[Illustration: 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] 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. [Illustration: FIG. 18--_Diastylis goodsiri_, ONE OF THE CUMACEA. ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.) _a´_, Antennule; _l_1-_l_5, the five pairs of walking legs; _m_, brood-pouch; _ps_, "pseudo-rostrum" formed by lateral plates of the carapace; _t_, telson; _ur_, uropods] The members of the second order of the Peracarida, the CUMACEA (Fig. 18), are small marine Crustacea in which the anterior part of the body is generally stout, while the abdomen is slender and very mobile. The short carapace does not cover more than the first three or four of the thoracic somites. The eyes are not stalked, and are usually fused together to form a single organ on the front part of the head. Swimming branches (exopodites) are usually present on some of the thoracic legs, at least in the males, which are more active swimmers than the females. In the males, also, the swimmerets of the abdomen are often more or less developed, but they are always absent in the females. The uropods do not form a tail-fan, but are slender forked rods carrying comb-like rows of spines, said to be used in cleaning the anterior appendages from the mud among which these animals generally live. The telson is often absent, or, rather, it is coalesced with the last somite of the abdomen. Under the side-fold of the carapace on each side lies, as in the Mysidacea, the epipodite of the maxilliped; but in this order it forms a gill, and usually carries a row of flattened gill lobes. [Illustration: FIG. 19--_Apseudes spinosus_, ONE OF THE TANAIDACEA. ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.) _ex_, Vestiges of exopodites on second and third thoracic limbs; _oc_, the small and immovable eye-stalks; _sc_, scale or exopodite of antenna; _ur_, uropod]