1885.] 



CLADOCERA. 



25 



extent of the aquary, changing only their course when striking 

 against the walls of the aquary. Both the frontal prolation and 

 the spine of the carapace, as also, perhaps, the peculiarly exserted 

 fornix, serve no doubt as a balancing and steering appara- 

 tus, and thus the present species is, it would seem, admirably 

 adapted for living in lakes of considerable extent. However, in 

 individuals of the later generations, this apparatus becomes, 

 as we have shown, very much reduced, and such reduction also 

 occasions a corresponding and very conspicuous alteration in 

 the movements of the animal, these being now rather slow and 

 abrupt, or jumping, as in our well-known species, D. pulex (de 

 Geer). Moreover, the animal is found to keep its body, when 

 moving, in a more vertical position, precisely as with the fore- 

 mentioned Northern species. It is obvious, that the movements 

 of the animal at such time merely serve to keep the body 

 suspended in the water, whereas any true locomotion does not, 

 as a rule, occur. A great number of specimens I saw from 

 day to day crowded in the same corner of the aquary, form- 

 ing there a dense assemblage, that retained its place well-nigh 

 unaltered. The development of the ephippium would seem to 

 restrict still further the agility of the animal, the weight of 

 its body thereby being considerably increased; and such indi- 

 viduals I found too, as a rule, congregated near the bottom of 

 the aquary. — The movements of the males are a triffe more 

 rapid, though made in a similar jerking or jumping manner, and 

 they often keep their bodies obliquely horizontal when in pursuit 

 of the females. 



This Daphnia was the Cladoceran first hatched; and in all 

 my aquaries a few individuals at first appeared; but in one only 

 they continued multiplying sufficiently to attain sexual differ- 

 entiation. In all the other aquaries, after some time the indi- 

 viduals disappeared. This fact I found to be in all probability 

 caused by the presence of another Australian Cladoceran, Moina 

 propinqua (to be described further on) this form being much 

 more tenacious of life and its power of reproduction truly mar- 

 vellous, so as to occupy after some time most of the space in the 



