1 2 CONK LIN. [Vol. XIII. 



ried about and supplied with food by the hermit, for here again 

 the Crepidula is unable to move about or change its position 

 in the least after it has reached adult size. When a hermit 

 dies, or leaves one shell for another, the Crepidulas in that 

 shell remain attached for some time, but sooner or later perish 

 without attempting to find another shell. Some doubt has 

 been expressed as to whether C. plana is a true species. ^ It 

 has been held that it belongs to the species fornicata, and that 

 those individuals living inside other shells have been slightly 

 modified by their environment, the shell becoming thinner and 

 flatter. There is no doubt, however, that C. plana is a well- 

 marked species, as is shown by its embryological as well as its 

 anatomical differences from C. fornicata. 



A very interesting variety of C. plana is found within those 

 gasteropod shells (Illyonassa, Litorina, etc.) inhabited by the 

 smaller hermit crab, Eupagurus longicarpus. This variety 

 resembles the type in all respects save size, being usually less 

 than one-thirteenth the size of adult female specimens found 

 within the larger shells. That this difference in size is not 

 due merely to age is shown by the fact that the dwarfs are 

 sexually mature, and they show by the shape and character of 

 their shells that they are several years old. Apparently, all 

 the organs are perfectly formed, and differ from those of the 

 larger variety only in size. The ova are of the same size as 

 those laid by the larger form, but are fewer in number. The 

 same thing is true of the cells constituting the other organs 

 of the body, so that it may be said that the difference in size 

 between those two varieties is due to the smaller number of 

 cells of which the body of the dwarf variety is composed, rather 

 than to the smaller size of those cells. 



There are many evidences that this dwarf form is not a per- 

 manent or persistent variety, but only a physiological one.^ 

 It, like the typical form of this species, is sedentary, and can- 

 not move about after it has reached a certain size. The shape 

 of the shell and body are modified, so that they fit one particu- 



1 Cf. Gould : The Inveitebrata of Massachusetts. 



2 It may be doubted whether the word " variety " should be used in this connec- 

 tion at all. However, for lack of a better term, it is employed in its colloquial 

 meaning rather than in a strictly scientific sense. 



