310 TRANSACTIONS OF THE [aPR. 12, 



of attachment of the foot muscles are the spoon shaped pro- 

 cesses, one for each valve, attached at the umbo on its inner 

 side. The shape and position of these two processes are such as 

 to bring the center of the mechanical system nearer to the longi- 

 tudinal axis of the burrow, and this is mechanically an advan- 

 tage. 



The burrow being completed (Pholadidea and ParapJiohif^) the 

 foot which is no longer for use, atrophies until not a trace of its 

 musculature remains, so that in every part of the abdomen, the 

 abdominal wall is so thin that the genitalia can be seen. So en- 

 tirely lost is the musculature that the crystalline style bulges out 

 the abdominal wall where once there was a large mass of muscle. 

 Concurrently the anterior plates (callum) have been secreted, 

 the dorsal by the antero-dorsal mantle pad which surrounds the 

 anterior adductor muscle and the ventral by the thick and now 

 fusing edges of the anterior mantle opening. 



But the question arises, namel}", why do not the adductor 

 muscles, which are clearly correlated with the work of drilling, 

 atrophy as does the foot. The answer to this will be found in 

 the fact that the animal still uses all the muscles in moving the 

 valves of his shell both for the indraught and expulsion of 

 water, the anterior adductor taking the place of the hinge liga- 

 ment which is entirely absent in all three forms. 



It is evident from the foregoing discussion that the Pholadi- 

 dea and Parapholas pass through a Zirphsea stage which I have 

 called the working form, during which the animals have an active 

 vigorous existence. Upon the cessation of their activity follows 

 degeneracy,* so that the relation between Pholadidea and 

 Zirpjhaea is joarallel in a high degree to that between the oyster 

 and its more energetic cousin, the clam. The oyster, however^ 

 tires of an active existence much sooner than the piddock, 

 which rests from his labors only when a safe retreat is made for 

 himself. 



In Teredo the shell presents in general the same features as 

 the forms already considered. It is, however, much contracted 

 antero-posteriorl}'. The valves articulate at the umbones, from 

 the inner surface of which spring the very long curved pro- 

 cesses already referred to in the other forms. At the extreme 

 ventral part of each valve is a tubercle, and at this point too, 

 the valves meet and articulate. A line passing through these 

 two points constitutes an axis of motion for the valves. In 

 front of this axis is the anterior adductor which extends above 

 and in front of the umbo, but not back of it. Behind the axis 

 is the large posterior adductor, whose homology' with that muscle 



* A. Lang: Text-Book of Comp. Anat., Eng. Ed., Vol. II., p. 115. 



