1897.] KEW YORK ACADEMY OF SCIENCES. 311 



in other dim3'arians is questioned. The view urged by Mr. J. 

 T. Cunningham is that the posterior adductor should be looked 

 for near the parieto-splancnic ganglion, and this is hard to con- 

 trovert. The passage of the rectum upward in front of this 

 muscle and then backward dorsal to it is a fact that must be 

 taken into consideration, however, together with the fact that 

 the anal opening occurs immediately posterior to it. There 

 seems to be no good reason why this muscle should not have 

 arisen dorsallly to the digestive tract if it is not the homologue 

 of the posterior adductor. 



The external surface of each valve is divided into two regions 

 by the axis of motion, an anterior and posterior. The anterior 

 area is again divided into two areas, each triangular in outline. 

 The la3'ers of shell accretion in the posterior triangle are parallel 

 to the axis of motion, and their margins are protruded into nu- 

 merous fine teeth which cut wood and even ivory when moved 

 in a direction perpendicular to the aforesaid axis. The anterior 

 triangle has its accretion layers plaited into a multitude of sharp 

 angular ridges whose cutting direction is the same as that of the 

 teeth of the posterior triangle. The accretion la3^ers themselves 

 are very regular and hard. 



It is not necessary to discuss further the anatom}' of Teredo, 

 since we have to do simply with the mechanical arrangements 

 correlated with the boring habit. 



The portion of the shell posterior to the axis of motion is 

 smooth. An epidermis is present in the Pholadidse and Teredo 

 except over the anterior part of the shell from which it is worn 

 away. 



The burrow of the Pholadidie has the shape of an elongate 

 cone, rounded at the base. The wall very frequently presents to 

 view sliort scratches which are segregated in series. Those in a 

 given series are parallel to each other and are clearly made by 

 the teeth of the valves. I have sometimes found, however, that 

 besides these there are other scratches confined to a small cir- 

 cular patch at the very basal extremity of the burrow. These 

 are made, it seems probable, by sand which gets between the 

 disk of the foot and the wall. The direction of the scratches is 

 such as to suggest a shearing twist on the part of the foot, and 

 it is hard to avoid the conclusion that the foot to some extent 

 aids in the attrition of the rocky wall by a drill-like action, using 

 sand as a cutting material. 



The burrow of Teredo is so well known that there is no need 

 of description. It is sufficient to quote Quatrefages, who says 

 the burrow " appears to me to have every mark of a direct me- 

 chanical activity." 



