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Two families of the Amphipoda differ so much in general appearance from the others that they deserve mention. The Caprellidæ ([Fig. 22]) have the body drawn out to a thread-like slenderness, and the abdomen reduced to a mere vestige. The fourth and fifth pairs of thoracic limbs are generally absent, though the corresponding gills remain. The animals live in the sea, clambering among sea-weeds or zoophytes in a fashion which recalls the movements of "looper" caterpillars. The Cyamidæ, or "Whale-lice" ([Fig. 23]), are, as the name implies, parasites on the skin of whales, and are closely related to the Caprellidæ. They have, however, a broad, flattened body, more like that of an Isopod than an ordinary Amphipod, and their legs have strong curved claws with which they cling to the skin of their host.

The third division of the Malacostraca, the Eucarida, consists of two orders of very unequal interest and importance. The first of these, the order Euphausiacea ([Fig. 24]), comprises only a single family of small, shrimp-like Crustacea found swimming freely at the surface or in the depths of the sea. In these the carapace fuses with all the thoracic somites, the eyes are stalked, the antennules have two flagella, and the antennæ have a broad scale. None of the thoracic limbs are modified into maxillipeds, and all carry swimming exopodites. The uropods and telson form a tail-fan. A single series of feathery gills are attached to the bases of the thoracic limbs. Nearly all the Euphausiacea possess the power of emitting light, and are furnished for the purpose with a number of organs which were formerly supposed to be "accessory eyes."

Fig. 24—Meganyctiphanes norvegica, One of the Euphausiacea. Twice Natural Size. (From Lankester's "Treatise on Zoology.")

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The second order of the Eucarida, the Decapoda, is by far the largest of the orders of Crustacea, and it includes all the larger and more familiar members of the class. It is necessary, therefore, to give a considerably fuller account of its subdivisions than has been given in the case of the other orders. The typical characters of the Decapoda are well illustrated by the Lobster, which has been already described. As in the Euphausiacea, the eyes are stalked, and the carapace fuses with all the thoracic somites. From the Euphausiacea the Decapoda differ in the fact that three pairs of the thoracic limbs are modified as maxillipeds, the remaining five pairs forming the "ten legs" to which the name of the order alludes. Further, the gills are arranged in more than one series, not all attached to the bases of the legs, as in the Euphausiacea, and covered over by the side-flaps of the carapace instead of being freely exposed. While agreeing in these essential characters, however, the members of the order Decapoda differ very widely among themselves in structure and in general form, and they are classified (in the arrangement adopted here) in two suborders, which are again subdivided into sections and tribes.

Order DECAPODA.
Subclass Natantia- { Tribe Penæidea.
{ " Stenopidea.
{ " Caridea.
" Reptantia.
Section Palinura- { " Scyllaridea.
{ " Eryonidea.
Section Astacura- " Nephropsidea.
" Anomura- { " Galatheidea.
{ " Thalassinidea.
{ " Paguridea.
{ " Hippidea.
" Brachyura- { " Dromiacea.
{ " Oxystomata.
{ " Brachygnatha.
Subtribe Brachyrhyncha.
"Oxyrhyncha.

The suborder Natantia includes the numerous species of what are commonly known as Prawns and Shrimps. These are characteristically powerful swimmers, with lightly armoured bodies, more or less flattened from side to side, with a thin, saw-edged rostrum, and with large swimmerets which are the chief organs of swimming; in addition, some of the more primitive Natantia have swimming branches, or exopodites, like those of the Euphausiacea, on the thoracic legs. This suborder is divided into three tribes. The Penæidea include the large Prawns of tropical seas (Penæus—[Plate IV].), which have the first three pairs of legs provided with chelæ, and not differing greatly in size. The Stenopidea are a small group of forms resembling the Penæidea in having chelæ on the first three pairs of legs, but the third pair are much larger than the others. The Caridea comprise our common Prawns (Leander, Pandalus) and Shrimps (Crangon), besides a host of less generally known forms; in these the third legs are never chelate, although the first and second often are.