384 SIZE AND NUMBER AS RELATED TO ORGANISMS 



vast majority of the parasitic species, whose "respiration" it would 

 seem natural to explain in other ways. 



In Spirina parasitifera these organs are very small, and it therefore 

 seems not unlikely that the present methods, when applied to more 

 suitable material, may give results much more detailed and intelligible. 

 The structure of one of these glands of S. parasitifera, so far as deter- 

 mined, is shown in Figs. 15 and 16. The great difference in the size 

 of the organs in the two cases, as well as the difference in structure 

 and space relationships, suggests the probability that the shape of 

 the organ changes materially, perhaps rapidly, under various condi- 

 tions. To this surmise it may be added that the method of collect- 

 ing the spirinas, and the varying length of time between their exis- 

 tence under natural conditions and the time of examination, would 

 result in a very material alteration in the environment; and it is 

 believed that the longer this time became the less free oxygen would 

 exist in the sea water in which the specimens were kept. This length 

 of time varied widely. 



SIZE AND NUMBER AS RELATED TO ORGANISMS 



The interesting behavior of the spermatidian granules as described 

 on pages 38 to 41 has suggested the following sketchy discussion of 

 the relationship of organisms to size and number. 



Why not vertebrates a mile long and a thousand feet high? Why 

 not vertebrates only a quarter of an inch long? The known facts 

 clearly indicate limits in both directions. 



Among the reasons for the existence of the upper limit are, circula- 

 tion difficulties due to friction in the blood vessels; accumulation of 

 an excess of excreta in the blood during the long journey to the dis- 

 tant extremities and back; the difficulty of maintaining the requisite 

 temperature at the extremities; limits set by the strength of ma- 

 terials, bone could not be strong enough or muscles efficient enough 

 properly to support and move so large an organism; food supply 

 difficulties; space limitations connected with protecting such an 

 organism from the elements, etc., etc. 



Reasons for the non-existence of exceedingly small vertebrates 

 also come readily to mind. The complicated vertebrate mechanism 

 would be in the way in an organism of such small size. Why an 

 elaborate pumping system to pump blood for a distance through which 

 it might diffuse without such a system? So with "centralized" 

 respiration. An internal skeleton plus the necessary protective 



