1911 Encyclopædia Britannica/Peripatus

PERIPATUS, a genus of animals belonging to the air-breathing division of the phylum Arthropoda. It differs, however, from all other Arthropoda in such important respects that a special class, equivalent in rank to the old-established Arthropod classes, had been created for its sole occupancy. This class has been named the Prototracheata or Onychophora (see Arthropoda), and may be most appropriately placed in the system in the 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.

EB1911 Peripatus - Peripatus capensis.jpg
(After Sedgwick.)
Fig. 1.—Peripatus capensis, drawn from life. Life size.
EB1911 Peripatus - P. capensis - ventral view of the head.jpg
(After Sedgwick.)

Fig. 2.—Ventral view of the head of P. capensis.

ant, Antennae, or.p, Oral papillae; F.1, First leg; T, Tongue.

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.

EB1911 Peripatus - Peripatus capensis - inner jaw-claw.jpg EB1911 Peripatus - Peripatus capensis - outer jaw-claw.jpg
(After Balfour.) (After Balfour.)

Fig. 3.—Inner jaw-claw of P. capensis.

Fig. 4.—Outer jaw-claw of P. capensis.

EB1911 Peripatus - P. capensis - ventral view of last leg of a male.jpg
(After Sedgwick.)

Fig. 5.—Ventral view of last leg of a male P. capensis.

f, Foot; l, leg; p, spiniferous pads. The white papilla of the proximal part of this leg is characteristic of the male of this species.

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.

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.

EB1911 Peripatus - P. capensis dissected.jpg
(After Balfour.)

Fig. 6.—Peripatus capensis dissected so as to show the alimentary canal, slime glands and salivary glands. The dissection is viewed from the ventral side, and the lips (L) have been cut through in the middle line behind and pulled outwards so as to expose the jaws (f), which have been turned outwards, and the tongue (T) bearing a median row of chitinous teeth, which branches behind into two. The muscular pharynx, extending back into the space between the first and second pairs of legs, is followed by a short tubular oesophagus. The latter opens into the large stomach with plicated walls, extending almost to the hind end of the animal. The stomach at its point of junction with the rectum presents an S shaped ventro-dorsal curve.

A, Anus; at, antenna; F.1, F.2, first and second feet; j, jaws; L, lips; oe, oesophagus; or.p, oral papilla; ph, pharynx; R, rectum; s.d, salivary duct; s.g, salivary gland; sl.d, slime reservoir; sl.g, portion of tubules of slime gland; st, stomach; T, tongue in roof of mouth.

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.

EB1911 Peripatus - P. capensis - brain and anterior nerves.jpg
(After Balfour.)

Fig. 7.—Brain and anterior part of the ventral nerve-cords of Peripatus capensis enlarged and viewed from the ventral surface.

atn, Antennary nerves; co, commissures between ventral cords; d, ventral appendages of brain; E, eye; en, nerves passing outwards from ventral cord; F.g.1, ganglionic enlargements from which nerves to feet pass off; jn, nerves to jaws; org, ganglionic enlargement from which nerves to oral papillae pass off; orn, nerves to oral papillae; pc, posterior lobe of brain; pn, nerves to feet; sy, sympathetic nerves.

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.

EB1911 Peripatus - P. capensis - tracheal pit and tubes.jpg
(After Balfour.)

Fig. 8.—Section through a tracheal pit and diverging bundles of tracheal tubes taken transversely to the long axis of the body.

tr, Tracheae, showing rudimentary spiral fibre; tr.c, Cells resembling those lining the tracheal pits, which occur at intervals along the course of the tracheae; tr.o, Tracheal stigma; tr.p, Tracheal pit.

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.

EB1911 Peripatus - P. capensis - nephridium.jpg
Fig. 9.—Nephridium from the ninth pair of legs of P. capensis.
o.s, External opening of segmental organ.
p.f, Internal opening of nephridium into the body cavity (lateral compartment).
s, Vesicle of segmental organ.
s.c.1, s.c.2, s.c.3, s.c.4,
Successive regions of coiled portion of nephridium.
s.o.t, Third portion of nephridium broken off at p.f from the internal vesicle, which is not shown.

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 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.

EB1911 Peripatus - P. capensis - Male Generative Organs.jpg
(After Balfour.)
Fig. 10.—Male Generative Organs of Peripatus capensis. Dorsal view.
a.g, Enlarged crural glands of last pair of legs.
F.16, 17,
Last pair of legs.
f, Small accessory glandular tubes.
p, Common duct into which vasa deferentia open.
te, Testes.
v, Seminal vesicles.
v.c, Nerve-cord.
v.d, Vas deferens.

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.

EB1911 Peripatus - P. capensis - Series of Embryos.jpg
(After Sedgwick.)

Fig. 11.—A Series of Embryos of P. capensis. The hind end of embryos B, C, D is uppermost in the figures, the primitive streak is the white patch behind the blastopore.

A, Gastrula stage, ventral view, showing blastopore.
B, Older gastrula stage, ventral view, showing elongated blastopore and primitive streak.
C, Ventral view of embryo with three pairs of mesoblastic somites, dumb-bell shaped blastopore and primitive streak.
D, Ventral view of embryo, in which the blastopore has completely closed in its middle portion. The anterior pair of somites have moved to the front end of the body.
E, side view of later embryo. At, Antenna; d, dorsal projection; p.s., praeoral somite.
F, Ventral view of head of embryo, intermediate between E and G. At, Antennae; c.g, cerebral groove; j, jaws; j.s. swelling at base of jaws; L, lips; M, mouth; or.p, oral papillae; o.s, opening of salivary gland.
G, side view of older embryo.

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, and becomes entirely transformed into a nephridium (fig. 12, D, 2′) The dorsal part shifts dorsal wards and diminishes relatively in size (fig. 12, C). Its fate differs in the different parts of the body. In the anterior somites it dwindles and disappears, but in the posterior part it unites with the dorsal divisions of contiguous somites of the same side, and forms a tube—the generative tube (fig. 12, D, 2) The last section of this tube retains its connexion with the ventral portion of the somite, and so acquires an external opening, which is at first lateral, but soon shifts to the middle line, and fuses with its fellow, to form the single generative opening. The praeoral somite develops the rudiment of a nephridium, but eventually entirely disappears. The jaw somite also disappears; the oral papilla somite forms ventrally the salivary glands, which are thus serially homologous with nephridia. The various divisions of the perivisceral cavity develop as a series of spaces between the ectoderm and endoderm, and later in the mesoderm The mesoderm seems to be formed entirely from the proliferation of the cells of the mesoblastic somites. It thus appears that in Peripatus the coelom does not develop a perivisceral portion, but gives rise only to the renal and reproductive organs.

EB1911 Peripatus - Series of Embryos.jpg
(After Sedgwick.)

Fig. 12.—A series of diagrams of transverse sections through Peripatus embryos to show the relations of the coelom at successive stages.

A, Early stage; no trace of the vascular space; endoderm and ectoderm in contact.

B, Endoderm has separated from the dorsal and ventral ectoderm. The somite is represented as having divided on the left side into a dorsal and ventral portion.

C, The haemocoele (3) has become divided up into a number of spaces, the arrangement of which is unimportant. The dorsal part of the somite has travelled dorsalwards, and now constitutes a small space (triangular in section) just dorsal to the gut. The ventral portion (2) has assumed a tubular character, and has acquired an external opening. The internal vesicle is already indicated, and is shown in the diagram by the thinner black line: 1, gut; 2, somite; 2′, nephridial part of coelom; 3, haemocoele; 3′, part of haemocoele which will form the heart—the part of the haemocoele on each side of this will form the pericardium; 4, nerve-cord; 4, slime glands.

D represents the conditions at the time of birth. The coelom is represented as surrounded by a thick black line, except in the part which forms the internal vesicle of the nephridium.

The genus Peripatus was established in 1826 by L. Guilding, who first obtained specimens of it from St Vincent in the Antilles. He regarded it as a mollusc, being no doubt deceived by the slug-like appearance given by the antennae. Specimens were subsequently obtained from other parts of the neotropical region, and from South Africa and Australia, and the animal was variously assigned by the zoologists of the day to the Annelida and Myriapoda. Its true place in the system, as a primitive member of the group Arthropoda, was first established in 1874 by H. N. Moseley who discovered the tracheae. Peripatus is an Arthropod, as shown by (1) the presence of appendages modified as jaws, (2) the presence of paired lateral ostia perforating the wall of heart and putting its cavity in communication with the pericardium; (3) the presence of a vascular body cavity and pericardium (haemocoelic body cavity), (4) absence of a perivisceral section of the coelom. Finally, the tracheae, though not characteristic of all the classes of the Arthropoda, are found nowhere outside that group, and constitute a very important additional reason for uniting Peripatus with it. Peripatus, though indubitably an Arthropod, differs in such important respects from all the old-established Arthropod classes, that a special class, equivalent in rank to the others, and called Prototracheata or Onychophora, has had, as we have seen, to be created for its sole occupancy. This unlikeness to other Arthropoda is mainly due to the Annelidan affinities which it presents, but in part to the presence of the following peculiar features: (1) the number and diffusion of the tracheal apertures; (2) the restriction of the jaws to a single pair, (3) the disposition of the generative organs, (4) the texture of the skin; and (5) the simplicity and similarity of all the segments of the body behind the head. The Annelidan affinities are superficially indicated in so marked a manner by the thinness of the cuticle, the dermo-muscular body-wall, the hollow appendages, that, as already stated, many of the earlier zoologists who examined Peripatus placed it among the segmented worms; and the discovery that there is some solid morphological basis for this determination constitutes one of the most interesting points of the recent work on the genus. The Annelidan features are: (1) the paired nephridia in every segment of the body behind the first two (Saenger, Balfour); (2) the presence of cilia in the generative tracts (Gaffron). It is true that neither of these features is absolutely distinctive of the Annelida, but when taken in conjunction with the Annelidan disposition of the chief systems of organs, viz. the central nervous system, and the main vascular trunk or heart, they may be considered as indicating affinities in that direction.

Synopsis of Species.

Peripatus (Guilding).—Soft-bodied vermiform animals, with one pair of ringed antennae, one pair of jaws, one pair of oral papillae, and a varying number of claw-bearing ambulatory legs. Dorsal surface arched and more darkly pigmented than the flat ventral surface. Skin transversely ridged and beset by wart-like spiniferous papillae. Mouth anterior, ventral; anus posterior, terminal. Generative opening single, median, ventral and posterior. One pair of simple eyes. Brain large, with two ventral hollow a pendages; ventral cords widely divaricated, without distinct ganglia. Alimentary canal simple, uncoiled. Segmentally arranged paired nephridia are present. Body cavity is continuous with the vascular system, and does not communicate with the paired nephridia. Heart tubular, with paired ostia. Respiration by means of tracheae. Dioecious; males smaller and generally less numerous than females. Generative glands tubular, continuous with the ducts. Viviparous. Young born fully developed. Distribution: Africa (Cape Colony, Natal, and the Gaboon), New Zealand, Australia and Tasmania, New Britain, South and Central America and the West Indies, the Malay Peninsula [and in Sumatra?].

The genus Peripatus, so far as adult conformation is concerned, is a very homogeneous one. It is true, as was pointed out by Sedgwick, that the species from the same part of the world resemble one another more closely than they do species from other regions, but recent researches have shown that the line between them cannot be so sharply drawn as was at first supposed, and it is certainly not desirable in the present state of our knowledge to divide them into generic or subgeneric groups, as has been done by some zoologists. (The following genera have been proposed: Peripatus for the neotropical species, Peripatoides for the Australasian, Perzpatopsis and Opisthopatus for the African. Paraperipatus for the New Britain, Eoperipatus for the Malayan species, and Ooperipatus for the suppose oviparous species of Australia and New Zealand.) The colour is highly variable in species from all regions; it is perhaps more constant in the species from the neotropical region than in those from elsewhere. The number of legs tends to be variable whenever it exceeds 19 praegenital pairs; when the number is less than that it is usually, though not always, constant. More constant points of difference are the form of the jaws, the position of the generative orifice, the presence of a receptaculum seminis and a receptaculum ovorum, the arrangement of the primary papillae on the distal end of the feet, and above all the early development.

South African Species.—With three spinous pads on the legs, and feet with two primary papillae on the anterior side and one on the posterior side; outer jaw with one minor tooth at the base of the main tooth, inner jaw with no interval between the large tooth and the series of small ones; last fully developed leg of the male with enlarged crural gland opening on a large papilla placed on its ventral surface, coxal organs absent; the nephridial openings of the 4th and 5th pairs of legs are placed in the proximal spinous pad. Genital opening subterminal, behind the last pair of fully developed legs; oviduct without receptacula seminis or receptacula ovorum; the terminal unpaired portion of vas deferens short. Ova of considerable size, but with only a small quantity of yolk. The embryos in the uterus are all nearly of the same age, except for a month or two before birth, when two broods overlap.

The following species are aberrant in respect of these characters: Peripatus (Opisthopatus) cinctipes, Purcell (Cape Colony and Natal), presents a few Australasian features; there is a small receptaculum seminis on each oviduct, some of the legs are provided with well developed coxal organs, the feet have one anterior, one posterior and one dorsal papilla, and the successive difference in the ages of the embryos in the uterus, though nothing like that found in the neotropical species, is slightly greater than that found in other investigated African species. Several pairs of legs in the middle region of the body are provided with enlarged crural glands which open on a large papilla. Male with four accessory glands, opening on each side of and behind the genital aperture. P. thollons, Bouvier, (Equatorial West Africa [Gaboon]), shows some neotropical features; there are 24 to 25 pairs of legs, the genital opening is between the penultimate legs, and though there are only three spinous pads the nephridial openings of the 4th and 5th legs are proximal to the 3rd pad, coxal organs are present, and the jaws are of the neotropical type, the oviducts have receptacula seminis. The following South African species may be mentioned: P. capensis (Grube), with 17 (rarely 18) pairs of claw-bearing legs; P. balfouri (Sedgw.) with 18 (rarely 19) pairs; P. moseleyi (Wood-M.), with 20 to 24 pairs.

Australasian Species.—With 14, 15 or 16 pairs of claw-bearing ambulatory legs, with three spinous pads on the legs, and nephridial opening of the 4th and 5th legs on the proximal pad, feet with one anterior, one posterior and one dorsal primary papilla; inner jaw without diastema, outer with or without a minor tooth. Last leg of the male with or without a large white papilla on its ventral surface for the opening of a gland, and marked papillae for the crural glands are sometimes present on other legs of the male; well-developed coxal glands absent. Genital opening between the legs of the last pair; oviducts with receptacua seminis, without receptacula ovorum; the terminal portion of the vas deferens long and complicated; the accessory male glands open between the genital aperture and the anus, near the latter. Ova large and heavily charged with yolk, and provided with a stoutish shell. The uterus appears to contain embryos of different ages. Specimens are recorded from West Australia, Queensland, New South Wales, Victoria and New Zealand. The Australasian species are in some confusion. The number of claw-bearing legs varies from 14 to 16 pairs, but the number most often found is 15. Whether the number varies in the same species is not clear. There appears to be evidence that some species are occasionally or normally oviparous, and in the supposed oviparous species the oviduct opens at the end of a papilla called from its supposed function an ovipositor, but the oviparity has not yet been certainly proved as a normal occurrence. Among the species described may be mentioned P. leuckarti (Saenger), P. insignis (Dendy), P. oviparus (Dendy), P. viridimaculatus (Dendy), P. novae zealandiae (Hutton), but it is by no means certain that future research will maintain these. Mr J. J. Fletcher, indeed, is of opinion that the Australian forms are all varieties of one species, P. leuckarti.

Neotropical Species.—With three to five spinous pads on the legs, nephridial opening of the 4th and 5th legs usually proximal to the 3rd pad, and feet either with two primary papillae on the anterior side and one on the posterior, or with two on the anterior and two on the posterior; outer jaw with small minor tooth or teeth at the base of the main tooth, inner jaw with diastema. A variable number of posterior legs of the males anterior to the genital opening with one or two large papillae carrying the openings of the crural glands; well-developed coxal organs present on most of the legs. The primary papillae usually divided into two portions. Genital opening between the legs of the penultimate pair; oviduct provided with receptacula seminis and ovorum; unpaired part of vas deferens long and complicated; accessory organs of male opening at the sides of the anus. Ova minute, with little food-yolk; embryos in the uterus at very different stages of development. The number of legs usually if not always variable in the same species; the usual number is 28 to 32 pairs, but in some species 40 to 43 pairs are found. The neotropical species appear to fall into two groups: (1) the so-called Andean species, viz. those which inhabit the high plateaus or Pacific slope of the Andes; in these there are 4 (sometimes 5) pedal papillae, and the nephridial openings of the 4th and 5th legs are on the third pad; and (2) the Caribbean species, viz. the remaining neotropical species, in which there are 3 papillae on the foot and the nephridial openings of the 4th and 5th legs are between the 3rd and 4th pads. The Andean species are P. eisenii (Wh.), P. tuberculatus (Bouv.), P. lankesteri (Bouv.), P. quitensis (Schm.), P. corradi (Cam.), P. cameranoi (Bouv.) and P. balzani (Cam.). Of the remaining species, which are the majority, may be mentioned P. edwardsii (Blanch), P. jamaicensis (Gr. and Cock.), P. trinidadensis (Sedgw.), P. torquatus (Ken.), P. im thurmi (Scl.).

New Britain Peripatus.—With 22 to 24 pairs of claw-bearing legs, with three spinous pads on the legs, and nephridial openings of legs 4 and 5 (sometimes of 6 also) on the proximal pad; feet with one primary papilla on the anterior, one on the posterior side, and one on the dorsal side (median or submedian); outer jaw with a minor tooth, inner jaw without diastema; crural glands absent; well-developed coxal organs absent. Genital opening subterminal behind the last pair of legs; oviduct with receptaculum seminis, without receptaculum ovorum; unpaired part of vas deferens very short; accessory glands two, opening medianly and dorsally. Ova small, .1 mm. in diameter, with little yolk, and the embryos provided with large trophic vesicles (Willey). Embryos in the uterus of very different ages, and probably born all the year round. One species only known, P. novae britanniae (Willey).

Sumatran Peripatus.—Peripatus with 24 pairs of ambulatory legs, and four spinous pads on the legs. The primary papillae of the neotropical character with conical bases. Generative opening between the legs of the penultimate pair. Feet with only two papillae. Single species. P. sumatranus (Sedgw.). The existence of this species is doubtful.

Peripatus from the Malay Peninsula.—With 23 to 25 pairs of claw-bearing legs, four spinous pads on the legs, and nephridial openings of legs 4 and 5 in the middle of the proximal pad or on its proximal side; feet with two primary papillae, one anterior and one posterior; outer jaw with two, inner jaw with two or three minor teeth at the base of the main tooth, separated by a diastema from the row of small teeth; crural glands present in the male only, in the two pairs of legs preceding the generative opening; coxal glands present. Genital opening between the penultimate legs; oviduct with receptacula seminis and ovorum; unpaired part of vas deferens long; male accessory glands two, opening medianly between the legs of the last pair. Ova large, with much yolk and thick membrane, like those of Australasian species; embryos with slit-like blastopore and of very different ages in the same uterus, probably born all the year round. The species are P. weldoni (Evans), P. horsti (Evans) and P. butleri (Evans). It will thus be seen that the Malay species, while resembling the neotropical species in the generative organs, differ from these in many features of the legs and feet, in the important characters furnished by the size and structure of the ovum, and by their early development.

Authorities.—F. M. Balfour, “The Anatomy and Development of P. capensis,” posthumous memoir, edited by H. N. Moseley and A. Sedgwick, Quart. Journ. Mic. Sci. vol. xxiii. (1883); E. L. Bouvier, “Sur l'organisation du Peripatus tholloni, Bouv.,” Comptes rendus, cxxvi. 1358-1361 (1898); “Contributions à l'histoire des Péripates Americains,” Ann. de la société entomologique de France, lxviii. 385-450 (1899); “Quelques observations sur les onychophores du musée britannique,” Quart. Journ. Mic. Sci xliii. 367 (1900); A. Dendy, “On the Oviparous Species of Onychophorea,” Quart. Journ. Mic. Sci. xlv. 362 (1902); R. Evans, “On Onychophora from the Siamese Malay States,” Quart. Journ. Mic. Sci. xliv. 473 (1901), and “On the Development of Ooperipatus,” ibid. xlv. 1 (1901); J. J. Fletcher, “On the Specific Identity of the Australian Peripatus, usually supposed to be P. leuckarti, Saenger,” Proc. Linn. Soc. New South Wales, x. 172 (1895); E. Gaffron, “Beiträge z. Anat. u. Physiol. v. Peripatus,” Th. 1 and 2, Zool. Beiträge (Schneider), i. 33, 145; L. Guilding, “Mollusca caribbaeana: an account of a new genus of Mollusca,” Zool. Journ. ii. 443, pl. 14 (1826); reprinted in Isis, xxi. 158, pl. ii. (1828); H. N. Moseley, “On the Structure and Development of Peripatus capensis,” Phil. Trans. (1874); R. I. Pocock, “Contributions to our Knowledge of the Arthropod Fauna of the West Indies,” pt. 2, Malacopoda, &c, Journ. Linn. Soc. xxiv. 518; W. F. Purcell, “On the South African Species of Peripatus,” &c., Annals of the South African Museum, i. 331 (1898-1899); and “Anatomy of Opisthopatus cinctipes,” ibid. vol. ii. (1900); W. L. Sclater, “On the Early Stages of the Development of a South American Species of Peripatus,” Quart. Journ. of Mic. Sci. xxviii. 343-361 (1888); A. Sedgwick, “A Monograph of the Development of Peripatus capensis” (originally published in various papers in the Quart. Journ. Mic. Sci., 1885-1888); Studies from the Morphological Lab. of the University of Cambridge, iv. 1-146 (1889); “A Monograph of the Species and Distribution of the Genus Peripatus, Guilding,” Quart. Journ. Mic. Sci. xxviii. 431-494 (1888); L. Sheldon, “On the Development of Peripatus novae zealandiae,” pts. 1 and 2, Quart. Journ. Mic. Sci. xxviii. and xxix. (1888 and 1889). The memoirs quoted by Sclater, Sedgwick and Sheldon are all reprinted in vol. iv. of the Studies from the Morphological Lab. of the University of Cambridge, vol. iv. (Cambridge University Press, 1889). T. Steel, “Observations on Peripatus,” Proc. Linn. Soc. New South Wales, p. 94 (1896); A. Willey, “The Anatomy and Development of P. novae britanniae,” Zoological Results, pt. 1, pp. 1-52 (Cambridge, 1898).

(A. Se.*)