PHOLADID^E. 95 



They usually burrow in a slanting direction. When 

 several individuals occupy the same layer, one of them 

 seldom interferes with another by breaking into the 

 hole of its neighbour ; but it pursues a parallel course. 

 Cases, however, now and then occur in which no such 

 forbearance is shown. The avoidance of each other's 

 burrow is probably owing to the extreme sensitiveness 

 of the foot or perforating organ, which is always pushed 

 out in advance to feel its way. The structure of this 

 organ is similar to that of the foot in Teredo and 

 Patella, being thinner and of finer texture in the middle 

 than at the circumference ; it is nearly circular and 

 truncated. Dr. Fischer believes that the foot in Phola- 

 didea, when it has ceased to perforate, becomes atro- 

 phied. It is then hindered from further action by the 

 shelly wrapper which closes the front gape of the shell, 

 and is therefore useless. As far as has been hitherto 

 observed, all the members of this family possess the 

 hyaline " style " or cuspidated process, which is found 

 in many of the other Conchifera. The use of this 

 curious internal apparatus is unknown. Some phy- 

 siologists consider it a digestive appendage of the 

 stomach ; but Cailliaud is of opinion that it is connected 

 with the fecundation of the eggs, in consequence of 

 these mollusks being hermaphrodite. Lacaze-Duthiers, 

 however, regards them as of distinct sexes. The splen- 

 did work of M. Emile-Blanchard, now in course of publi- 

 cation, and entitled ' ( I/organisation du Begne Animal," 

 ought to be consulted with respect to the internal 

 structure of the Pholadida. He has shown that each 

 lobe of the mantle on the anterior side is extended, and 

 reflected behind, where they are united and form a long 

 and muscular expansion apart from the rest of the body. 

 Siebold thought that at the base of the siphonal tentacles 



