The chitinous membrane between the plates is very extensible, rendering measurements of the body in these animals of little value. In a female full of eggs the dorsal plates may be separated by a considerable interval, while after egg-laying they may actually overlap. The four stigmata are not situated on the plates, but ventro-laterally, at the level of the hinder borders of the first and second abdominal plates.
The first ventral abdominal plate bears the genital orifice. In the same plate there are two other orifices, an anterior and a posterior, which belong to the “abdominal glands.” They were taken by some authors for the spinning organs, but their function is probably to supply material for the capsule by which the eggs are suspended from the body of the mother (see p. [434]).
The Chernetidea possess chelicerae, pedipalpi, and four pairs of ambulatory legs, all articulated to the cephalothorax.
Fig. [223].—Chelicera of Garypus. f, Flagellum; g, galea; s, serrula. (After Simon.)
The chelicerae are two-jointed, the upper portion of the first joint being produced forward into a claw, curving downward. The second joint is articulated beneath the first, and curves upward to a point, the appendage being thus chelate. This second joint, or movable digit, bears, near its extremity, the opening of the spinning organ, and is furnished, at all events in the Garypinae and Cheliferinae (see p. [437]), with a pectinate projection, the “galea,” arising at its base, and extending beyond the joint in front. In the Obisiinae it is only represented by a slight prominence.
Two other organs characterise the chelicerae of all the Chernetidea; these are the “serrula” and the “flagellum.” Their minute size and transparency make them very difficult of observation, and for a long time they escaped notice. The serrula is a comb-like structure attached to the inner side of the distal joint. The flagellum is attached to the outer side of the basal joint, and recalls the antenna of a Lamellicorn beetle, or the “pectines” of scorpions, a resemblance which gave rise to the supposition that they are olfactory organs. It is more likely, however, that they are of use in manipulating the silk.
The pedipalpi are six-jointed and are very large, giving these animals a superficial resemblance to scorpions. According to Simon,[[334]] the patella is absent, and the joints are coxa, trochanter, femur, tibia, tarsus, with an apophysis forming the fixed digit of the chela, while the sixth joint is the movable digit, and is articulated behind the tarsus. These joints, especially the tarsus, are often much thickened, but however strongly developed, they are always narrow and pediculate at the base. The coxae of the pedipalps are closely approximated, and are enlarged and flattened. They probably assist in mastication, but there is no true maxillary plate articulated to the coxa as in some Arachnid groups.
The legs are usually short and feeble, and the number of their articulations varies from five to eight, so that it is not easy to be certain of the homologies of the individual joints to those of other Arachnids. The coxae are large, and form the floor of the cephalothorax. They are succeeded by a short trochanter, which may be followed by another short joint, the “trochantin.” Then come the femur and tibia, elongated joints without any interposed patella, and finally the tarsus of one or two joints, terminated by two smooth curved claws, beneath which is situated a trumpet-shaped membranous sucker.
Internal Structure.[[335]]—The internal structure of the Chernetidea, as far as their small size has permitted it to be made out, bears a considerable resemblance to that of the Phalangidea. The alimentary canal dilates into a small sucking pharynx before passing through the nerve-mass into the large many-lobed stomach, but the narrow intestine which terminates the canal is convoluted or looped, and possesses a feebly-developed stercoral pocket.