No. I.] THE EMBRYOLOGY OF CREPIDULA. 1 1 



below low-water mark, where it usually occurs in curious chains 

 often containing ten or twelve individuals. In these chains 

 the foot of one individual is firmly fastened to the dorsal side 

 of the shell of the next one, and the heads of all the animals 

 are turned in the same general direction ; the first or oldest 

 individual in a chain is usually attached by its foot to a stone 

 or dead shell. Even those which live upon Limulus sometimes 

 show a tendency to pile one upon another, though in this case 

 there are seldom more than two or three in a pile. C. fornicata 

 also occurs, but in comparatively small numbers, on submerged 

 portions of stones, buoys, and wharves. In none of these 

 cases, however, is it able to change its position after it is about 

 half grown, and it obtains all its food from the particles which 

 float to it in the water. The fact that the large Crepidulas are 

 immovably fixed to one spot is shown not only by the shells, 

 which have in many cases become greatly distorted in order 

 that they may perfectly fit the spot of fixation, but I have again 

 and again observed that in old Crepidulas the sole of the foot 

 secretes a calcareous substance by which the animal is so firmly 

 fixed that the foot is often torn to pieces before it can be freed 

 from its attachments. Unlike most prosobranchs, the foot in 

 Crepidula is plainly divided into two portions, a broad and thin 

 propodiitm which is deeply notched in the middle, and a thick, 

 muscular mesopodiimi or sole, by which the animal is attached. 

 The sole of the foot forms a powerful sucker, and when the 

 animal is removed from its attachment so as not to injure the 

 foot, the latter immediately becomes deeply concave on the 

 ventral side, showing that considerable muscular tension was 

 being exerted in order to produce the suction. 



C. plana is smaller and much flatter than C. fornicata, and 

 its shell, which is quite fragile, is nearly white in color. It is 

 found most abundantly within those gasteropod shells (Neve- 

 rita, Lunatia, etc.) inhabited by the larger hermit crab, Eupa- 

 gurus Bernhardus, and while it may be found in this position 

 either at the outer or inner lip of the shell, it is nearly always so 

 situated that its head is directed toward the opening of the shell 

 in which the crab lives. It is evident that in this case also the 

 Crepidula has taken this position in order that it may be car- 



