The Mysidacea, although pelagic, are not very often met with in the true plankton on the surface; they generally swim some way below the surface, going down in many cases into the abysses. For this reason they thrive excellently in aquaria, and the common _Mysis vulgaris_ is often present in such numbers in the tanks at the Zoological station at Naples as to damage the other inmates by the mere press of numbers. The Mysidacea, like the majority of the Peracarida, undergo a direct development, and hatch out with the structure of the adult fully formed. Many of the Mysidacea bear auditory sacs upon the sixth pair of pleopods, a characteristic not found in the Euphausiacea. =Fam. 1. Eucopiidae.=—The curious form _Eucopia australis_ (Fig. 79) described by Sars,[95] may be chosen as an example of the Mysidacea. The peculiarity of this form consists chiefly in the immense elongation of the endopodites of the fifth, sixth, and seventh thoracic appendages. Characteristic of the Mysidacea is the freedom of the hinder thoracic segments from fusion with the carapace, otherwise this animal is seen closely to resemble the _Euphausia_ figured (Fig. 102). _Eucopia australis_, like so many of the Mysidacea, is a deep-sea animal, being brought up with the dredge from over 1000 fathoms; it is very widely distributed over the Atlantic Ocean. [Illustration: FIG. 79.—_Eucopia australis_, young female, × 3. _A_, 1st antenna; _Ab.1_, 1st abdominal segment; _Ab.6_, 6th abdominal appendage; _E_, eye; _T_, telson; _Th_, 5th thoracic appendage. (After Sars.) ] =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. [Illustration: 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]