ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 257 



mandible has an unbranched palp ; the first maxilla forms a simple 

 piercer ; the first foot is reduced in size, but resembles in form that of 

 Idya ; the second foot is two-branched, but with its joints and setge 

 reduced ; the third and fourth feet are absent ; the fifth feet are highly 

 chitinized and ventral in position, connected by a chitinized ventral 

 plate. The abdomen is not chitinized, and has feebly-marked segmenta- 

 tion. There is one egg-sac. The new form bears some resemblance to 

 Balsenophilus, described by Aurivillius from the baleen plates of the blue 

 whale, but in Balsenophilus the adaptation to the peculiar mode of life 

 has not gone so far, and the relationship with Harpacticus is at once 

 apparent. Both are parasites in the making. 



Reproduction in Cladocera.* — W. E. Agar has experimented with 

 Simocephalus vetulus, and gives the following summary of his con- 

 clusions. Certain not yet fully elucidated factors in the environment 

 influence the onset of sexuality. Certain factors likewise bring about 

 " degeneration," or high rate of mortality. Certain factors of the 

 environment may act cumulatively over a number of generations. 



Therefore the increasing sexuality and " degeneration " (or high 

 mortality) observed under certain supposedly constant experimental 

 conditions receive a ready explanation in the supposition that the 

 environment is one favourable to the development of these phenomena. 

 This explanation is made much more probable when we find that under 

 other environmental conditions there is no tendency to increasing 

 sexuality or degeneration. 



Many species exhibit the phenomenon of specially labile periods, 

 when sexuality is easily influenced by certain factors in the environ- 

 ment. This labile condition is usually ascribed to the fact that the line 

 is in about the middle of the reproductive cycle, the diminishing 

 tendency to parthenogenesis being about equally balanced by the 

 increasing tendency to sexual reproduction. Such a balanced condition 

 must, however, be passed through equally, whether the tendency to 

 sexuality is being increased by the progress of the " cycle," or by the 

 cumulative effect of an unfavourable environment. Hence the existence 

 of labile periods is as readily explained on the one hypothesis as on the 

 other. 



There is no justification for retaining the hypothesis of an inherent 

 reproductive cycle— that is to say, the hypothesis that the number of 

 generations or lapse of time since the last fertilized egg influences, as 

 such, the production of sexual or degenerate forms. For the production 

 of these forms is under certain conditions not influenced even by the 

 lapse of an enormous number of parthenogenetic generations, while their 

 production certainly is influenced by environment in other cases. The 

 residuum of cases being equally well explicable on either hypothesis 

 (cycle or environment), it is most reasonable to suppose that the factor 

 that was effective in the one case (environment) was the one that was 

 effective in the other, and conversely, that the ineffective factor of the 

 one case (" reproductive cycle ") was ineffective in the other. 



* Journ. Genetics, iii. (1914) pp. 179-94. 



