10 



G. O. SARS. 



[No. 8. 



highly chitinized corneous rod, constituting the hinge of the 

 ephippium, and posteriorly projecting as a short mucroniform 

 spine, anteriorly as two rather narrow and slightly curved riband- 

 shaped processes, diverging a little at their extremities. This 

 rod, moreover, is armed, throughout its entire length, with a 

 double row of small, posteriorly directed denticles, those at the end 

 of the anterior processes assuming the shape of recurved hooks, 

 by means of which the ephippium readily affixes itself to extrane- 

 ous objects. Viewed from above (fig. 2), the ephippium is found to 

 be strongly compressed and, as it were, constricted in the middle. 

 The two valves of the ephippium fit close together round the 

 edges, and exhibit, each of them, two succeeding umboniform promi- 

 nences, placed obliquely to the longitudinal axis of the ephip- 

 pium, so as to be nearly parallel to the posterior margin. These 

 prominences indicate the chambers, in which the ova are lodged. 

 The colour of the ephippia, except the corneous hinge, was a 

 uniform dirty grey, much the same as that of the mud in which 

 they lay imbedded. 



With two fine needles it is not very difficult to open the 

 valves from the ventral side, especially when the ephippium 

 has been for some time immersed in water. This effected, 

 another capsule (fig. 4) is found to lie within the ephippium, 

 connected but loosely therewith and exhibiting a somewhat 

 elliptic form. The structure of this internal capsule is very 

 delicate, forming, as it does, a soft and flexible envelope for 

 the ova, and, moreover, projecting at its periphery to a thin 

 rim, developed most along the ventral face. Also this inner cap- 

 sule is composed of two symmetrical valves fitting close together 

 round the periphery, and in the centre lie imbedded the two 

 ova, side by side, in two corresponding chambers (see fig. 3), 

 much as seed in the pod. 



As regards the microscopical structure of the ephippium, it 

 exhibits everywhere, saving the dorsal rod, or hinge, an exceed- 

 ingly dense net-work of minute polygonal cells, that easily be- 

 come filled with air and thus contribute in buoying up the ephip- 

 pium on the surface of the water. These cell-like cavities are 



