=The Merostomata.=—The only living form, Limulus, is undoubtedly a very primitive type, as the genital glands and ducts are double, opening wide apart on the basal pair of abdominal legs (Fig. 3). Moreover, their head-appendages, which are single, with spines on the basal joint, are very primitive and morphologically nearer in shape to those of the worms (Syllidæ, etc.) than even those of the Crustacea. Besides, their four pairs of coxal glands, with an external opening at the base of the fifth pair of head-appendages, and which probably are modified nephridia (Crustacea having but a single pair in any one form, either opening out on the second antennal, green gland, or second maxillary, shell-gland, segment), indicate a closer approximation to the polynephrous worms. Limulus has other archaic features, especially as regards the structure of the simple and compound eyes and the simple nature of the brain. =The Trilobita.=—These archaic forms are still more generalized and primitive than the Merostomata and Crustacea, and probably were the first Arthropoda to be evolved from some unknown annelid worm. They had jointed biramose limbs of nearly uniform shape and size on each segment of the body, which were not, as in Crustacea, differentiated into antennæ, jaws (mandibles), maxillæ, maxillipeds, and two kinds of legs (thoracic and abdominal), showing that they are a much more primitive type, and nearer to the annelids than any other Arthropoda. Their gills, as shown by the researches of Walcott and of Beecher, were attached to nearly if not every pair of limbs behind the antennæ (Figs. 1, 2). The fact that in Trilobita the first pair of limbs is antenniform does not prove that they are Crustacea, since Eurypterus has a similar pair of appendages. [Illustration: FIG. 2.—Restored section of Calymene: _C_, carapace; _en_, endopodite; _en′_, exopodite; with the gills on the epipodal or respiratory part of the appendage.—After Walcott. ] The limbs in trilobites, as well as the abdominal ones of merostomes, and all those of Crustacea, except the first antennæ, are biramose, consisting of an outer (exopodite) and an inner division (endopodite). In this respect the terrestrial air-breathing tracheate forms, Arachnida, Myriopoda, and Insecta, differ from the branchiate forms, as their legs are single or undivided, being adapted for supporting the body during locomotion upon the solid earth. It is to be observed that when, as in Limulus, the body is supported by cephalic ambulatory limbs, they are single, while the abdominal limbs, used as they are in swimming, are biramose, much as in Crustacea.