Mysidacea
General morphology and main diagnostic features

The Mysidacea are characterized morphologically as follows (typic. mysid, lateral v.,female, typic. mysid, lateral view,male, typical mysid, dorsal view and detail typi. mysid, dorsal view). Almost all of the cephalon and the thoracic somites are covered by the carapace. The carapace is shield-like, fused dorsally with the head region and not more than four anterior thoracic somites. The eyes are usually stalked and movable, the cornea generally developed with visual elements (detail typi. mysid, dorsal view). The cephalon bears five pairs of appendages; antennule, antenna (antenna), mandible (mandible with 3-segm. palp), maxillule (maxillule) and maxilla (maxilla).
The thorax has 8 pairs of pereopods, all of which are divided into two branches, endopod and exopod (first pereopod). Endopods of the first and second pereopods are usually transformed into gnathopods, which differ considerably from the remaining limbs. All exopods are natatory. Females bear a marsupium composed of less than seven pairs of lamellae, in which the embryos grow. The abdomen consists of six somites, generally of the same form, although the last somite is longer than the others (cross section of abdomen). Each abdominal somite, except for the last, is provided with a pair of pleopods. Pleopods have a flagelliform exopod and endopod, frequently reduced in the female (typical female pereopod) and sometimes in the male, often sexually modified in the male (typical male pereopod). The posterior extremity of the body is a tail fan formed by the telson (telson) and a pair of uropods (uropods). A statocyst is usually present in the endopod of the uropods (uropods).

—Main differences between mysids and euphausiids
Mysids are similar in their general shrimp-like appearance to the euphausiids. Actually, these two animal groups were combined in the order "Schizopoda" for some time in the past. Differences between their adults are shown in Table 1: table 1.

The key to mysid genera starts at Page 1603: Mysidacea. Features of importance for identification of the animals include:

—Integument
External skeleton covering the body. It is usually thin and usually poorly calcified, but calcification is conspicuous in species of the families Lophogastridae and Gnathophausiidae. Usually the surface of the integument is smooth, but in some species the body is partly or totally covered with minute spinules, bristles or scales. In rare cases the carapace and the abdominal somites can be armed with long spines (Echinomysis chuni, Arachnomysis leuckarti), or a row of short spines on the posterior somites of the abdomen.

—Carapace (typic. mysid, lateral v.,female and typical mysid, dorsal view)
The anterior margin is produced frontally in various degrees to form a rostrum. The posterior margin is usually emarginate and sometimes furnished with spines (Gastrosaccus namibensis) or lobes (Bowmaniella recifensis). In a few cases humps or protuberances are present along the mid-dorsal line (Mysidopsis camelina).

—Eyes (detail typi. mysid, dorsal view)
The eye is usually composed of two portions, the visual part (cornea), and the stalk. The stalk is frequently provided with a digitate to papilliform process on the dorsal surface. The cornea is developed to varying degrees, tending to be reduced with increasing habitat depth. In deep-sea mysids the eyes are frequently modified into plate-like structures without a definite stalk and visual elements (Pseudomma magellanensis). Some species have two corneas in a single eye (Euchaetomera zurstrasseni 1).

—Antennal scale (antennal scale, antenna)
The antennal scale consists of a pair of thin lobes, which are modifications of the exopod of the antenna. The shape varies variously according to species, such as oval, lanceolate, styliform, triangular, etc., with rounded or pointed apex. The inner margin is usually setose. The outer margin varies specifically: naked, setose, dentate, or spinose. Sometimes this scale is divided distally into two segments by a more or less distinct suture.

—Endopods of pereopods (first pereopod, 8th male pereopod)
Those of the first and second, and sometimes third, pereopods are modified into feeding appendages, called gnathopods or maxillipeds. In the third to eighth pereopods, the carpus and propodus are usually fused with each other to form a carpopropodus. The carpopropodus is secondarily divided into subsegments. Sometimes the number of subsegments is specifically diagnostic. In some species the second or third endopod has a subchelate termination (Boreomysis bispinosa 2).

—Uropods (uropods)
These are composed of two branches, endopod and exopod. The endopod is usually shorter than the exopod, setose all around, and furnished with a few or many spines on part of, or throughout, the inner margin; it may also lack spines. The outer margin of the exopod is armed with spines, or setae, or, in rare cases, with both.

—Telson (telson)
The telson is the most important diagnostic character. Its shape is triangular or trapezoidal, with broad or narrow apical margin, or linguiform with broadly or narrowly rounded apical margin. Its lateral margins are naked, or entirely or partly armed with spines which may be dense or sparse. The distal margin is armed with one or several pairs of spines and sometimes a pair of apical plumose setae. A deep or shallow distal cleft is frequently present. The cleft is unarmed or armed with spines on the margin, and sometimes bears a pair of plumose setae at the anterior end of the cleft.