The floor of the cephalothorax is for the most part formed by the coxae of the appendages, and the sternum is hardly recognisable in many species. In Solpuga, however (see p. [429]), it exists in the form of a long narrow plate of three segments, ending anteriorly in a lancet-shaped labium.
A pair of large simple eyes are borne on a prominence in the middle of the anterior portion of the cephalothorax, and there are often one or two pairs of vestigial lateral eyes.
The first pair of tracheal stigmata are to be found behind the coxae of the second legs.
The mouth-parts take the form of a characteristic beak, consisting of a labrum and a labium entirely fused along their sides. The mouth is at the extremity of the beak, and is furnished with a straining apparatus of complicated hairs.
The abdomen possesses ten free segments, marked off by dorsal and ventral plates, with a wide membranous lateral interval. The ventral plates are paired, the first pair forming the genital opercula, while behind the second and third are two pairs of stigmata. Some species have a single median stigma on the fourth segment, but this is in some cases permanently closed, and in the genus Rhagodes entirely absent, so that it would seem to be a disappearing structure.
The appendages are the six pairs common to all Arachnids—chelicerae, pedipalpi, and four pairs of legs. The chelicerae, which are enormously developed, are two-jointed and chelate, the distal joint being articulated beneath the produced basal joint. In the male there is nearly always present, on the basal joint, a remarkable structure of modified hairs called the “flagellum,” and believed to be sensory. It differs in the different genera, and is only absent in the Eremobatinae (see p. [429]). The pedipalpi are strong, six-jointed, leg-like appendages, without terminal claw. They end in a knob-like joint, sometimes movable, sometimes fixed, which contains a very remarkable eversible sense-organ, which is probably olfactory. It is concealed by a lid-like structure, and when protruded is seen to be furnished, on its under surface, with a pile of velvet-like sensory hairs.
The legs differ in the number of their joints, as the third and fourth pairs have the femora divided, and the tarsus jointed. The first pair has only a very small terminal claw, but two well-developed claws are borne by the tarsi of the other legs. Each of the last legs bears, on its under surface, five “racket organs,” believed to be sensory.
Internal Structure.—The alimentary canal possesses a sucking chamber within the beak, after which it narrows to pass through the nerve-mass, and after an S-shaped fold, joins the mid-gut. This gives off four pairs of thin diverticula towards the legs, the last two entering the coxae of the third and fourth pairs.
At the constriction between the cephalothorax and the abdomen there is no true pedicle, but there is a transverse septum or “diaphragm,” through which the blood-vessel, tracheal nerves, and alimentary canal pass. The gut narrows here, and, on entering the abdomen, proceeds straight to a stercoral pocket at the hind end of the animal, but gives off, at the commencement, two long lateral diverticula, which run backwards parallel with the main trunk. These are furnished with innumerable secondary tube-like diverticula, which proceed in all directions and fill every available portion of the abdomen. The caeca, which are so characteristic of the Arachnidan mid-gut, here reach their extreme development. A pair of Malpighian tubules enter the main trunk in the fourth abdominal segment.
Other excretory organs are the coxal glands, which form many coils behind the nerve-mass, and open between the coxae of the third and fourth legs. They have been taken for poison-glands.