CRUSTACEANS. 239 



The body is formed of seven ti'ansverse plates, distinctly separate, 

 with curving ends, as sufficiently exposed by inspection from above : 

 besides which are the first segment, with the eyes and the antennae, and 

 the last of diiierent formation from any of the rest, much larger, and 

 bearing the styles. Numerous short bristles invest the margin and many 

 parts of the body. 



The whole aspect of the creature is very peculiar, and very difterent 

 from any other in these volumes, though much correspondence may be 

 traced between the number, proportion, and distribution of the parts 

 among the whole. This animal feeds upon vegetable matter exclusively, 

 and, unless the next, it is the only one of the crustacean race with which 

 I am acquainted that does so. The leaves of the beech in decay are pre- 

 ferred, and in such parts of the pond as these are most abundant there 

 the Asellus is most numerous. The animal does not swim, it runs freely 

 and expeditiously over the decaying leaves, but the smoothness of glass 

 is so adverse to the structure of its limbs that it cannot ascend a cyclinder 

 without the assistance of some other substance. 



The breeding season extends through several months of the year. 

 The ovarium, in common with that of the other subjects of this section, 

 spreads over the lower part of the thorax and abdomen, consisting of 

 about forty ova, contained perhaps in a sac, or confined by a delicate 

 integument. 



That of a specimen, being white, was far advanced on March 20th. 

 When mature it is yellowish. Many of the young, very minute, and 

 almost white, which had been recently hatched, were running about the 

 vessel, over and under the leaves, on the 22d of May. They had sen- 

 sibly grown on July 10th, and were then darker than previously. The 

 young animal is about half a line long, and transparent under the mici'o- 

 scope. On July 15th I endeavoured to secure a specimen ascending the 

 side of the vessel, which bore a quantity of spawn on the breast and ab- 

 domen, it escaped, however, to retreat among the refuse below ; but, by 

 means of a long glass tube, it was caught next morning. 



This animal, being prolific, was well adapted for elucidating some 

 investigation regarding the ova. Two watch glasses, of unequal convex- 



