ASELLUS AQUATICUS. 345 



on its underside, followed by a pair of small crustaceous 

 sac-like appendages, constricted (ib. fig. 67 p. and De 

 Geer, vii. pi. 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 

 plates, 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 Edin- 

 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 Asellus 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 



