ASELLUS AQUATICUS, 345 
on its underside, followed by a pair of small crustaceous 
sac-like appendages, constricted (ab. fig. 67 p. and De 
Geer, vii. pl. 31, fig. 12 e, e), which partially conceal a bi- 
partite, complex, horny apparatus, each portion consisting 
of a large basal joint (partly visible in the upper part of 
our figure, P), which supports two small knob-like ap- 
pendages, the innermost of which is armed with a small 
horny spur, directed backwards (De Geer, fig. 13 x, and 
Treviranus, fig. 67). ‘This apparatus was first very carefully 
described and figured in detail by De Geer, and sub- 
sequently more satisfactorily by Treviranus. In the 
female, this apparatus is replaced by two small semi-oval 
plates, strongly ciliated along the terminal outer edge, 
which rest upon the base of the outer pair of branchial 
platés, the latter being divided into two nearly equal 
portions, as in the male. 
The following excellent account of the habits of this 
animal is given by Sir J. Dalyell, in his fine work on the 
* Powers of the Creator” :— 
“The species abounds in remarkable profusion in a 
pond at Red Braes, in the immediate vicinity of Kdin- 
burgh. None exceed six lines in length, and little more 
than half a line in breadth. It feeds upon vegetable 
matter exclusively. The leaves of the beech in decay 
are preferred, and in such parts of the pond as these are 
most abundant, there the Asel/lus is most numerous. 
The animal does not generally swim, it runs freely and 
expeditiously over the decaying leaves. 
‘‘ The breeding season extends through several months 
of the year. The ovarian pouch occupies the under 
surface of the thorax and abdomen, and contains about 
forty ova. Many of the young, very minute and almost 
white, recently hatched, were running about over and 
under the leaves on the 22nd May. The young animal 
