold whereby it can draw itself down. By a repetition of the process 

 it still further increases its distance from the surface. The razor-clam can start this 

 burrow when lying on the surface ; at first its progress is slow, but as soon as it gets 

 the shell in a vertical position, it goes much more rapidly. 



The razor-clams are used for food, but to a far less extent than many other bivalves. 

 For this there are several reasons. First, it is not so common as the quahog or the 

 soft-shell clam, and, again, it is not so easily procured. We have already alluded to 

 the rapidity with which it disappears when alarmed, and the recent investigations of 

 Dr. Sharp show that it does not depend, for its warning, upon the senses of sight and 

 feeling alone for its warnings of impending danger. On the ends of the siphons Dr. 

 Sharp finds organs of vision, very rudimentary, it is true, but still sufficient to recog- 

 nize various degrees of light and shade. His attention was called to this subject by 

 Ihe fact that a shadow cast upon razor-clams exposed for sale caused them to imme- 

 diately withdraw their siphons. Histological investigation followed, which showed 

 that the essential parts of optic organs were present. 



The fishermen, in procuring them, walk quickly up to the animal, and grasp it 



