=The Crustacea.=—These Arthropoda are in many most important characteristics unlike the insects; they have two pairs of antennæ, five pairs of buccal appendages, and they are branchiate Arthropoda. They have evidently originated entirely independently, and by a direct line of descent from some unknown annelid ancestor which was either a many-segmented worm, with parapodia, or the two groups together with the Rotifera may have originated from a common appendigerous Trochosphæra. Their segments in the higher forms are definite in number (23 or 24) and arranged into two regions, a head-thorax (cephalothorax) and hind-body (abdomen). Nearly all the segments, both of the cephalothorax and abdomen, bear a pair of jointed limbs, and to them at their base are, in the higher forms, appended the gills (branchiæ). The limbs are in the more specialized forms (shrimps and crabs) differentiated into eye-stalks, two pairs of antennæ, a pair of palpus-bearing jaws (mandibles), two pairs of maxillæ and three pairs of maxillipeds; these appendages being biramose, and the latter bearing gills attached to their basal joints. The legs are further differentiated into ambulatory thoracic legs and into swimming or abdominal legs, and in the latter the first pair of the male is modified into copulatory organs (gonopoda). The male and female reproductive organs as a rule are in separate individuals, hermaphrodites being very unusual, and the glands may be paired or single. The sexual outlets are generally paired, and, as in the male lobster and other Macrura, open in the basal joint of the last pair of legs, and in the female in the third from the last; while originally in all Crustacea the sexual organs were most probably paired (Fig. 3, _B_). They are, except a few land Isopoda, aquatic, mostly marine, and when they have a metamorphosis, pass through a six-legged larval stage, called the Nauplius, the shrimps and crabs passing through an additional stage, the Zoëa. Crustacea also differ much from insects in the highly modified nature of the nephridia, which are usually represented by the green gland of the lobster, or the shell-glands of the Phyllopoda, which open out in one of the head-segments; also in the possession of a pair of large digestive glands, the so-called liver. Intermediate in some respects between the Crustacea and insects, but more primitive, in respect to what are perhaps the most weighty characters, than the Crustacea, are the Trilobita, the Merostomata (Limulus), and, finally, the Arachnida, these being allied groups. In the Trilobita and Merostomata (Limulus), the head-appendages are more like feet than jaws, while they have in most respects a similar mode of embryonic development, the larval forms being also similar. [Illustration: FIG. 1.—Restoration of under side of a trilobite (_Triarthrus becki_), the trunk limbs bearing small triangular respiratory lobes or gills.—After Beecher. ]