Page:EB1911 - Volume 21.djvu/182

Rh Breeding.—As already stated, Peripatus is viviparous. The Australasian species come nearest to laying eggs, inasmuch as the eggs are large, full of yolk, and enclosed in a shell, but development normally takes place in the uterus, though abnormally, incompletely developed eggs are extruded. The uterus always contains several young, which are usually at different stages of development and are born at different times of the year. In most of the African species, however, the embryos of the uterus are almost of the same age and are born at a definite season. The young of P. capensis are born in April and May. They are almost colourless at birth, excepting the antennae, which are green, and their length is 10 to 15 mm. A large female will produce thirty to forty young in one year. The period of gestation is thirteen months, that is to say, the ova pass into the oviducts about one month before the young of the preceding year are born.

Anatomy.—The alimentary canal (fig. 6). The buccal cavity, as explained above, is a secondary formation around the true mouth, which is at its dorsal posterior end. It contains the tongue and the jaws, which have already been described, and into the hind end of it there open ventrally by a median opening the salivary glands. The mouth leads into a muscular pharynx, which is connected by a short oesophagus with the stomach. The stomach forms by far the largest part of the alimentary canal. It is a dilated soft-walled tube, and leads behind into the short narrow rectum, which opens at the anus. There are no glands opening into the alimentary canal. The central nervous system, the anterior part of which is shown in fig. 7, is of the “rope-ladder” type, and the ventral cords meet over the rectum.

The cuticle is a thin layer, of which the spines, jaws and claws are special developments. Its surface is not, however, smooth, but is everywhere, with the exception of the perioral region, raised into minute secondary

papillae, which in most instances bear at their free extremity a somewhat prominent spine. The epidermis, placed immediately within the cuticle, is composed of a single row of cells. The pigment which gives the characteristic colour to the skin is deposited in the protoplasm of the outer ends of the cells in the form of small granules. Beneath the epidermis is a thin cutis, which is followed by the muscular layers (external circular and internal longitudinal). The muscular fibres of the jaws are transversely striated, the other muscles are unstriated.

The apertures of the tracheal system are placed in the depressions between the papillae or ridges of the skin. Each of them leads into a tube, which may be called the tracheal pit (fig. 8); the walls of this are formed of epithelial cells, bounded towards the lumen of the pit by a very delicate cuticular membrane continuous with the cuticle covering the surface of the body. Internally it expands in the transverse plane, and from the expanded portion the tracheal tubes arise in diverging bundles. The tracheae are minute tubes exhibiting a faint transverse striation which is probably the indication of a spiral fibre. They appear to branch, but only exceptionally. The tracheal apertures are diffused over the surface of the body, but are especially developed in certain regions.

The vascular system consists of a dorsal tubular heart with paired ostia leading into it from the pericardium. Of the pericardium, and the various other divisions of the perivisceral cavity (fig. 12, D). As in all Arthropoda, the perivisceral cavity is a haemocoele, i.e. contains blood, and forms part of the vascular system. It is divided by septa into chambers (fig. 12, D), of which the most important are the central chamber containing the alimentary canal and the dorsal chamber or pericardium. Nephridia are present in all the legs. In all of them (except the first three) the followlng parts may be recognized (fig. 9): (1) a vesicular portion (s) opening to the exterior on the ventral surface of the legs by a narrow passage (s.d); (2) a coiled portion, which is again subdivided into several sections (s.c); (3) a section with closely-packed nuclei ending by a somewhat enlarged opening (p.f); (4) the terminal portion, which consists of a thin-walled vesicle. The nephridia of the first three pairs of legs are smaller than the rest, consisting only of a vesicle and duct. The fourth and fifth pairs are larger than those behind, and are in other respects peculiar; for instance, they open on the third pad (counting from the distal end of the