38 FIRST BOOK OF ZOOLOGY. 



Many differences will be observed in the muscular impres- 

 sions, as well as in the teeth and the position of the ligament. 



37. As the common soft-shelled clam can be readily pro- 

 cured in the fish-markets, it will be well to study this first. 

 A live specimen must be selected, and, as the clam lives a 

 long time after it has been removed from the water, there 

 will be no difficulty in getting the proper specimen. Upon 

 pressing the valves together, or touching the soft parts which 

 partly protrude from between the valves, the creature will 

 show signs of life, by drawing the shells closer together, and 

 this will assure the pupil that the specimen is alive. 



A large shallow dish may now be filled with pure sea- 

 water, and in this the clam may be placed. After it has 

 remained there for some time, the black end of the animal, 

 which is incorrectly called the head, will slowly stretch out 

 from between the shells, and the end, unfolding, will display 

 two openings fringed with little feelers (see Fig. 42). Into 

 one of these openings the water will be seen flowing, while 

 from the other a cun-ent of water will be seen issuing. And 

 these openings are called the incurrent and excurrent orifices^ 

 and correspond to similar parts previously described in the 

 fresh-water mussels. In the latter creature, the openings 

 just protruded beyond the edge of the shell. In some very 

 small species of fresh-water bivalves, one of which was shown 

 in Fig. 3Y, these openings were at the end of separate tubes. 

 In the clam the tubes are inclosed in one sheath. 



The clam can protrude this apparatus to a length equal- 

 ing that of the shell two or three times. As the clam lives 



