ACEPHALA. 33 



The Cardium. Other bivalves are furnished with an 

 instrument shaped somewhat 

 like a foot and leg, with which 

 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 muscu- 

 lar fibres, woven together in a 

 very complex manner, and 

 capable of motion in every 

 direction. By retracting, and then forcing this instru- 

 ment 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 pur- 

 pose 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 down- 

 wards. By a reverse of this motion, that is, by first 

 drawing up the foot, and then pushing it downwards 

 against the sand, the shell is again forced towards the 

 surface. In this manner does the Cardium 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. 



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 direc- 

 tion, and made in this manner. 



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

 itself in the sand 7 



