Page:EB1911 - Volume 21.djvu/183

Rh leg) and the external vesicular portion is not dilated. The external opening of the other nephridia is laced at the outer end of a transverse groove at the base of the legs. The salivary glands are the modified nephridia of the segment of the oral papillae.

The male generative organs (fig. 10) consist of a pair of testes (te), a pair of seminal vesicles (v), vasa deferential (v.d.), and accessory glandular tubules (f). All the above parts lie in the central compartment of the body cavity. The ovaries consist of a pair of tubes closely applied together, and continued posteriorly into the oviducts. Each oviduct, after a short course, becomes dilated into the uterus. The two uteri join behind and open to the exterior by a median opening. The ovaries always contain spermatozoa, some of which project through the ovarian wall into the body cavity. Spermatozoa are not found in the uterus and oviducts, and it appears probable, as we have said, that they reach the ovary directly by boring through the skin and traversing the body cavity. In all the species except the African species there is a globular receptaculum seminis opening by two short ducts close together into the oviduct, and in the neotropical species there is in addition a small receptaculum ovorum, with extremely thin walls, opening into the oviduct by a short duct just in front of the receptaculum seminis. The epithelium of the latter structure is clothed with actively moving cilia. There appear to be present in most, if not all, of the legs some accessory glandular structures opening just externally to the nephridia. They are called the crural glands.

Development.—Peripatus is found in Africa, in Australasia, in South America and the West Indies, in New Britain, and in the Malay Peninsula and Sumatra. The species found in these various localities are closely similar in their anatomical characters, the principal differences relating to the structure of the female generative organs and to the number of the legs. They, however, differ in the most striking manner in the structure of the ovum and the early development. In all the Australasian species the egg is large and heavily charged with food-yolk, and is surrounded by a tough membrane. In the Cape species the eggs are smaller, though still of considerable size; the yolk is much less developed, and the egg membrane is thinner though dense. In the New Britain species the egg is still smaller (.1 mm.), and there is a large trophic vesicle. In the neotropical species the egg is minute, and almost entirely devoid of yolk. The unsegmented uterine ovum of P. novae zealandiae measures 1.5 mm. in length by 8 mm. in breadth, that of P. capensis is .56 mm. in length; and that of P. trinidadensis .04 mm. in diameter. In correspondence with these differences in the ovum there are differences in the early development, though the later stages are closely similar.

The development has been worked out in P. capensis, to which species the following description refers. The segmentation is peculiar, and leads to the formation of a solid gastrula, consisting of a cortex of ectoderm nuclei surrounding a central endodermal mass, which is exposed at one point—the blastopore. The enteron arises as a space in the endoderm, and an opacity—the primitive streak—appears at the hind end of the blastopore (fig. 11, B). The elongation of the embryo is accompanied by an elongation of the blastopore, which soon becomes dumb-bell shaped (fig. 11, C). At the same time the mesoblastic somites (embryonic segments of mesoderm) make their appearance in pairs at the hind end, and gradually travel forwards on each side of the blastopore to the front end, where the somites of the anterior pair soon meet in front of the blastopore (fig. 11, D). Meanwhile the narrow middle part of the blastopore has closed by a fusion of its lips, so that the blastopore is represented by two openings, the future mouth and anus. A primitive groove makes its appearance behind the blastopore (fig. 11, D). At this stage the hind end of the body becomes curved ventrally into a spiral (fig. 11, E), and at the same time the appendages appear as hollow processes of the body-wall, a mesoblastic somite being prolonged into each of them. The first to appear are the antennae, into which the praeoral somites are prolonged. The remainder appear from before backwards in regular order, viz. jaw, oral papillae, legs 1-17. The full number of somites and their appendages is not, however, completed until a later stage. The nervous system is formed as an annular thickening of ectoderm passing in front of the mouth and behind the anus, and lying on each side of the blastopore along the lines of the somites. The praeoral art of this thickening, which gives rise to the cerebral ganglia, becomes pitted inwards on each side (fig. 11, F, c.g.). These pits are eventually closed, and form the hollow ventral appendages of the suprapharyngeal ganglia of the adult (fig. 7, d). The lips are formed as folds of the side wall of the body, extending from the praeoral lobes to just behind the jaw (fig. 11, F, L). They enclose the jaws (j), mouth (M), and opening of the salivary glands (o.s), and so give rise to the buccal cavity. The embryo has now lost its spiral curvature, and becomes completely doubled upon itself, the hind end being in contact with the mouth (fig. 11, G). It remains in this position until birth. The just-born young are from 10 to 15 mm. in length, and have green antennae, but the rest of the body is either quite white or of a reddish colour. This red colour differs from the colour of the adult in being soluble in spirit. The mesoblastic somites are paired sacs formed from the anterior lateral portions of the primitive streak (fig. 11, C). As they are formed they become placed in pairs on each side of the blastopore. The somites of the first pair eventually obtain a position entirely in front of the blastopore (Fig. 11, D). They form the somites of the praeoral lobes. The full complement of somites is acquired at about the stage of fig. 11, E. The relations of the mesoblastic somites are shown in fig. 12, A, which represents a transverse section taken between the mouth and anus of an embryo of the stage of fig. 11, D. The history of these somites is an exceedingly interesting one, and may be described shortly as follows: They divide into two parts—a ventral part which extends into the appendage, and a dorsal part (fig. 12, B). Each of the ventral parts acquires an opening to the exterior, just outside the nerve-cord,