1898.] NATURAL SCIENCES OF PHILADELPHIA. 437 



II. An interesting case of environmental dimorphism to which I 

 wish to call attention is found in a race of dwarfs which is specifi- 

 cally identical with C. plana (Plate XXII, Rows 3 and 4). 



This species is found most abundantly inside dead shells of Never- 

 ita inhabited by the large hermit crab, Eupagurus Bernhardus. In 

 this position individuals grow to a large size, mature females fre- 

 quently reaching a length of 2 inches and a breadth of H inches. 

 On the other hand, the dwarfs referred to are found within dead 

 shells of Illyonassa or Littorina inhabited by the little hermit, Eupa- 

 gurus longicarpus, and never exceed f inch in length by f in breadth, 

 i. e., they are about J the linear dimensions of the larger form. I 

 removed from their shells a large number of individuals of both the 

 common and the dwarfed forms, and estimated the volume of the 

 body in the following way : The individuals were first placed on 

 blotting paper to remove any excess of water, and then a given 

 number were dropped into a known volume of water in a finely gradu- 

 ated tube. In this way the average body volume could be deter- 

 mined with sufficient accuracy. The results of very many such 

 measurements in which mature females of all sizes were taken with- 

 out any conscious selection of large or small individuals show that 

 the average body volume of a mature female of G. plana is f cc. 

 while the average volume of a mature female of the dwarf variety is 

 sV cc, i.e., the average body volume of the typical form is about 

 thirteen times that of the dwarf. This disproportion in size would 

 be much greater if comparison were made between the largest indi- 

 viduals obtainable in the two classes since the dwarfs are much more 

 uniform in size than the type forms. 



This difference in size is not due merely to differences in the age 

 of individuals compared, since only sexually mature females were 

 chosen for purposes of measurement; all the individuals measured 

 were carrying egg masses, and unless we assume that sexual matur- 

 ity appears much earlier in the dwarfs than in the giants, we must 

 conclude that they were of approximately the same age. A careful 

 study of the shells of the dwarfs and giants also strengthens the view 

 that the former are, on the whole, as old as the latter ; for while the 

 dwarf shells are much smaller and more delicate than those of the 

 giants, they are, in no sense, immature in shape or character; the 

 lines of growth are closely crowded together, the margin of the shell 

 is frequently thickened, and its general shape differs from that of an 

 immature shell. 



