28 Our Food Mollusks 



The descendants of these ancient forms, adapting 

 themselves to various new modes of living, found new 

 uses for the foot which gradually became modified to 

 perform these functions more perfectly. Burrowing was 

 one of these habits. It is well demonstrated by the 

 young of the long neck and little neck clams, that thrust 

 and worm the sharpened anterior end of the foot into the 

 bottom, then expand it so that it acts as an anchor. 

 Special muscles (shown in the figure of Venus) extend- 

 ing from the base of the organ to an attachment on the 

 shell, then contract and draw the body down into the 

 bottom. This is repeated until the desired depth is 

 reached. 



In the black mussel and the young of the shallow 

 water scallop of the Atlantic, the foot performs a very 

 special function, that of shaping and attaching the an- 

 chor threads of the byssus. The byssus gland is an 

 organ developed in the ventral tissues of the foot, near 

 its posterior margin. It produces a viscous secretion 

 that hardens in coming in contact with water, forming 

 threads of great strength. It is also present and func- 

 tional in young little neck and long neck clams. 



In the smaller scallop and black mussel, there is a 

 groove on the under side of the foot, extending from the 

 opening of the byssus gland to the end of the foot, where 

 it expands into a diamond-shaped cavity. This cavity 

 remains open below, while the groove is temporarily 

 converted into a closed tube by the folding together of 

 its edges. The foot is stretched out from the body, the 

 end placed against some foreign object, and the fluid 

 is poured out through the tube. When this is opened, 

 water comes in contact with the secretion, which instantly 

 hardens into a thread, and the foot is withdrawn to be 



