The fifth order of the Peracarida, the Amphipoda, is also a very large one. The "Sand-hoppers," which are very common on sandy coasts, belong to this order, as do also a very large number of other forms found in the sea and in fresh water, which have no popular names. A common species is Gammarus pulex, sometimes called the "Fresh-water Shrimp," which is found everywhere in streams and ditches. Several closely allied species, such as G. locusta ([Fig. 21]), are found in the sea. The body is flattened from side to side, and the abdomen is generally bent upon itself. There is no carapace, but, as in the Isopods, the first thoracic somite is fused with the head. The eyes are sessile on the sides of the head. The antennules have a small inner branch, and the antenna have no exopodites. The thoracic limbs, of which the first pair form maxillipeds, have no exopodites, and are partly hidden by a row of shield-like plates along the sides of the thorax. These plates are formed by the enlarged and flattened basal segments of the limbs themselves, and on the inner side they carry a series of oval plates, which are the gills. The abdominal appendages are divided into two sets: the first three pairs have each two slender, many-jointed branches, and are used in swimming; the last three pairs are short, stiff, and directed backwards, and are used in pushing the animal through mud or among water-weeds. In many Amphipods, such as the Sand-hoppers, the last three pairs of abdominal limbs are used in jumping by sudden backward strokes of the abdomen.
Fig. 22—Two Species of Caprellidæ. (From Lankester's "Treatise on Zoology," after Sars.)
A, Phtisica marina, a species which retains the fourth and fifth pairs of thoracic limbs (prp′, prp″); B, Caprella linearis, in which these limbs are represented only by the gills (br). (Enlarged.) a′, Antennule; a″, antenna; abd, vestigial abdomen; gn, gnathopods; m, brood-pouch; IV, V, fourth and fifth thoracic somites
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Fig. 23—Paracyamus boopis, the Whale-louse of the Humpback Whale. (From Lankester's "Treatise on Zoology," after Sars.)
A, Male, dorsal view, enlarged; B, the maxillipeds detached and further enlarged. a′, Antennule; a″, antenna; abd, vestigial abdomen; br, gills; gn, gnathopods; IV, V, fourth and fifth thoracic somites