This statement has been copied into a good many books, without criticism. At last Mr. David Robertson tried various experiments with a view to discover if these creatures would feed on each other, or, failing other food, put up with cambric or muslin. The specimens upon which he made his observations did neither the one nor the other. Mr. Robertson embodied the results of his experiments in a paper which he read before the Natural History Society of Glasgow[54]. And the story may be read in an abbreviated form in the Rev. T. R. R. Stebbing’s _Naturalist of Cumbrae_, p. 329. In Gammarus we have a standard with which to compare our Sand-hopper. The first thing to notice is the difference in the antennae. Here the superior pair are very short, and carry no secondary appendage, and the lower pair have no denticle or tooth-like process. There is also considerable difference in the gnathopods, or jaw-feet, the sixth joint of which, in the Sand-hopper, does not form a ‘hand.’ The pleopods, or swimming feet, are small, and are used for leaping. We shall also find some difference in the details of the mouth parts, especially in the toothing of the mandibles. We now come naturally to the Isop´oda, which are distinguished by the nearly uniform size of the seven segments which constitute the trunk, and the seven pairs of limbs borne by these segments. The head is distinct, and the breathing apparatus is carried on the under side of the pleon--in these animals not the ‘swimming’ part--five pairs of plates lying one over another, sometimes covered by a larger outer pair. A normal I´sopod may be represented [image:--·······--], where the small dash will stand for the cephalon, or head; the seven dots for the segments of the perei´on, and the long dash for the pleon. The Common Asellus (_Asellus aquaticus_) of ponds and ditches is an excellent subject. It lives well in confinement, and if the conditions are fairly favourable, will increase and multiply. These animals will forage for themselves, and pick up a comfortable living from the vegetable _débris_ that always accumulates at the bottom of an aquarium, and they are not averse from an occasional meal of animal food. [Illustration: FIG. 74. Water Woodlouse.] While our specimens of Asellus are moving about in any convenient vessel, we may verify with the hand lens what has been said about the general form. Then we may notice the antennae, the inner pair being much the smaller. There can be no difficulty in discriminating the head and the eyes; the seven segments of the perei´on, each bearing a pair of limbs; and the pleon with its two terminal appendages. These last consist of a stalk bearing two longer filaments, armed with spines, and ending in a small pencil of hairs. It is easy to see that the segments of the pleon have coalesced, so as to form a continuous plate or shield on the upper surface.