Fam. 2. Lophogastridae.—The members of this family (Lophogaster, Gnathophausia) agree with the Eucopiidae in the possession of branched gills on some of the thoracic limbs, in the absence of auditory sacs on the sixth pair of pleopods, in the presence of normally developed pleopods in both the male and female, and in the brood-lamellae being developed on all seven of the thoracic limbs. The endopodites of the posterior thoracic limbs are, however, of a normal size.
Fig. [80].—Dorsal view of male Diastylis stygia, × 12. A, 2nd antenna; Ab.6, 6th abdominal appendage. (After Sars.)
Fam. 3. Mysidae.—These differ from both the foregoing families in the absence of gills, in the presence of an auditory sac on the sixth pleopods, in the reduction of the other pleopods in the female, and in the brood-lamellae being developed only on the more posterior pairs of thoracic limbs. A number of closely related genera compose this family, of which Mysis, Boreomysis, and Siriella may be mentioned. Mysis oculata, var. relicta, is a fresh-water form from the lakes of northern and central Europe.
Order II. Cumacea.[[96]]
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.