W. E. Agar 307 



Species of Daphnia are mostly differentiated from one another by 

 small quantitative characters which are difficult to define precisely, 

 though no one who has brought into cultivation numbers of clones of 

 this genus could doubt for a moment that an enormous amount of 

 genetic diversity exists within the genus, and indeed within each species, 

 or even each named variety (cf Agar, 1914). 



The eleven clones with which this experiment was started can easily 

 be divided into two groups, one referable to D. pulex and the other to 

 D. ohtusa. The females of these two species are very similar in regard 

 to their morphological characteristics. Indeed, the only even fairly 

 satisfactory distinguishing feature, capable of precise definition, which 

 I could find, is the relative sizes of the three dorsal abdominal processes. 

 In the females, the differences in these processes are not very great, and 

 transgressive fluctuation takes place. In the males however the middle 

 of the three processes is six or seven times as long in D. pulex as in 

 D. ohtusa, although the body length of the male D. pulex is only greater 

 than that of D. ohtusa in the proportion of about 5 : 4. Thus it is never 

 possible to confuse adult males of these two species. 



The relations of these abdominal processes are shown in Figs. 1 and 2 

 (pp. 6, 7). 



In the female, process G has obviously the function of closing the 

 brood pouch posteriorly and so of preventing the premature falling out of 

 the eggs and embryos. When the moment arrives for the young to be 

 bom, the mother flexes her abdomen ventrally, thus withdrawing the 

 process from the brood pouch, and leaving a way of escape for the young. 

 Processes B and A may possibly help indirectly in closing the brood 

 pouch by strengthening process C. 



As the figures show, the same three processes are present in the 

 male, but only B is well developed. It does not seem possible to ascribe 

 any function to this single well developed process. It is probably an 

 example of the appearance in one sex of an organ which is only functional 

 in the other. Its hypertrophy in D. pulex as compared with D. ohtusa 

 also points to the conclusion that it is a functionless organ. 



Owing to the great influence of environmental differences, especially 

 of food and temperature, on the size of these animals, the absolute 

 lengths of these abdominal processes would be unsatisfactory to use. 

 Consequently a ratio was always employed which eliminated the greater 

 part of this environmental influence. The ratios in question are in the 

 female : (1) the ratio between length of body and length of process B 



ratio J, and (2) the ratio between the lengths of processes B and A 



