The first order of the Peracarida, the Mysidacea, consists of small, free-swimming, shrimp-like animals ([Fig. 16]). Many species are common in the sea round the British coasts, and from their possession of a brood-pouch, in which the young are carried, they are sometimes known as "Opossum Shrimps." The eyes are stalked, and the carapace is well developed, although it does not unite with all the thoracic somites. The antennæ have a flattened, scale-like exopodite, probably of use for keeping the animal balanced in swimming. Only one pair of the thoracic limbs are modified to form maxillipeds, and all the legs (as in the larval Lobster) have exopodites which form the chief swimming organs. The uropods and telson form a "tail-fan." One of the most curious points in the organization of some Mysidacea is the possession of a pair of statocysts in the endopodites of the uropods. Each statocyst consists of a small cavity containing a cake-shaped concretion known as a "statolith," resting on a group of sensory hairs. There is reason to believe that these organs have the same function as the statocysts of the Lobster, although they are placed at the other end of the body. The statolith serves the same purpose as the sand-grains found in the Lobster's statocyst, although, unlike these, it is not introduced from outside, but is formed in position by secretion from the walls of the sac.
Fig. 17—Gnathophausia willemoesii, One of the Deep-sea Mysidacea. Half Natural Size. (From Lankester's "Treatise on Zoology," after Sars.)
gr, A groove dividing the last abdominal somite
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Most of the Mysidacea have no special organs of respiration, that function being discharged (as in many of the smaller Crustacea) by the general surface of the body, and especially by the thin carapace; but certain deep-sea Mysidacea ([Fig. 17]) have tufted gills attached at the base of the thoracic legs. In all cases the maxilliped has a plate-like epipodite, which lies under the side-fold of the carapace, and no doubt assists respiration, causing by its movements a current of water to flow under the carapace.
Fig. 18—Diastylis goodsiri, One of the Cumacea. Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)