No. I.] THE EMBRYOLOGY OF CREPIDULA. 13 



lar spot and no other ; therefore, the animal cannot migrate to 

 larger quarters after it has grown to its maximum size in the 

 smaller ones. The eggs, embryos, and larvae of the two vari- 

 eties cannot be distinguished, and since both live together on the 

 same beach, under about the same conditions of food, tempera- 

 ture, and water, it seems probable that the later development 

 of both would be the same if one was not forced by the 

 smaller size of the shell which it inhabits, or by the smaller 

 quantity of food supplied to it, to remain smaller than the 

 typical form. But what is absolutely conclusive is the fact 

 that the dwarfs, when placed in positions where they can 

 obtain a new foothold and increase in size, become almost, if 

 not quite, as large as the common form. A few specimens 

 were found which showed by the shape and character of their 

 shells that for several years they had lived in the shells in- 

 habited by the smaller hermit crab, and had been typical dwarfs ; 

 afterward, having been detached, they by rare good fortune 

 gained a new foothold on a larger surface, and their shells 

 began to increase in size, the new portions of the shell be- 

 coming shaped so as to fit the surface upon which they had 

 found a new home. In every such shell one can recognize 

 both the dwarf and the normal forms. The dwarfs are what 

 they are by reason of external conditions, and not because of 

 inheritance. In such a case the shape and size of the body, 

 and the number of cells in the entire organism are greatly 

 modified by the direct action of environment. There is no 

 evidence, however, that these modifications of the shape and 

 size of the body and the number of cells have become in the 

 least degree heritable. 



C. convexa is smaller than either of the preceding species, 

 and as its name indicates, its shell is more convex, while its 

 color is much darker than either of the others. It is found 

 upon the outside of those gasteropod shells (chiefly Litorina 

 and Illyonassa) inhabited by the smaller hermit crab, Eupa- 

 gurus longicarpus ; and it undoubtedly obtains its food, as do 

 the others, from the fragments left by its messmate. Unlike 

 the others, however, it can move about to a limited extent, and, 

 if removed from the surface to which it is fastened, can attach 



