BODY SIZE AND CELL SIZE 181 



2. CELL SIZE AND BODY SIZE IN TYPICAL AND DWARFED 

 INDIVIDUALS OF CREPIDULA PLANA 



I have already discussed in detail an interesting case of en- 

 vironmental dimorphism in C. plana (Conklin, '96, '97, '98), but 

 since this work has apparently escaped the attention of other 

 workers on this subject I shall here quote from one of these papers 

 ('98) at some length. The ordinary or typical form of 



Crepidula plana is found most abundantly in dead shells of Neverita 

 inhabited by the large hermit crab, Pagurus polycarpus. In this posi- 

 tion individuals grow to a large size, mature females frequently reaching 

 a length of two inches and a breadth of one and one-quarter inches. 

 A dwarf race of C. plana occurs in the dead shells of Nassa or Littorina, 

 inhabited by the little hermit crab, Pagurus longicarpus; the largest 

 individuals of this race never exceed three-quarters inch in length and 

 three-eighths inch in breadth, i.e., they are about one-third the linear 

 dimensions of the larger form. 



There is good evidence that these dwarfs are not a permanent variety 

 or race. In the first place there are no anatomical differences between 

 the two varieties save size only; secondly the eggs, embryos and larvae 

 of the two cannot be distinguished; .... finally, a few specimens 

 were found which showed by the shape and character of their shells 

 that at one time they had been typical dwarfs; afterwards, having been 

 detached, they obtained a new foothold on a larger surface, and their 

 shells increased in size, the new portions of the shell being 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 be- 

 cause of inheritance; they are, in short, a physiological and not a mor- 

 phological variety. 



The average body volume of a mature female of C. plana is f cc, 

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

 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 individuals ob- 

 tainable in the two classes, since the dwarfs are much more uniform in 

 size than the type forms. 



The dwarfs are perfectly formed in all respects, and all organs of the 

 body seem to be reduced in about the same proportions. It is interest- 

 ing to note that certain organs, or parts of organs, which are formed in 

 considerable numbers in the course of development, are reduced in num- 

 ber but not in size in the smaller individuals;- this is true of the number 



- In agreement with these observations are the experiments of Miss Peebles 

 ('97) on the regeneration of small pieces of hydra; in such cases one, two, three 

 or four tentacles are formed, depending upon the size of the regenerating piece. 



