THE SIZE OF ORGANISMS 181 



as ether, alcohol, tobacco, lecithin, etc., (d) temperature, (e) oxygen, 

 (/) presence or absence of waste products, (g) conditions of normal or 

 abnormal stimulation and irritability. These extrinsic factors which 

 influence growth have been studied by many investigators, but owing 

 to lack of time I shall pass over all of them except the last named. In 

 the case of the dwarf Crepidulce which are found in the small shells with 

 the small hermit crabs there is practically no evidence that any of the 

 other factors except the last named, are involved in this dwarfing. 

 These animals live in open shells on sandy sea beaches along with the 

 giant forms; so far as I can determine, the food supply is super- 

 abundant, while the conditions of temperature, aeration and freedom 

 from waste products are identically the same for dwarfs and giants. 

 The only difference which I have been able to detect is the size of the 

 shells to which the animals are attached ; those which are attached to the 

 small shells of Nassa or Litorina live and die as dwarfs, reaching only 

 about one thirteenth the volume of those which are attached to the 

 larger shells of Natica; however, if they are removed from the smaller 

 shells and placed on the larger ones they may grow to typical size. The 

 dwarfs, however, are continually hampered by their limited quarters; 

 they are unable fully to expand the foot or the mantle, and they are 

 more frequently irritated by the movements of the hermit crabs than 

 are those in the larger shells. Under these circumstances they probably 

 take less food than those in larger quarters, and although they become 

 perfectly differentiated and sexually mature they are dwarfed in size. 

 Similarly I have found that Paramecium confined in capillary tubes 

 never grows nor divides, though it may live indefinitely, and although 

 precautions may be taken to change the medium frequently and thus to 

 remove waste products and to supply abundant food and oxygen. In 

 such tubes Paramecium is continually irritated and presumably takes 

 less food than when in unconfined spaces. 



II. Body Size, Cell Size and Cell Number 



Is the size of an organism due to the size of its constitutent parts, or 

 to the number of those parts, or to both of these causes combined? 

 Evidently different organisms differ in this regard. In many plants 

 and lower animals the number of constituent parts is directly corre- 

 lated with the body size ; branches and leaves, segments and organs may 

 increase in number indefinitely with the growth of the organism. In 

 tapeworms and many annelids the number of segments, with their 

 characteristic organs, increases throughout life; but in more highly 

 differentiated forms the number of body segments and organs is con- 

 stant, and does not increase in number after embryonic stages. In spite 

 of the information occasionally conveyed by examination papers, the 



