368 M. F. Plateau on the Freshwater 



The head has undergone a remarkable curvature, which 

 separates certain parts and brings others nearer together. The 

 cephalic ring presents the median region (stomachal region of 

 Desmarest), covering the anterior part of the digestive tube ; 

 and we may recognize the existence of lateral regions. The 

 facial regions are : — the frontal region, in the middle, much 

 reduced in size and covering the organ of vision (it is deve- 

 lojsed into a rostrum only in D. mucronata) ; and the orbital 

 regions on each side of this. 



We may comit as cephalic somites : — the first, characterized 

 by the presence of the eyes ; the second, by the antennules 

 (ra?m); the third, by the antenna (smaller antennae of 

 Strauss), and its posterior margin bears the labrum ; the 

 fourth is marked by the protognaths (mandibles), and bears 

 the labium, Avhilst its hypertrophied epimera constitute the 

 valves, as MM. Milne-Edwards and G. O. Sars have abeady 

 shown. 



The thorax, which, like a great part of the abdomen, is 

 enclosed between the valves, includes six somites : there is a 

 fifth somite bearing the deutognaths (maxilla?.), and a sixth 

 bearing the tritognaths (first pair of feet, 'of authors) and 

 terminating the anterior pereion. The posterior pereion is 

 formed by four somites, each bearing a pair of pereiopods. 



The abdomen consists of six somites, namely : — the eleventh, 

 twelfth, and thirteenth ; the fourteenth, provided with mamillse 

 which close the incubatory cavity ; the fifteenth, bearing the 

 caudal setje ; and the sixteenth, or last, which is a true 

 telson. 



Hitherto we have had scarcely any exact data as to the moult- 

 ing of the Cladocera. I have been able to observe this pheno- 

 menon in the female of D. mucronata. A long transverse 

 fissure is formed along the branchio-cardiac furrow which se- 

 parates the valves from the head ; and the scapular buckler 

 splits along the median line or dorsal crest of the valves. 

 The head bends down in front, and a new cephalic extremity 

 makes its appearance towards the back through the transverse 

 fissm'e. The Daj?hnia shakes itself rapidly; the antennules 

 escape from the old ones as if these were actual sheaths ; then 

 the animal, by a few last efforts, finally escapes from its old 

 skin through the longitudinal opening of the crest of the 

 valves. The phenomenon takes place with extreme rapidity, 

 the whole change only lasting two seconds. 



The circulator)^ apparatus presents some curious peculiari- 

 ties. Thus the venous sinus which surrounds the heart is by 

 no means always circular, as has been supposed. In D. pulex, 

 when seen from the dorsal surface, it is polygonal, Avith seven 



