ACEPHALA. 2& 



65. The Cardium. Other bivalves are furnished with an 

 instrument shaped somewhat 

 like a foot and leg, with which Fig. 21. 



they give themselves a slow, 

 but continued motion through 

 the sand. The form of this in- 

 strument in the cardium or 

 code, is shown by Fig. 21. 

 This organ is a hard mass of 

 muscular fibres, woven together 

 in a very complex manner, and 

 capable of motion in every di- 

 rection. By retracting, and then forcing this instrument 

 forward, a contrary motion is given the shell, for the 

 same reason that a boatman in shallow water, pushes 

 his craft along with an oar from the stern. With 

 his foot, the cardium also contrives to bury himself to any 

 depth he chooses in the sand or mud. For this purpose 

 the leg is elongated, and by a sort of vermicular motion is 

 forced deep into the sand ; then turning up the toe, and 

 forming it into a kind of hook, the animal by an alternate 

 retraction and elongation of the leg, raises and depresses 

 the shell, and by the resistance of the sand on the hook 

 . gradually draws the whole downward. By a reverse of 

 this motion, that is, by first drawing up the foot, and then 

 pushing it downward against the sand, the shell is again 

 forced toward the surface. In this manner does the car- 

 dium bury itself in the sand, in the course of a minute or 

 two, to avoid danger, and as quickly emerges from its 

 hiding-place when the danger is past. 



66. With an instrument similar to that belonging to the 

 cardium, many species of bivalve mollusca move along on 

 the sandy bottoms of the water in which they live, with 

 greater or less facility. In nearly every still pond or river, 

 the furrows left by the passage of unios, or fresh-water 

 clams, may be seen running in every direction, and made 

 in this manner. 



In what manner does the cardium move? How does the cardium burr 

 tself in the sand ? 



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