438 PROCEEDINGS OF THE ACADEMY OF [1898. 



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

 the body seem to be reduced in size in about the same proportion. 5 

 Strangely enough, however, the cells composing the various organs 

 of the dwarfs are not reduced in size. It must follow, therefore, that 

 a smaller number of cells are present in the various organs and also 

 in the entire body of the dwarf than in the giant. It is an almost 

 impossible task to count the actual number of cells present, even in 

 a very small organ. I have, however, been able to count the num- 

 ber of cells in a cross section of the rectum, and while the size of the 

 cells here, as everywhere, is the same in both varieties, the number 

 of the cells in the sections is greater in the giants than in the dwarfs. 

 Of all the cells of the body the ova are most readily enumerated ; 

 they are laid in capsules which can be easily counted, and each of 

 which contains a nearly constant number of eggs. Oft repeated ob- 

 servations show that, without exception, the fertilized but unseg- 

 mented eggs of the dwarfs are of exactly the same size as those of 

 the giants, but are very much fewer in number ; e. g., the following 

 table of averages has been obtained from a large number of obser- 

 vations : — 



Diam. of egg. No. of caps. Eggs in caps. Total No. 

 C. plana (type) .136 mm. 51 176 9,000 



C. plana (dwarf). 136 mm. 48 64 3,070 



It is notable that the number of capsules formed is nearly the 

 same in the two varieties, though there is a great difference in the 

 number of eggs inclosed in each capsule. 



In Crepidula, therefore, the cell size is constant, and variations in 

 the size of the body are due to variations in the number of cells 

 present. 



This conclusion leads naturally to an inquiry as to the cause of 

 the smaller number of cells, and hence the smaller size of the body 

 of the dwarfs as compared with the giants. In this connection it 

 will be remembered that Semper 6 long ago observed that the pond 



5 It is worthy of note that certain organs, particularly the gill filaments, are 

 reduced in number in the smaller individuals but not in size, e. g., the num- 

 bers of gill filaments in three different individuals were as follows: — 



Mature female . . Vol. of body, .75 cc, Gill filaments, 204. 

 Immature female . Vol. of body, .05 cc, Gill filaments, 53. 

 Dwarf female . . Vol. of body, .05 cc, Gill filaments, 58. 



6 Semper, Ueber die Wachsthumsbedingungen der Linnattss tagnalis. Arb. 

 aus dem. Zool. Zoot. Inst. Wurzburg, Vol. 1, 1874, also Animal Life as af- 

 fected by the Natural Conditions of Existence, 1879. 



