, 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.
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.
If we now take our dissecting microscope and place an Asellus in some water in an excavated 3 in. by 1 in. slip on the stage, examination with an inch lens will show us a considerable amount of detail. With the half-inch Leitz lens (see p. 18) one may see quite clearly the beautifully annulated form of the flagella of the antennae, the sensory hairs with which these organs are set, and the circulation of the blood in the limbs and the antennae—the corpuscles moving in a continuous stream. More than this: we shall be able with the same power to distinguish tufts of Vorticellids that settle on the Asellus, and the commensal rotifers that roam about on the body of their host, generally on the limbs and under surface.
Now we may turn the Asellus on its back, to examine the breathing apparatus more closely than we were able to do when the creature was moving about in the bottle. It will be easy to make out the opercular plates—modified tail appendages—that open and shut to admit water to, or allow it to flow out from, the true breathing-plates which function as gills, and correspond to the swimming feet of the Amphipods.
In the female there is a pouch beneath the perei´on, in which the eggs are carried till they are hatched, and which serves as a nursery and refuge for the young.