a′, Antennule; a″, antenna; ab1-ab6, the abdominal limbs; ad, muscle joining the two valves of the shell; f, tail-fork; p, palp of maxillula; r, rostral plate; t, telson; 1-7, the seven somites of the abdomen

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The first order of the Malacostraca, the Nebaliacea, comprises a few Crustacea of small size, which differ in some very important characters from all the other orders. Nebalia bipes ([Fig. 15]), which occurs on the southern coasts of the British Isles, has a large bivalved carapace enclosing most of the limbs. In front, a small "rostral plate" is joined to the carapace by a movable hinge, and partly covers the stalked eyes. The eight pairs of thoracic feet are all alike, and are flattened and leaf-like in form, resembling those of the Branchiopoda. The first four pairs of abdominal limbs are large two-branched swimming feet, but the last two pairs are reduced to small vestiges. Two of the most important points in which the Nebaliacea differ from all the other Malacostraca are that there are seven instead of six somites in the abdomen (the last somite has no appendages), and that the telson has connected with it a pair of movable rods forming a "caudal fork" like that of the Branchiopoda. On account of the leaf-like thoracic feet and the possession of a caudal fork and other features, the Nebaliacea were formerly classified with the Branchiopoda, but a closer examination of their structure has shown that they are true Malacostraca. In having an additional somite in the abdomen and in other points, however, they may be regarded as forming a link between the Malacostraca and the lower forms of Crustacea, and for this reason they are set apart as a series Leptostraca, while the other orders form a series Eumalacostraca.

The orders of the Eumalacostraca, again, are grouped, as shown in the table of classification, into four divisions. The first of these, the Syncarida, includes only one order, comprising a few small Crustacea (see [Fig. 84], p. 264) which have recently been discovered in fresh water in Tasmania and Australia. They have no carapace, and all the thoracic somites, or all but the first, are distinct. The antennules are two-branched, the antennæ may have a scale-like exopodite, and the last pair of abdominal appendages form, with the telson, a tail-fan. The eyes are sometimes stalked, but in one species they are sessile. The thoracic limbs, which are not clearly divided into maxillipeds and legs, carry a double series of plate-like gills or epipodites. As will be shown later, the living Syncarida are especially interesting on account of their resemblance to certain very ancient fossil Crustacea.

The second division of the Eumalacostraca, the Peracarida, includes five orders, the members of which differ very greatly in appearance. They all agree, however, in certain important points of structure, of which the most conspicuous is the possession, in the female sex, of a brood-pouch for carrying the eggs and young. This brood-pouch is formed by a series of overlapping plates attached to the bases of the thoracic limbs.

Fig. 16—Mysis relicta, One of the Mysidacea. Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)

cs, Cervical groove of the carapace; m, brood-pouch

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