Page:EB1911 - Volume 21.djvu/181

Rh neighbourhood of the Myriapoda, though it must not be forgotten that it differs from the Myriapoda more than the Myriapoda differ from other Arthropoda, and that in some respects it presents features which recall the segmented worms (Annelida). The genus has a wide distribution (see below), but it has not been found in Europe or in North America. There is but little variety of structure in the genus, and the species are limited in number. They live beneath the bark of trees, in the crevices of rock and of rotten stumps of trees, and beneath stones. They require a moist atmosphere, and are exceedingly susceptible to drought. They avoid light, and are therefore rarely seen. They move slowly, picking their course by means of their antennae. When irritated they eject with considerable force the contents of their slime reservoirs by means of the sudden contraction of the muscular body-wall. The slime, which appears to be harmless, is extremely sticky, but it easily comes away from the skin of the animal itself. Locomotion is effected by means of the legs, with the body fully extended. Hutton describes his specimens as sucking the juices of flies, which they had stuck down with their slime, and they have been observed in captivity to devour the entrails which have been removed from their fellows, and to eat raw sheep's liver. They move their mouths in a suctorial manner, tearing the food with their jaws. They have the power of extruding their jaws from the mouth, and of working them alternately backwards and forwards. They are viviparous; the young are fully formed at birth, and differ from the adult only in size and colour. The mother does not appear to pay any special attention to her offspring, which wander away and get their own living. It has lately been stated that some of the Australian species are normally oviparous, but this has not been fully proved. Sexual differences are not strongly marked, and are sometimes absent. There does not appear to be any true copulation. In some species the male deposits small oval spermatophores indiscriminately on any part of the body of the female. It seems probable that in such cases the spermatozoa make their way from the adherent spermatophore through the body-wall into the body, and so by traversing the tissues reach the ovary. In other species which possess receptacula seminis it is probable that fertilization is effected once only in early life before any ova pass into the uterus.

External Features.—The anterior part of the body may be called the head, though it is not sharply marked off from the rest of the body (fig. 1). The head carries three pairs of appendages, a pair of simple eyes, and a ventrally placed mouth. The body is elongated and vermiform; it bears a number of paired appendages, each terminating in a pair of claws, and all very much alike. The number varies in the different species. The anus is always at the posterior end of the body, and the generative opening is on the ventral surface, just in front of the anus; it may be between the legs of the penultimate pair, or between the legs of the last pair, or it may be subterminal. The colour varies considerably in the different species, and even in different individuals of the same species. The skin has a velvety appearance, and is thrown into I a number of transverse ridges, along which wart-like papillae are placed. These papillae, which are found everywhere, are the primary papillae; they are covered with small, scale-like projections called secondary papillae, and are specially developed on the dorsal surface, less so on the ventral. Each papilla carries at its extremity a well-marked spine. Among the primary papillae smaller accessory papillae are sometimes present.

The appendages of the head are the antennae, the jaws and the oral papillae. The mouth is at the hinder end of a depression

called the buccal cavity, and is surrounded by an annular tumid lip, raised into papilliform ridges and bearing a few spines (fig. 2). Within the buccal cavity are the two jaws. They are short, stump-like, muscular structures, armed at their free extremities by a pair of cutting blades or claws, and are placed one on each side of the mouth. In the median line of the buccal cavity in front is placed a thick muscular protuberance, which may be called the tongue, though attached to the dorsal instead of to the ventral wall of the mouth (fig. 2). The tongue bears a row of small, chitinous teeth. The jaw-claws (figs. 3 and 4), which resemble in all essential points the claws borne by the feet, and, like these, are thickenings of the cuticle, are sickle-shaped. They have their convex edge directed forwards, and their concave, or cutting edge, turned backwards. The inner cutting plate (fig. 3) usually bears a number of cutting teeth. The oral papillae are placed at the sides of the head (fig. 2). The ducts of the slime-glands open at their free end. They possess two main rinlgs of projecting tissue, and their extremities bear papillae irregularly arranged. The ambulatory appendages vary in number. There are seventeen pairs in P. capensis and eighteen in P. balfourz, while in P. jamaicensis the number varies from twenty-nine to forty-three. They consist of two main divisions, which we may call the leg and the foot (fig. 5). The leg (l) has the form of a truncated cone, the broad end of which is attached to the ventro-lateral wall of the body, of which it is a prolongation. It is marked by a number of rings of papillae placed transversely to its long axis, the dorsal of which are pigmented like the dorsal surface of the body, and the ventral like the ventral surface. At the narrow distal end of the leg there are on the ventral surface three or four (rarely five) spiniferous pads each of which is continued dorsally into a row of papillae. The foot is attached to the distal end of the leg. It is slightly narrower at its attached extremity than at its free end. It bears two sickle-shaped claws, and at its distal end three (rarely four) papillae. The part of the foot which carries the claws is especially retractile, and is generally found more or less telescoped into the proximal part. The legs of the fourth and fifth pairs differ from the others in the fact that the third pad (counting from the distal end of the leg) carries the opening of the enlarged nephridia of these segments. In some species certain of the legs bear on their ventral sides furrows with tumid lips and lined by smooth non-tuberculate epithelium; they are called coxal organs, and it appears that they can be everted. The males are generally rather smaller and less numerous than the females. In those species in which the number of legs varies the male has a smaller number of legs than the female.