=External Structure.=—The external features which the members of this Order have in common are the segmented pediculate abdomen (9 to 12 segments), the two-jointed non-chelate chelicerae, the antenniform first pair of legs, and the presence of two pairs of lung-book stigmata beneath the abdomen. The constituent families differ so much in outward form that they must be dealt with separately. The Thelyphonidae or “Whip Scorpions” (see p. 312) have a long-oval carapace bearing well-developed eyes, two in front, and a group of three or five on either side some distance behind. The pedipalpi are chelate, and have their basal joints fused beneath the mouth, being thus incapable of any masticating motion. The first legs are six-jointed, and have multi-articulate tarsi; the others are seven-jointed, and their tarsi, in some species at least, are tri-articulate. The abdomen consists of two portions, a wide nine-jointed pre-abdomen and a short narrow three-jointed post-abdomen, to which a filiform tail is articulated. Beneath the cephalothorax, between the coxae of the legs, is a distinct sternal plate in two portions (Fig. 171). The first abdominal ventral plate is largely developed, and covers two segments. Behind it are the median genital opening and two pulmonary stigmata, while the other stigmata are behind the second ventral plate, which corresponds to the third abdominal segment. On the last abdominal segment there are often two or four light-coloured spots called “ommatoids,” and considered by some authors to be organs of sight. Laurie, however (_vide infra_), thinks it more probable that they are olfactory in function. The Schizonotidae (see p. 312) have a two-jointed carapace, and do not possess more than two eyes. There is a short unjointed tail-piece. In the Tarantulidae (Phrynidae) the whole body is much flattened and extended laterally, the undivided carapace being reniform, and broader than long. The long non-chelate pedipalps have their basal joints free and movable, and there are several sternal plates. There are nine abdominal tergal plates, the last three diminishing rapidly in size, and the last plate covering a button-like terminal portion of the abdomen. The first abdominal ventral plate is largely developed, as in the Thelyphonidae, and the genital orifice and pulmonary stigmata are in the same situation as in that group. The Tarantulidae have glutinous glands in the first abdominal segment which are capable of spinning a few irregular threads. In the whole group paired circular depressions are conspicuous dorsally on all the abdominal segments. These indicate the points of attachment of the dorso-ventral muscles.