DIVISION 2. PERACARIDA.
The carapace, when present, leaves at least four of the thoracic somites distinct; the first thoracic segment is always fused with the head. The eyes are pedunculate or sessile.
The mandible possesses a lacinia mobilis. A brood-pouch is formed in the female from oostegites attached to the thoracic limbs. The hepatic caeca are few and simple; the heart is elongated and tubular; the spermatozoa are filiform, and development takes place without a complicated metamorphosis.
Order I. Mysidacea.
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.
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.)