The Cumacea are a group of small marine animals rarely attaining an inch in length, which agree with the Mysidacea in the characters noted above as diagnostic of the Division Peracarida; they possess, however, in addition a number of peculiar properties, and Sars believes them to be of a primitive nature showing relationship to _Nebalia_, and possibly to an ancestral Zoaea-like form. They follow a habit similar to that of the Mysidacea, being caught either in the surface-plankton or in great depths, many of the deep-sea forms being blind. They are, however, not true plankton forms, and they appear to attain a greater development both in point of variety and size in the seas of the northern hemisphere. The thoracic limbs may be biramous, but there is a tendency among many of the genera to lose the exopodites of some of the thoracic legs, an exopodite never being present on the last few thoracic limbs of the female and on the last in the male. In the Cumidae the four posterior pairs in both sexes have no exopodites. The first three thoracic appendages following the maxillae are distinguished as maxillipedes; they are uniramous, and the first pair carries an epipodite and a large gill upon the basal joints. Pleopods are only developed in the male sex. The flagellum of the second antennae in the male may be enormously elongated, as in the Atlantic deep-sea species shown in Fig. 80, so as to exceed in length the rest of the body. =Fam. 1. Cumidae.=—No sharp demarcation between thorax and abdomen. Four posterior pairs of legs in both sexes without exopodites. Male with five well-developed pleopods in addition to the uropods. Telson wanting. _Cuma_, _Cyclaspis_, etc. =Fam. 2. Lampropidae.=—Body-form resembles that of Cumidae. All the thoracic limbs except the last have exopodites. The male has three pairs of pleopods. Telson present. _Lamprops_, _Platyaspis_, etc. =Fam. 3. Leuconidae.=—Body-form similar to above. Male has only two pairs of pleopods. Mouth-parts peculiar, much less setose than in other families. Telson absent. _Leucon_, _Eudorella_. =Fam. 4. Diastylidae.=—Anterior part of thorax sharply marked off from posterior part. Male has two pairs of pleopods. Telson present. _Diastylis_ (Fig. 80). _D. goodsiri_ from the Arctic ocean measures over an inch in length. =Fam. 5. Pseudocumidae.=—Rather similar to Diastylidae, but differ in reduced size of telson and presence of exopodites on third and fourth thoracic legs of female. This family is represented by three very similar marine forms of the genus _Pseudocuma_; but, as Sars has shown,[97] the Caspian Sea contains thirteen peculiar species, only one of which can be referred to the genus _Pseudocuma_, while the rest may be partitioned among four genera, _Pterocuma_, _Stenocuma_, _Caspiocuma_, _Schizorhynchus_. =Order III. Isopoda.=