Page:Popular Science Monthly Volume 83.djvu/182

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ODY 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 insignificant, 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 alimentary 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 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 33 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 hernhardus); the dwarfs live in small shells occupied by a species of little hermits (Pagurus longicarpus). The dwarfs are sexually 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