524 H. S. Jennings and George T. Hargitt. 



same environments ; (2) whether the form of the posterior half of 

 the body differs in the two races. ^ 



The method of measurement was as follows: Twenty-five 

 specimens of each race, taken at random, were outlined at a 

 magnification of 500 diameters, by the use of the Edinger projec- 

 tion apparatus. The following dimensions were then taken from 

 these outlines: (1) total length; (2) greatest breadth; (3) breadth 

 at the middle; (4) breadth at certain distances behind the middle, 

 the distances selected being respectively i, I, 1 and I of the dis- 

 tance from the middle to the posterior end (the greatest breadth 

 in most cases coincided with the breadth at the middle). 



It is evident that the breadths measured in the posterior half 

 of the body will enable us to determine whether caudatum tapers 

 backward more rapidly from the middle than does aurelia, as 

 has been supposed to be the case. This is done by reducing the 

 breadths found at the intervals behind the middle to percentages 

 of the breadth at the middle. These percentages of course 

 decrease more rapidly backward in the animals that taper more 

 rapidty backward.^ 



As to the relative breadth, in proportion to the length, the mean 

 ratio of greatest breadth to greatest length ^ for the twenty-five 

 specimens of L^ (caudatum), was 21.9 per cent; for the twenty- 

 five specimens of k (aurelia) 24.3 per cent. A similar result 

 was reached later when the race L2 (caudatum) was compared 

 with the very small aurelia race ^, living in th6 same culture, 

 under conditions of excessive nutriment (see p. 527). Here 

 twenty-five specimens of L^ gave a mean ratio of breadth to 

 length amounting to 34.1 per cent while twenty-five i gave a 



* As we have before mentioned, the two races could easily be distinguished, 

 even though living rogether, by the great difference in size. See p. 516. 



^ Owing to the fact that the apparent shape of the anterior half of the body 

 differs, depending on how the specimens happen to lie (as shown later), charac- 

 teristic differences between the two species in the anterior half, if such exist, can- 

 not readily be tested in this way. 



* These mean ratios are of course obtained by dividing for each specimen separ- 

 ately the breadth by the length, and taking the mean of the ratios so obtained. 

 Dividing the mean breadth of all by the mean length of all would give a different 

 result. 



