The fifth and last subclass, that of the MALACOSTRACA, is by far the largest and most important, and will require to be considered in more detail than any of the others. The animals composing the various orders into which the subclass is divided differ very greatly in structure, but they all agree in having typically the same number of appendages as the Lobster--namely, nineteen pairs (or twenty, if the eye-stalks be included). They also agree in the very important character that the trunk limbs are divided into two sets, thoracic and abdominal, the former of eight, and the latter of six pairs. [Illustration: FIG. 15--_Nebalia bipes._ ENLARGED. (From Lankester's "Treatise on Zoology," after Claus.) _a´_, Antennule; _a´´_, antenna; _ab_1-_ab_6, 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] 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.