All species of Gammarus, whether living in the sea or fresh water, may be readily distinguished by the rows of small spines on the three hinder segments of the pleon, for this is one of the characteristic marks of the genus. After we have kept specimens in the aquarium for a time, so as to become familiar with their general appearance and habits, we will put them to practical use by breaking them up. Our first task is to work over the animal as a whole, and to make out the three regions--ceph´alon, or head; pereī´on, or body; and plē´on, or swimming part, or tail--into which it is divisible. It will not be sufficient to do this once, and then to imagine we have the whole matter fixed in our memory. It should be repeated over and over again, with every specimen that comes into our hands, till we know these regions practically, and the number and kind of appendages they carry. And then the three rows of spines are to be looked for. For all this the inch lens will be quite sufficient. Now let us separate the head. When this is done, and if we use the lens, we shall at once be able to account for the name ‘Sessile-eyed Crustacea,’ for the difference between the eyes of our specimen and those of a shrimp or a crab will be evident. Nor can there be any doubt that they are compound eyes, though the outer integument is not divided into facets. The antennae are next to be considered. Of these there are two pairs, the superior, or inner, pair being the longer. These antennae consist of three basal joints and a many-jointed flagellum, or lash, and on each of the inner pair is a secondary appendage, arising from the distal (or outer) end of the third basal joint. We may represent one of the superior antennae thus: [image]. The dashes represent the three basal joints, the long row of dots the many-jointed flagellum, and the slanting row of dots stands for the secondary appendage. The sensory-hairs on the joints of the flagellum should be looked for, and may be made out with the inch lens. The same power will show the denticle, or tooth-like projection at the base of the lower antennae. Next come the mouth parts. As compared with Crabs, Amphipods seem badly off in this respect; for the second and third maxillipedes of the former become the first and second gnathopods of the latter, so that instead of six pairs of mouth organs the Amphipods have only four. It is not an easy matter for a beginner to separate the mouth parts of an Amphipod, but the difficulty is not insuperable, and will grow ‘small by degrees and beautifully less’ with practice. We have to make out four pairs of organs arranged in the order given at the side of the page, the mandibles being the innermost. MANDIBLES. FIRST MAXILLAE. SECOND MAXILLAE. MAXILLIPEDES.