IN DAPSmA, AND OF THE STETJCTIJEE OF THE EPHIPPIUM. 
87 
carapace and the back, the animal leaves off SAvimming, sinks slowly to the bottom, rests 
there for a few seconds, and then giving a sudden spring forwards, leaves behind it the 
inner case, with the rest of the moulted skin. 
This experiment renders evident, I think, the true nature of the inner case of the 
ephippium, which is in fact the inner layer of the carapace, modified in the same manner 
as the outer. If a thin section of the carapace be made, it Avill be found to consist of 
two folds, containing probably between them another, most likely double membrane, the 
corium. 
According to analogy, we may safely conclude that the corium forms round itself the 
new carapace, which therefore arises between the two layers of the old carapace. More- 
over, it is only when fully developed that the lower and side margins of the “ inner 
case ” are free ; before maturity they may easily be found to be continuous with a very 
dehcate membrane, the inner layer of the carapace. 
The diagram (Plate VII. fig. 4) will serve to make this explanation clearer. The lines 
marked w represent the outer and inner layer of the old carapace, in the thickness of 
which the two layers of the new carapace x are formed. The thickness of the carapace 
is much exaggerated in this diagram, as well as in Plate VII. fig. 6, in order to make the 
different layers more distinct. 
The manner in which the “ inner case ” with the two eggs is deposited inside the outer 
one does not need much explanation : it will be seen by the above-mentioned diagram 
that the lower margins of the carapace are pulled in towards one another, so that the 
section of each Aulve, instead of presenting, as at first, a segment of a circle, has a bow 
shape, the upper or ephippial half being smaller than the other. Plate VII. fig. 4 is a 
diagram of the section of the carapace along the line Plate VII. fig. I m, the body of the 
animal being omitted; and it is therefore evident that the old skin being slipped off, the 
inner case would naturally come out under and remain enclosed in the outer one. 
The receptacle of a specimen which has at the last moult cast an ephippium, will 
generally he found to be traversed by certain folds : these are caused by the fiattening of 
that part of the carapace which corresponded to the projection in which the eggs were 
situated. 
Plate VII. fig. 3 represents one half of the outer ephippium magnified 250 times, and 
only partly finished. It consists of small angular cells, about -000125 in diameter, gra- 
dually passing, near the centre of the holloAV which is to contain the egg, into small round 
pit-hke cells. 
The inner ephippium (Plate VII. fig. 2) is very similar. The lower side margins consist 
of a plain membrane, in which angular cells gradually become visible : round the central 
protuberances these cells become elongated, but on the surface of the protuberance 
regain their first shape. A dotted membrane is also evident here. 
I have not succeeded in either obtaining ephippial eggs from isolated specimens, or in 
clearly proving that these eggs require impregnation, and the facts bearing on this sub- 
ject may be summed up as follows. In the first place, the common eggs of D. Schwfferi, 
N 2 
