At this point we may conveniently take leave of the Stalk-eyed, and pass on to the Sessile-eyed, Crustacea. Leaving the Cuma´cea out of the question, we have two Sub-orders from which to choose our subjects—the Amphip´oda and the Isop´oda—conveniently Englished, Am´phipods and I´sopods. We learn from Mr. Stebbing[49] that ‘the Amphip´oda, which are common in fresh as well as in salt water, were so named by the French naturalist Latreille, as having feet extending in all directions, their limbs at the same time having much diversity of form, in correspondence with diversity of function. The Isop´oda, or equalfooted animals, besides being found both in fresh and salt water, have more decidedly than the Amphip´oda extended their range to the dry land. The name was invented by Latreille in ignorance of the great number of species, since investigated, in which the feet are strikingly unlike and unequal. Nevertheless, the name may stand, just as a rose remains a rose even when it is not rose-coloured.’ These last two sentences must be borne in mind, for they throw great light on a subject that may give us some trouble—the question of priority in nomenclature.
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
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.
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.
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: