1. Section shewing a deep pit in front of primitive streak, probably an early indication of the neurenteric canal. 2. Section immediately following 1.

Fig. G. Section through blastoderm with well-developed primitive streak, shewing an exceptionally deep slit-like primitive groove.

Series H, 1 and 2. Sections through a blastoderm with a fully-developed primitive streak.

1. Section through the anterior part of area pellucida, shewing the cubical granular hypoblast cells in this region. 2. Section slightly behind 1, shewing the primitive hypoblast cells differentiated into stellate cells, which can hardly be resolved in the middle line into hypoblast and mesoblast.

Plate 45.

Series I, 1, 2, 3, 4 and 5. Sections through blastoderm somewhat older than Series H.

1. Section through area pellucida well in front of primitive streak. 2. Section through area pellucida just in front of primitive streak. 3. Section through the front end of primitive streak. 4. Section slightly behind 3. 5. Section slightly behind 4.

Series K, 1, 2, 3, 4 and 5. Sections through a blastoderm in which the first traces of notochord and medullary groove have made their appearance. Rather more than half the section is represented in each figure, but the right half is represented in 1 and 3, and the left in 2 and 4.

1. Section through notochord immediately behind the head fold. 2. Section shewing medullary groove a little behind 1. 3. Section just in front of the primitive streak. 4 and 5. Sections through the front end of the primitive streak.

Fig. L. Surface view of blastoderm with a very young primitive streak.

PART I.

Description of the Species.

Peripatus capensis (fig. 1).

[The body is elongated, and slightly flattened dorso-ventrally. The dorsal surface is arched, and darkly pigmented; while the ventral surface is nearly flat, and of a lighter colour.

The mouth is placed at the anterior end of the body, on the ventral surface.

The anus is posterior and terminal.

The generative opening is single and median, and placed in both sexes on the ventral surface, immediately in front of the anus.

There are a pair of ringed antennæ projecting from the anterior end of the head, and a pair of simple eyes, placed on the dorsal surface at the roots of the antennæ.

The appendages of the body behind the antennæ are disposed in twenty pairs.

1. The single pair of jaws placed within the buccal cavity in front of the true mouth opening, and consisting each of a papilla, armed at its termination with two cutting blades.

2. The oral papillæ placed on each side of the mouth. At their apices the ducts of the slime glands open.

3. The seventeen pairs of ambulatory appendages, each provided with a pair of chitinous claws at its extremity.

4. The anal papillæ placed on each side of the generative opening.

Colour.—The following statements on this head are derived from observations of spirit specimens. The colour varies in different individuals. It always consists of a groundwork of green and bluish grey, with a greater or less admixture of brown. The chief variations in the appearance of the animal, so far as colour is concerned, depend on the shade of the green. In some it is dark, as in the specimen figured (fig. 1); in others it is of a lighter shade.

There is a dark line running down the middle of the dorsal surface, in the middle of which is a fine whitish line.

The ventral surface is almost entirely free from the green pigment, but possesses a certain amount of light brown. This brown pigment is more conspicuous and of a darker shade on the spinous pads of the foot.

In parts of the body where the pigment is scarce, it is seen to be confined to the papillæ. This is especially evident round the mouth, where the sparse green pigment is entirely confined to the papillæ.

In some specimens a number of white papillæ, or perhaps light brown, are scattered over the dorsal surface; and sometimes there is a scattering of green papillæ all over the ventral surface. These two peculiarities are more especially noticeable in small specimens.

Ridges and Papillæ of the Skin.—The skin is thrown into a number of transverse ridges, along which the primary wart-like papillæ are placed.

The papillæ, which are found everywhere, are specially developed on the dorsal surface, less so on the ventral. The papillæ round the lips differ from the remaining papillæ of the ventral surface in containing a green pigment. Each papilla bears at its extremity a well-marked spine.

The ridges of the skin are not continued across the dorsal middle line, being interrupted by the whitish line already mentioned. Those which lie in the same transverse line as the legs are not continued on to the latter, but stop at the junction of the latter with the body. All the others pass round to the ventral surface and are continued across the middle line; they do not, however, become continuous with the ridges of the other side, but passing between them gradually thin off and vanish.

The ridges on the legs are directed transversely to their long axes, i.e. are at right angles to the ridges of the rest of the body.

The rings consist essentially of a number of coalesced primary papillæ, and are, therefore, beset by a number of spines like those of the primary papillæ (described below). They are more deeply pigmented than the rest of the antenna.

The free end of the antenna is covered by a cap of tissue like that of the rings. It is followed by four or more rings placed close together on the terminal enlargement. There appears to be about thirty rings on the antennæ of all adults of this species. But they are difficult to count, and a number of small rings occur between them, which are not included in the thirty.

The antennæ are prolongations of the dorso-lateral parts of the anterior end of the body.

The eyes are paired and are situated at the roots of the antennæ on the dorso-lateral parts of the head. Each is placed on the side of a protuberance which is continued as the antenna, and presents the appearance of a small circular crystalline ball inserted on the skin in this region.

The rings of papillæ on that part of the head from which the antennæ arise lose their transverse arrangement. They are arranged concentrically to the antennal rings, and have a straight course forwards between the antennæ.

The oral papillæ are placed at the side of the head. They are attached ventro-laterally on each side of the lips. The duct of the slime gland opens through their free end. They possess two main rings of projecting tissue, which are especially pigmented on the dorsal side; and their extremities are covered by papillæ irregularly arranged.

The buccal cavity, jaws, and lips are described below.

The Ambulatory Appendages.—The claw-bearing legs are usually seventeen in number; but in two cases of small females we have observed that the anal papillæ bear claws, and present all the essential features of the ambulatory appendages. In one small female specimen there were twenty pairs of claw-bearing appendages, the last being like the claw-bearing anal papillæ last mentioned, and the generative opening being placed between them.

The leg has the form of a truncated cone, the broad end of which is attached to the ventro-lateral body-wall, of which it appears to be, and is, a prolongation. It is marked by a number of rings of primary papillæ, placed transversely to the long axis of the leg, the dorsal of which contain a green and the ventral a brown pigment. These rings of papillæ, at the attachment of the leg, gradually change their direction and merge into the body rings. At the narrow end of the cone there are three ventrally placed pads, in which the brown pigment is dark, and which are covered by a number of spines precisely resembling the spines of the primary papillæ. These spinous pads are continued dorsally, each into a ring of papillæ.

The papillæ of the ventral row next the proximal of these spinous pads are intermediate in character between the primary papillæ and the spinous pads. Each of these papillæ is larger than a normal papilla, and bears several spines (fig. 2). This character of the papilla of this row is even more marked in some of the anterior legs than in the one figured; it seems probable that the pads have been formed by the coalescence of several rows of papillæ on the ventral surface of the legs. On the outer and inner sides of these pads the spines are absent, and secondary papillæ only are present.

The foot is attached to the distal end of the leg. It is slightly narrower at its attached extremity than at its free end, which bears the two claws. The integument of the foot is covered with secondary papillæ, but spines and primary papillæ are absent, except at the points now to be described.

On each side of the middle ventral line of the proximal end of the foot is placed an elliptical elevation of the integument covered with spines. Attached to the proximal and lateral end of this is a primary papilla. At the distal end of the ventral side of the foot on each side of the middle line is a group of inconspicuous pale elevations, bearing spines.

On the front side of the distal end of the foot, close to the socket in which the claws are placed, are two primary papillæ, one dorsal and the other ventral.

On the posterior side of the foot the dorsal of these only is present. The claws are sickle-shaped, and placed on papillæ on the terminal portion of the foot. The part of the foot on which they are placed is especially retractile, and is generally found more or less telescoped into the proximal part (as in the figure).

The fourth and fifth pairs of legs exactly resemble the others, except in the fact that the proximal pad is broken up into three, a small central and two larger lateral. The enlarged segmental organs of these legs open on the small central division.

The last (17) leg of the male (Pl. 47, fig. 4) is characterized by possessing a well-marked white papilla on the ventral surface. This papilla, which presents a slit-like opening at its apex, is placed on the second row of papillæ counting from the innermost pad, and slightly posterior to the axial line of the leg.

The anal papillæ, or as they should be called, generative papillæ, are placed one on each side of the generative aperture. They are most marked in small and least so in large specimens. That they are rudimentary ambulatory appendages is shewn by the fact that they are sometimes provided with claws, and resemble closely the anterior appendages.]

PART II.

Alimentary Canal.

The alimentary canal of Peripatus capensis forms, in the extended condition of the animal, a nearly straight tube, slightly longer than the body, the general characters of which are shewn in figs. 6 and 7.

For the purposes of description, it may conveniently be divided into five regions, viz. (1) the buccal cavity with the tongue, jaws, and salivary glands, (2) pharynx, (3) the œsophagus, (4) the stomach, (5) the rectum.

The Buccal Cavity.—The buccal cavity has the form of a fairly deep pit, of a longitudinal oval form, placed on the ventral surface of the head, and surrounded by a tumid lip.

[The buccal cavity has been shewn by Moseley to be formed in the embryo by the fusion of a series of processes surrounding the true mouth-opening, and enclosing in their fusion the jaws.]

The lip is covered by a soft skin, in which are numerous organs of touch, similar to those in other parts of the skin having their projecting portions enclosed in delicate spines formed by the cuticle. The skin of the lips differs, however, from the remainder of the skin, in the absence of tubercles, and in the great reduction of the thickness of the dermis. It is raised into a series of papilliform ridges, whose general form is shewn in fig. 5; of these there is one unpaired and median behind, and a pair, differing somewhat in character from the remainder, in front, and there are, in addition, seven on each side.

The greater part of the buccal cavity is filled with the tongue and jaws just described. It opens dorsally and behind by the mouth into the pharynx, there being no sharp line of demarcation between the buccal cavity and the pharynx. Behind the opening into the pharynx there is a continuation of the buccal cavity shewn in transverse section in fig. 13, and in longitudinal and horizontal section in fig. 17, into which there opens the common junction of the two salivary glands. This diverticulum is wide at first and opens by a somewhat constricted mouth into the pharynx above (Pl. 49, fig. 13, also shewn in longitudinal and horizontal section in fig. 17). Behind it narrows, passing insensibly into what may most conveniently be regarded as a common duct for the two salivary glands (Pl. 49, fig. 17).

The cells lining the ducts are considerably less columnar than those of the gland proper. Their nuclei (fig. 14) are situated at the free extremities instead of at the base of the cells, and they are without secretory globules. The cells lining the ducts of the salivary glands pass, without any sharp line of demarcation, into those of the oral epithelium, which are flatter and have their nuclei placed in the middle.

The Pharynx.—The Pharynx is a highly muscular tube (fig. 7) with a triangular lumen (figs. 14, 15), which extends from the mouth to about half way between the first and second pair of legs. It is lined by a flattish epithelium bounded by a cuticle continuous with that of the mouth. On the dorsal side is a ridge projecting into the lumen of the pharynx. This ridge may be traced forwards (Pl. 49, figs. 11-14) into the tongue, and the two grooves at the side of this ridge, forming the two upper angles of the triangular lumen, may be followed into those at the sides of the tongue. The muscles of the pharynx are very highly developed, consisting of an intrinsic and an extrinsic set. The former consists, as is best seen in longitudinal sections, of (Pl. 51, fig. 23) radial fibres, arranged in somewhat wedge-shaped laminæ, between which are rings of circular fibres. The latter are thicker externally than internally, and so also appear wedge-shaped in longitudinal sections. Very characteristic of the pharynx are the two sympathetic nerves placed close to the two dorsal angles of the triangular lumen (fig. 14, sy).

The pharynx of Peripatus is interesting in that it is unlike, so far as I know, the pharynx of any true Arthropod, in all of which the region corresponding with the pharynx of Peripatus is provided with relatively very thin walls.

The pharynx of Peripatus has, on the other hand, a very close and obvious resemblance to that of many of the Chætopoda, a resemblance which is greatly increased by the characteristic course of the sympathetic nerves.

The form of the lumen, as already pointed out by Grube, resembles that of the Nematoda.

(1) A peritoneal investment.

(2) A layer of longitudinal fibres.

(3) A layer of circular fibres, amongst which are numerous nuclei.

(4) A connective-tissue layer supporting (5) a layer of fairly columnar hyaline epithelium, bounded on its inner aspect by a cuticle continued from that of the pharynx. In front it passes insensibly into the pharynx, and beyond the region where the dorsal walls of the pharynx have clearly commenced, the ventral walls still retain the characters of the œsophageal walls. The œsophagus is vertically oval in front, but more nearly circular behind. Characteristic of the œsophagus is the junction of the two sympathetic nerves on its dorsal wall (fig. 16). These nerves cannot be traced far beyond their point of junction.

The Stomach.—The next section of the alimentary tract is the stomach or mesenteron (fig. 6). It is by far the largest part of the alimentary tract, commencing at about the second pair of legs and extending nearly to the hind end of the body. It tapers both in front and behind, and is narrowest in the middle, and is marked off sharply both from the œsophagus in front and the rectum behind, and is distinguished from both of these by its somewhat pinker hue. In the retracted condition of the animal it is, as pointed out by Moseley, folded in a single short dorsal loop, at about the junction of its first with its second third, and also, according to my observations, at its junction with the rectum; but in the extended condition it is nearly straight, though usually the posterior fold at the junction of the rectum is not completely removed. Its walls are always marked by plications which, as both Moseley and Grube have stated, do not in any way correspond with the segmentation of the body. In its interior I have frequently found the chitinous remains of the skins of insects, so that we are not justified in considering that the diet is purely vegetable. It lies free, and is, like the remainder of the alimentary tract, without a mesentery. The structure of the walls of the stomach has not hitherto been very satisfactorily described.

The layer supporting the epithelium is reduced to a basement membrane. The epithelial part of the wall of the stomach is by far the thickest (fig. 20), and is mainly composed of enormously elongated, fibre-like cells, which in the middle part of the stomach, where they are longest, are nearly half a millimètre in length, and only about .006 mm. in breadth. Their nuclei, as seen in fig. 20, are very elongated, and are placed about a quarter of the length from the base.

The cells are mainly filled with an immense number of highly refracting spherules, probably secretory globules, but held by Grube, from the fact of their dissolving in ether, to be fat. The epithelial cells are raised into numerous blunt processes projecting into the lumen of the stomach.

In addition to the cells just described there are present in the anterior part of the stomach a fair sprinkling of mucous cells. There are also everywhere present around the bases of the columnar cells short cells with spherical nuclei, which are somewhat irregularly scattered in the middle and posterior parts of the stomach, but form in the front part a definite layer. I have not been able to isolate these cells, and can give no account of their function.

Nervous System.

The central nervous system consists of a pair of supra-œsophageal ganglia united in the middle line, and of a pair of widely divaricated ventral cords, continuous in front with the supra-œsophageal ganglia.

It will be convenient in the first instance to deal with the general anatomy of the nervous system and then with the histology.

Ventral Cords.—The ventral cords at first sight appear to be without ganglionic thickenings, but on more careful examination they are found to be enlarged at each pair of legs (Pl. 48, fig. 8). These enlargements may be regarded as imperfect ganglia. There are, therefore, seventeen such pairs of ganglia corresponding to the seventeen pairs of legs. There is in addition a ganglionic enlargement at the commencement of the œsophageal commissures, where the nerves to the oral papillæ are given off (Pl. 51, fig. 22, or.g.), and the region of junction between the œsophageal commissures with the supra-œsophageal ganglia, where another pair of nerves are given off to the jaws (Pl. 51, fig. 22, jn), may be regarded as the anterior ganglion of the ventral cords. There are, therefore, according to the above reckoning, nineteen pairs of ganglia connected with the ventral cords.

The ventral cords are placed each in the lateral compartments of the body-cavity, immediately within the longitudinal layer of muscles.

In Peripatus novæ zealandiæ, and probably also in P. capensis, two of these nerves, coming off from each pair of ganglia, are distinguished from the remainder by the fact that they are provided with numerous nerve-cells, instead of being composed of nerve-fibres only, like the remaining commissures (Pl. 52, fig. 26 gco). In correlation with the nerves given off from them to the skin the commissures are smaller in the middle than at the two ends.

Posteriorly the two nerve-cords nearly meet immediately in front of the generative aperture, and between this aperture and the last pair of feet there are about six commissures passing between them (Pl. 48, fig. 8). Behind the generative aperture the two cords bend upwards, and, as is shewn in fig. 8, fall into each other dorsally to the rectum. The section of the two cords placed dorsally to the rectum is solely formed of nerve-fibres; the nerve-cells, present elsewhere, being here absent.

In front of the ganglion of the first foot the commissures have a more dorsal situation than in the remainder of the body. The median longitudinal ventral muscle here gradually thins out and comes to an end, while the commissures pass immediately below the wall of the pharynx (Pl. 49, figs. 14, 15). The ventral cords themselves at first approach very close to each other in this region, separating again, however, to envelope between them the pharynx (Pl. 51, fig. 22).

There are eleven commissures in front of the first pair of legs (Pl. 51, fig. 22). The three foremost of these are very close together, the middle one arising in a more ventral position than the other two, and joining in the median ventral line a peculiar mass of cells placed in contact with the oral epithelium (fig. 14). It is probably an organ of special sense.

In front of each of the pair of pedal nerves a fairly large nerve may be seen passing outwards towards the side of the body (fig. 22). In addition to this nerve there are a number of smaller nerves passing off from the main trunk, which do not appear to be quite constant in number, but which are usually about seven or eight. Similar nerves to those behind are given off from the region in front of the first pair of legs, while at the point where the two ventral cords pass into the œsophageal commissures two large nerves (fig. 22), similar to the pairs of pedal nerves, take their origin. These nerves may be traced forwards into the oral papillæ, and are therefore to be regarded as the nerves of these appendages. On the ventral side of the cords, where they approach most closely, between the oral papillæ and the first pair of legs, a number of small nerves are given off to the skin, whose distribution appears to be to the same region of the skin as that of the branches from the commissures behind the first pair of legs.

From the œsophageal commissures, close to their junction with the supra-œsophageal ganglia, a nerve arises on each side which passes to the jaws, and a little in front of this, apparently from the supra-œsophageal ganglion itself, a second nerve to the jaws also takes its origin (Pl. 51, fig. 22, jn). These two nerves I take to be homologous with a pair of pedal nerves.

Between the nerves to the jaws and those to the oral papillæ a number of small nerves take their origin. Three of these on each side pass in a dorsal direction and one or two in a ventral one.

A number of nerves arise from the supra-œsophageal ganglia, mainly from their dorsal surface.

In front are the immense antennary nerves extending along the whole length of each antenna, and giving off numerous lateral twigs to the sense organs. Near the origin of the antennary nerves, and rather on the dorsal surface, there spring a few small twigs, which pass to the skin, and are presumably sensory. The largest of them is shewn in Pl. 50, fig. 19A. About one-third of the way back the two large optic nerves take their origin, also arising laterally, but rather from the dorsal surface (Pl. 50, fig. 19D and E). Each of them joins a large ganglionic mass placed immediately behind the retina. Nearly on a level with the optic nerves and slightly nearer the middle dorsal line a pair of small nerves (fig. 19D) spring from the brain and pass upwards, while nearly in the same line with the optic nerves and a little behind them a larger pair of nerves take their origin.

Behind all these nerves there arises from the line of suture between the two supra-œsophageal ganglia a large median nerve which appears to supply the integument of the dorsal part of the head (Pl. 48, fig. 8; Pl. 49, figs. 11-14, dn).

Sympathetic System.—In addition to the nerves just described there are two very important nerves which arise near the median ventral line, close to the hind end of the supra-œsophageal ganglia. The origin of these two nerves is shewn in the surface view (fig. 22, sy, and in section in fig. 11). They at first tend somewhat forwards and pass into the muscles near the epithelium lining the groove on each side of the tongue. Here they suddenly bend backwards again and follow the grooves into the pharynx.

Supra-œsophageal Ganglia.—In the present state of our knowledge a very detailed description of the histology of the supra-œsophageal ganglia would be quite superfluous, and I shall confine myself to a description of the more obvious features in the arrangement of the ganglionic and fibrous portions (Pl. 50, fig. 19A-G).

The ganglion cells are in the first place confined, for the most part, to the surface. Along the under side of each ganglion there is a very thick layer of cells, continuous behind, with the layer of ganglion cells which is placed on the under surface of the œsophageal commissures. These cells have, moreover, an arrangement very similar to that in the ventral cords, so that a section through the supra-œsophageal ganglia has an obvious resemblance to what would be the appearance of a section through the united ventral cords. On the outer borders of the ganglia the cells extend upwards, but they end on about the level of the optic nerve (fig. 19D). Immediately dorsal to this point the fibrous matter of the brain is exposed freely on the surface (fig. 19A, B, &c., a). I shall call the region of fibrous matter so exposed the dorso-lateral horn of white matter.

This tongue of ganglion cells sends in, slightly behind the level of the eyes, a transverse vertical prolongation inwards into the white matter of the brain, which is shewn in the series of transverse sections in fig. 19E, and also in the vertical longitudinal section (Pl. 51, fig. 21), and in horizontal section in Pl. 51, fig. 22.

On the ventral aspect of each lobe of the brain there is present a very peculiar, bluntly conical protuberance of ganglion cells (Pl. 51, fig. 22), which was first detected by Grube (No. 10), and described by him as “a white thick body of a regular tetrahedral form, and exhibiting an oval dark spot in the middle of two of the faces.” He further states that it is united by a delicate nerve to the supra-œsophageal ganglion, and regards it as an organ of hearing.

In Peripatus capensis the organ in question can hardly be described as tetrahedral. It is rather of a flattened oval form, and consists, as shewn in sections (Pl. 50, fig. 19C and D, d), mainly of ganglion cells. In its interior is a cavity with a distinct bounding membrane: the cells of which it is composed vary somewhat in size, being smallest near the point of attachment. At its free end is placed a highly refractive, somewhat oval body, probably forming what Grube describes as a dark spot, half embedded in its substance, and kept in place by the sheath of nervous matter surrounding it. This body appears to have fallen out in my sections. The whole structure is attached to the under surface of the brain by a very short stalk formed of a bundle of cells and nervous fibres.

It is difficult to offer any interpretation of the nature of this body. It is removed considerably from the surface of the animal, and is not, therefore, so far as I can see, adapted to serve as an organ of hearing.

The distribution of the white or fibrous matter of the ganglia is not very easy to describe.

The median lobe of the brain is also prolonged into a dorso-lateral lobe (fig. 19, a), which, as already mentioned, is freely exposed on the surface. On its ventral border there springs the optic nerve, and several pairs of sensory nerves already described (fig. 19D, E), while from its dorsal border a pair of sensory nerves also spring, nearly in the same vertical plane as the optic nerves.

Posteriorly where the dorsal surface of the brain is not covered in with ganglion cells the dorso-lateral horn and median lobe of the brain become indistinguishable.

The Skin.

The skin is formed of three layers.

1. The cuticle.

2. The epidermis or hypodermis.

3. The dermis.

The cuticle is a layer of about 0.002 mm. in thickness. Its surface is not, however, smooth, but is everywhere, with the exception of the perioral region, raised into minute secondary papillæ, the base of which varies somewhat in diameter, but is usually not far from 0.02 mm. On the ventral surface of the body these papillæ are for the most part somewhat blunt, but on the dorsal surface they are more or less sharply pointed. In most instances they bear at their free extremity a somewhat prominent spine. The whole surface of each of the secondary papillæ just described is in its turn covered by numerous minute spinous tubercles. In the perioral region, where the cuticle is smooth, it is obviously formed of two layers which easily separate from each other, and there is I believe a similar division elsewhere, though it is not so easy to see. It is to be presumed that the cuticle is regularly shed.

The epidermis, placed immediately within the cuticle, is composed of a single row of cells, which vary, however, a good deal in size in different regions of the body. The cells excrete the cuticle, and, as shewn in fig. 32, they stand in a very remarkable relation to the secondary papillæ of the cuticle just described. Each epidermis cell is in fact placed within one of these secondary papillæ, so that the cuticle of each secondary papilla is the product of a single epidermis cell. This relation is easily seen in section, while it may also be beautifully shewn by taking a part of the skin which is not too much pigmented, and, after staining it, examining from the surface.

At the apex of most, if not all, the primary wart-like papillæ there are present oval aggregations, or masses of epidermis cells, each such mass being enclosed in a thickish capsule (fig. 31). The cells of these masses appear to form the wall of a cavity which leads into the hollow interior of a long spine. These spines when carefully examined with high objectives present a rather peculiar structure. The base of the spine is enveloped by the normal cuticle, but the spine itself, which terminates in a very fine point, appears, as shewn in fig. 31, to be continuous with the inner layer of the cuticle. In the perioral region the outer layer of the cuticle, as well as the inner, appear to be continued to the end of the spines. Within the base of the spine there is visible a finely striated substance which may often be traced into the cavity enclosed by the cells, and appears to be continuous with the cells. Attached to the inner ends of most of the capsules of these organs a delicate fibrillated cord may be observed, and although I have not in any instance succeeded in tracing this cord into one of the nerve-stems, yet in the antennæ, where the nerve-stems are of an enormous size, I have satisfied myself that the minute nerves leaving the main nerve-stems and passing out towards the skin are histologically not to be distinguished from these fibrillated cords. I have therefore but little hesitation in regarding these cords as nerves.

The arrangement of the cells in the bodies just described led me at first to look upon them as glands, but a further investigation induced me to regard them as a form of tactile organ. The arguments for this view are both of a positive and a negative kind.

The positive arguments are the following:

(1) The organs are supplied with large nerves, which is distinctly in favour of their being sense organs rather than glands.

(2) The peculiar striæ at the base of the spines appear to me like the imperfectly preserved remains of sense hairs.

(3) The distribution of these organs favours the view that they are tactile organs. They are most numerous on the antennæ, where such organs would naturally be present, especially in a case like that of Peripatus, where the nerve passing to the antennæ is simply gigantic. On the other hand, the antennæ would not be a natural place to look for an enormous development of dermal glands.

The lips, oral papillæ, and under surface of the legs, where these bodies are also very numerous, are situations where tactile organs would be of great use.

Under the head of negative arguments must be classed those which tell against these organs being glandular. The most important of these is the fact that they have no obvious orifice. Their cavities open no doubt into the spines, but the spines terminate in such extremely fine points that the existence of an orifice at their apex is hardly credible.

Another argument, from the distribution of these organs over the body is practically the converse of that already used. The distribution being as unfavourable to the view that they are glands, as it is favourable to that of their being sense organs.

The Tracheal System.

Transverse sections (fig. 30) through the trunk containing a tracheal opening shew that the walls of the pit expanded internally in a mushroom-like fashion, the narrow part being, however, often excentric in relation to the centre of the expanded part.

Although it was clear that the tracheæ started from the expanded region of the walls of the pit, I could not find that the lumen of the pit dilated into a large vesicle in this part, and further investigation proved that the tracheæ actually started from the slightly swollen inner extremity of the narrow part of the pit, the expanded walls of the pit forming an umbrella-like covering for the diverging bundles of tracheæ.

I have, in fig. 30, attempted to make clear this relation between the expanded walls of the tracheal pits and the tracheæ. In longitudinal sections of the trunk the tracheal pits do not exhibit the lateral expansion which I have just described, which proves that the divergence of the bundles of tracheæ only takes place laterally and not in an antero-posterior direction. Cells similar in general character to those of the walls of the tracheal pits are placed between the branches of tracheæ, and somewhat similar cells, though generally with more elongated nuclei, accompany the bundles of tracheæ as far as they can be followed in my sections. The structure of these parts in the adult would, in fact, lead one to suppose that the tracheæ had originated at the expense of the cells of pits of the epidermis, and that the cells accompanying the bundles of tracheæ were the remains of cords of cells which sprouted out from the blind ends of the epidermis pits and gave rise in the first instance to the tracheæ.

Situation of the tracheal apertures.—Moseley states (No. 13) that the tracheæ arise from the skin all over the surface of the body, but are especially developed in certain regions. He finds “a row of minute oval openings on the ventral surface of the body,” the openings being “situate with tolerable regularity in the centres of the interspaces between the pairs of members, but additional ones occurring at irregular intervals. Other similar openings occur in depressions on the inner side of the conical foot protuberance.” It is difficult in preserved specimens to make out the exact distributions of the tracheal apertures, but I have been able to make out certain points about them.

There is a double row of apertures on each side of the median dorsal line, forming two sub-dorsal rows of apertures. The apertures are considerably more numerous than the legs. There is also a double row of openings, again more numerous than the legs, on each side of the median ventral line between the insertions of the legs. Moseley speaks of a median row in this position. I think this must be a mistake.

Posteriorly the two inner rows approach very close to each other in the median ventral line, but I have never seen them in my section opening quite in the middle line. Both the dorsal and ventral rows are very irregular.

I have not found openings on the ventral or dorsal side of the feet but there are openings at the anterior and posterior aspects of the feet. There are, moreover, a considerable number of openings around the base of the feet.

The dorsal rows of tracheal apertures are continued into the head and give rise in this situation to enormous bundles of tracheæ.

In front of the mouth there is a very large median ventral tracheal pit, which gives off tracheæ to the ventral part of the nervous system, and still more in front a large number of such pits close together. The tracheæ to the central nervous system in many instances enter the nervous system bound up in the same sheath as the nerves.

The Muscular System.

The general muscular system consists of—(1) the general wall of the body; (2) the muscles connected with the mouth, pharynx, and jaws; (3) the muscles of the feet; (4) the muscles of the alimentary tract.

The muscular wall of the body is formed of—(1) an external layer of circular fibres; (2) an internal layer of longitudinal muscles; (3) a layer of transverse fibres.

The layer which I have spoken of as formed of circular fibres is formed of two strata of fibres which girth the body somewhat obliquely (Pl. 51, fig. 25). In the outer stratum the rings are arranged so that their ventral parts are behind, while the ventral parts of the rings of the inner stratum are most forward. Both in the median dorsal and ventral lines the layer of circular fibres become somewhat thinner, and where the legs are attached the regularity of both strata is somewhat interfered with, and they become continuous with a set of fibres inserted in the wall of the foot.

The longitudinal muscles are arranged as five bands (vide fig. 16), viz. two dorsal, two lateral, and three ventral. The three ventral may be spoken of as the latero-ventral and medio-ventral bands.

The transverse fibres consist of (1) a continuous sheet on each side inserted dorsally in the cutis, along a line opposite the space between the dorsal bands of longitudinal fibres, and ventrally between the ventro-median and ventro-lateral bands. Each sheet at its insertion slightly breaks up into separate bands. They divide the body-cavity into three regions—a median, containing the alimentary tract, slime glands, &c., and two lateral, which are less well developed, and contain the nervous system, salivary glands, segmental organs, &c.