The majority of the Amphipods live in salt water, but a few are found in ponds and streams, and some dwell on the shore, near, but not in, the sea. The animals of this Sub-order are distinctly segmented, and three regions may be distinguished thus [image], where C stands for the Cephalon, or head, Per. for the Perei´on, or body, and Pl. for the Ple´on (literally, the swimming part), or tail. On the head we shall find two pairs of antennae, the eyes, and the mouth appendages. Each segment of the perei´on bears a pair of appendages; the first two pairs are called respectively the first and second gnath´opods (or jaw-feet), and the other five pairs perei´opods, or walking feet. The pleon carries three pairs of ple´opods, or swimming feet, on the first three segments, and each of the following three has a pair of uropods or tail appendages. It is well to make out these parts in every specimen that comes in our way. More is learnt by breaking up one specimen than by reading the clearest description so often that one knows it by heart. [Illustration: FIG. 71.--Gammarus. (After Sars.)] We may begin with the Fresh-water Shrimp (_Gam´marus pulex_), which may be taken abundantly in running water where there is plenty of vegetation. Willow-moss affords these Crustaceans a favourite hiding-place. It is an excellent plan to gather a quantity of weed and shake it over a newspaper or a piece of mackintosh. The ‘Shrimps’--which, by the way, are not really Shrimps--will be dislodged from the weed, and we shall see them wriggling along on their sides, from which habit they and their near relatives are often called ‘Scuds,’ and ‘Screws.’ They are useful inmates of an aquarium, because they feed on decaying animal matter, and so keep the water pure and sweet. Opinions are divided as to whether these animals feed on water-plants; it is probable that when their natural food fails them, they take what comes in their way. I have kept marine and fresh-water species of Gammarus (the genus to which the Fresh-water Shrimp belongs) in tanks which contained no other animals, but plenty of vegetation, and both lived and did well for a considerable time. They are by no means unwilling to make a meal off the dead body of one of their own species; but it is exceedingly doubtful if they prey on each other, as some old writers have asserted. This notion probably arose from the fact that the male carries the female, which is much smaller, about with him, during the period of courtship, holding her tightly beneath his body by means of the fingers of its first two pairs of hands. The habit is not confined to this genus, nor even to this Sub-order; for some, if not all, the species of Idotea carry on their courtship in similar fashion, as does also the Water Woodlouse. For the verification of statements such as these, a small aquarium is necessary, but the animals will not be under observation long before the observer will be convinced of their truth.