Tracheate Arthropoda with soft, worm-like bodies, a pair of antennae, and a series of paired imperfectly jointed limbs. Tracheal stigmata nnmeroiis and scattered; nephridia or segmental organs present. A single genus Peripatus.

The body is not segmented, and is flat ventrally, convex dorsally. The ringed antennae are large. There is a buccal cavity inclosed by soft lips and containing the first pair of limbs, which are 2-clawed and act as jaws. The second pair of limbs form the oral papillae placed at the side of the mouth, at the summits of which open the receptacles of the slime glands. The remaining limbs are placed at equal distances as far as the posterior extremity of the body. The number of pairs varies in the different species (from 14-30)1. In P. capensis the limb is originally 5-jointed, but in the adult the jointing becomes obscure. In all the species it is terminated by two chitinoid claws. A pair of anal papillae by the side of the genital aperture appears to represent the last pair of limbs, as in some specimens of P. capeitsis they possess the two typical claws.

In P. capensis there is a delicate cuticle, a single layer of ectoderm (hypo-dermis) cells; an outer layer of circular muscles; an inner layer of longitudinal muscles arranged in five bands, two dorsal, two lateral, and three ventral; and a layer of transverse, i. e. dorsoventral muscles placed obliquely and dividing the coelome into three longitudinal cavities, a median containing the digestive tract, slime glands, and genitalia, and two lateral containing the salivary glands, nerve cords, and in the male the last enlarged pair of crural glands. The limbs contain a separate division of the coelome which lodges the crural glands and nephridia. The middle compartment of the coelome is lined by an endothelium. The surface of the body is covered with papillae. Closed capsules prolonged externally into a spine, lined by hypodermis cells and supplied by a nerve, are found in the primary papillae. They are very numerous on the antennae, lips, oral papillae, and certain regions of the ventral surface of the feet, and are probably tactile in function.

The muscles, with the exception of those attached to the jaws, are unstriped.

1 Ernst states (Nature, xxiii. p. 447) that a young Peripatus (? P. Edwardsi) possessed at birth 29 pairs of feet, the adult 31; consequently new pairs of feet must be added during growth. He also observed a skin cast by the young, animal. The fact that there are jaws and claws in reserve points to the probability of a moult occurring from time to time. For observed variations in the number of feet, cf. Moseley, 'On the Species of Peripatus,' A. N. H. (5), iii. 1879, p. 263.

The nervous system consists of a pair of supra-oesophageal ganglia united medianly, of commissures surrounding the pharynx, and a pair of widely separated ventral nerve-cords united dorsally and posteriorly by a fibrous commissure above the anus. The two cords are united across the middle ventral line by numerous transverse commissures. The an-tennary nerves arise from the supra-oesophageal ganglia, those for the jaws from the oesophageal commissures. The cords give off numerous lateral nerves, those for the feet being especially stout. The transverse commissures give off nerves to the skin. There is a ventral layer of ganglion cells more especially aggregated at the origin of the nerves to the feet. The sympathetic system consists of two nerves rising from the supra-oesophageal ganglia and running dorsally on the walls of the pharynx and oesophagus, where they unite. The eyes are paired, and lie dorso-laterally, one at the base of each antenna. They are formed as invaginations from the nervous thickenings of the prae-oral lobes of the embryo. The vesicle thus formed lies just below the skin. Its anterior cells become somewhat flattened, its lateral and especially its posterior cells elongated, forming visual cells.

These cells are pigmented at their outer ends; at their inner they are clear, i. e. bear retinidia, and touch the oval gelatinous lens, which must be formed originally by secretion from the cells. There is an optic ganglion. This eye resembles that of Chaetopoda and Gastropoda1.

The digestive tract consists of the buccal depression above mentioned, a muscular pharynx into which opens ventrally and posteriorly the common duct of the two salivary glands, a narrow oesophagus, all lined by the chitinoid cuticle; of a wide mesenteron stretching nearly the whole length of the body, and a short rectum lined by cuticle and opening by a terminal anus. The rectum has an external circular and an internal longitudinal layer of muscle fibres. The position of these layers is reversed in the other sections of the tract. A dorsal blood-vessel or heart in the shape of a muscular tube with a pair of valved ostia to each somite of the body lies in a pericardial cavity, closed below by a horizontal septum and placed between the two dorsal muscle bands. It gives off no vessels. A delicate ventral vessel lies externally to the circular layer of muscles. There is a double row of stigmatic apertures on either side the median dorsal line, and similar rows on either side the median ventral line; other apertures are found on the anterior and posterior aspects of the feet, at their base, on the dorsal aspect of the head, and a single large median ventral aperture in front of the mouth.

Each stigma leads into a pit dilated internally, from which proceed bundles of unbranched extremely minute tracheal tubes, lined by a cuticle which has a faint transverse striation. They are distributed to the muscles, viscera, nervous system, etc. A nephridium opens ventrally near the base of each foot. It consists of a vesicle opening externally; a coiled tubular portion; and a terminal funnel (Pciliated) which opens into the section of the coelome in the foot, from which it is stated to be differentiated (cp. the Elasmobranch excretory system). The three first pairs of nephridia are rudimentary, the fourth and fifth enlarged and somewhat peculiar. The sexes are separate. The male has a pair of testes, each with a prostatic gland. The two vasa deferentia unite, and then receive terminally the ducts of a pair of tubular accessory glands. The ovary is single, but divided by a longitudinal vertical septum into two halves. The two oviducts dilate into uteri in which the embryo is developed. They unite in a terminal vestibule. The generative aperture is ventral and in front of the anus. The filiform spermatozoa are united into rodlike spermatophores, which the male attaches to the body of the female in any region. Segmentation is complete.

The blastopore in P. capensis is elongated and divides, forming both mouth and anus. The young differ at birth from the adult only in size and colour.

1 Patten considers that it represents the primitive eye of the Arthropoda, from which other forms have been derived by abbreviation of development.

P. Edwardsi differs in some respects from P. capensis. Crural glands are absent in the female, present in the male only on the 7-8 pairs of prae-genital limbs. Bean-shaped vesicles to which muscles are attached and nerves distributed are lodged on the dorsal aspect near the claw of each limb. A pair of glands open ventrally in the male, one on either side of the anus (? = accessory male organs of P. capensis). The stigmata are irregularly scattered. There are no anal papillae. The sexual aperture lies in the penultimate somite between the last pair of limbs. The prostate gland of Moseley is the real testis and gives origin to the spermatospores, which undergo development in the dilated portion or testis of Moseley. The united portion of the vasa deferentia is divisible into three regions, an anterior in which the sperm collects, a middle in which the spermatophore is formed, and a terminal, the muscular ductus ejaculatorius. The spermatophore is about 1 1/2 inch ( = 4 cm.) long, has several envelopes, and contains the agglutinated spermatozoa in 3-5 dilatations.

The female organs consist of two ovaries, each connected by a ligament to the pericardial septum; of two oviducts, which communicate anteriorly and have each a funnel-shaped aperture into the coelome, and carry a horseshoe-shaped receptaculum seminis communicating at each end with the oviduct; and two uteri, with a single short muscular vagina. Ciliated epithelium is found in the ducts of the receptaculum, and in the section of the vas deferens that secretes the spermatophore.

In P. Novae-Zealandiae the tracheae are said to branch. The end of the male duct is exceedingly muscular ( = penis?), and contains unicellular glands in its walls. There is a spermatophore (Moseley).

The various species of Peripatus are found at the Cape of Good Hope, in Australia and New Zealand, in Central and South America, and the West Indies. They live under stones, in rotting wood, etc. in moist places, are nocturnal in habit, and feed on insects, etc, which they ensnare by the ejection of slime from the oral papillae. Their distribution and anatomy point to an extreme archaic origin.

Moseley, 'Myriapoda,' Encyclopaedia Britannica (ed. ix), xvii; Balfour, Q. J. M. xxiii. 1883 (with lit.); Gaffron, Schneider's Zool. Beitrage, i. 1885. Eye, Carriere, 'Sehorgane der Thiere,' 1885, p. 121. Development, Sedgwick, Q. J. M. xxv. 1855; xxvi. 1886; von Kennel, Arb. Zool. Zoot. Inst. Wurzburg, vii. 1885; viii. (I), 1886.