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. [Illustration: FIG. 24--_Meganyctiphanes norvegica_, ONE OF THE EUPHAUSIACEA. TWICE NATURAL SIZE. (From Lankester's "Treatise on Zoology.")] 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."