178 THE POPULAR SCIENCE MONTHLY 



THE SIZE OF OKGANISMS AND OE THEIE CONSTITUENT 



PAETS IN EELATION TO LONGEVITY, SENESCENCE 



AND KEJUVENESCENCE* 



Br Peofessoe EDWIN G. CONKLIN 



PEINCBTON UNIVEBSITX 



I. Body Size 



BODY size is one of the most variable properties of organism; the 

 smallest living things are probably invisible to the highest powers 

 of the microscope, the largest are gigantic beasts weighing many tons. 

 Within the same class, and in animals equally complex in structure, 

 variations in size are enormous, as, for example, in the elephant and the 

 mouse. Within the same species, where structural differences are insig- 

 nificant, size differences may be very great. In some species there are 

 great differences of size between males and females; in extreme cases 

 males may be minute and rudimentary forms, without mouths and ali- 

 mentary canals, and capable of living for only a few hours, as in certain 

 rotifers, worms and arthropods, whereas the females are relatively large 

 and perfect individuals capable of an extended existence. 



In Crepidula a genus of marine gasteropod which I have studied 

 and to which I must particularly direct your attention, I have found 1 

 great differences of body size in the mature individuals of different 

 species and also in different individuals of the same species. The 

 volume of the average adult male of C. fornicata is 125 times that of 

 the average male of C. convexa; the volume of the female of the former 

 species in 32 times that of the latter. In these gasteropods the males 

 are always much smaller than the females; the volume of the average 

 female of C. plana is about 15 times that of the average male. All 

 mature animals of this genus are sedentary, and many of them live in 

 or on dead shells which are the homes of hermit crabs. In the species 

 C. plana I have found an interesting class of dwarfs; the animals of 

 usual size live in large shells inhabited by a species of large hermit crabs 

 (Pagurus bernhardus) ; the dwarfs live in small shells occupied by a 

 species of little hermits (Pagurus longicarpus) . The dwarfs are sex- 

 ually mature and, unless forcibly removed, live their whole life long in 

 the small shells, where they attain an average size only one thirteenth 

 that of the normal forms but if the dwarfs are forcibly taken out of the 



* Lecture before the Harvey Society, New York, March 7, 1913. 

 ^Conklin, "Body Size and Cell Size," Jour. Morph., 12, 1912. 



