22 



Gr. 0. SAES. 



[No. 8. 



No essential difference in the general habitus could in any case 

 be detected on comparing the specimens of these genera tions. On 

 returning from my excursion at the end of September, I found 

 however, that a very conspicuous alteration had tåken place 

 with the individuals swarming in one of my aquaries, apparently 

 arising from a less complete nutrition along with an incipient 

 formation of ephippia. The size of these individuals was, as a 

 rule, somewhat inferior to that of the summer-generations, and 

 their reproduction by no means so great, more than two eggs, 

 or embryos, being rarely found in the matrix. Meanwhile, a 

 very striking deviation in the form of the head was observed, 

 giving to the individuals (see Pl. 4, fig. 2) a physiognomy very 

 different from that in the earlier generations. Thus, the enor- 

 mously developed frontal prolation, apparently so characteristic of 

 the present species, had become to that extent reduced in size 

 as to be well-nigh obsolete, the head having assumed a form 

 agreeing more closely with that, as a rule, found to occur 

 in the typical species of the genus, though still exhibiting in 

 front a slight angle. Moreover, the spine of the carapace had 

 comparatively become both much shorter and feebler than in 

 the earlier examined individuals. In some of the specimens, 

 too, as mentioned above, a most conspicuous formation of ephip- 

 pia had supervened, the dorsal part of the carapace having been 

 so modified as to receive the two winter-eggs. As will appear 

 on comparing fig. 2, Pl. 4 with figs 1, 2, 3, Pl. 1, the recently 

 developed ephippium, which still forms part of the carapace in 

 the specimen figured here, agrees in every respect with the 

 ephippia found in the dried mud, and described more in detail 

 in the first part of this Memoir. Now, it may be easily seen, 

 that the valves of the ephippium merely represent peculiarly 

 modified segments of the dorsal part of the carapace limiting 

 the so-called matrix, and that the hinge is formed by the 

 inspissated dorsal edge, which still retains its armature of denticles. 



The presence of individuals provided with ephippia naturally 

 induced me to seek for male specimens, knowing, as I did, that 

 the development of male Cladocera is mostly simultaneous with 



