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 structures within the buccal cavity are shewn as they appear in surface
views in figs. 5 and 7, but their real nature is best seen in sections, and
is illustrated by Pl. 49, figs. 11 and 12, representing the oral cavity in
transverse section, and by Pl. 49, figs. 17 and 18, representing it in
horizontal longitudinal sections. In the median line of the buccal cavity
in front is placed a thick muscular protuberance, which may perhaps
conveniently be called the tongue, though attached to the dorsal instead of
the ventral wall of the mouth. It has the form of an elongated ridge, which
ends rather abruptly behind, becoming continuous with the dorsal wall of
the pharynx. Its projecting edge is armed by a series of small teeth, which
are thickenings of the chitinous covering, prolonged from the surface of
the body over the buccal cavity. Where the ridge becomes flatter behind,
the row of teeth divides into two, with a shallow groove between them
(Pl. 48, fig. 7).
The surface of the tongue is covered by the oral epithelium, in parts of
which are organs of special sense, similar to those in the skin; but its
interior is wholly formed of powerful muscles. The muscles form two groups,
intermingled amongst each other. There are a series of fibres inserted in
the free edge of the tongue, which diverge, more or less obliquely, towards
the skin at the front of the head anteriorly, and towards the pharynx
behind. The latter set of fibres are directly continuous with the radial
fibres of the pharynx. The muscular fibres just described are clearly
adapted to give a sawing motion to the tongue, whose movements may thus, to
a certain extent, be compared to those of the odontophore of a mollusc.
In addition to the above set of muscles, there are also transverse muscles,
forming laminæ between the fibres just described. They pass from side to
side across the tongue, and their action is clearly to narrow it, and so
cause it to project outwards from the buccal cavity.
On each side of the tongue are placed the jaws, which are, no doubt, a pair
of appendages, modified in the characteristic arthropodan manner, to
subserve mastication. Their structure has never been satisfactorily
described, and is very complicated. They are essentially short papillæ,
moved by an elaborate and powerful system of muscles, and armed at their
free extremities by a pair of cutting blades or claws. The latter
structures are, in all essential points, similar to the claws borne by the
feet, and, like these, are formed as thickenings of the cuticle. They have
therefore essentially the characters of the claws and jaws of the
Arthropoda, and are wholly dissimilar to the setæ of Chætopoda. The claws
are sickle-shaped and, as shewn in Pl. 47, fig. 5, have their convex edge
directed nearly straight forwards, and their concave or cutting edge
pointed backwards. Their form differs somewhat in the different species,
and, as will be shewn in the systematic part of this memoir[570], forms a
good specific character. In _Peripatus capensis_ (Pl. 48, fig. 10) the
cutting surface of the outer blade is smooth and without teeth, while that
of the inner blade (fig. 9), which is the larger of the two, is provided
with five or six small teeth, in addition to the main point. A more
important difference between the two blades than that in the character of
the cutting edge just spoken of, is to be found in their relation to the
muscles which move them. The anterior parts of both blades are placed on
two epithelial ridges, which are moved by muscles common to both blades
(Pl. 49, fig. 11). Posteriorly, however, the behaviour of the two blades is
very different. The epithelial ridge bearing the outer blade is continued
back for a short distance behind the blade, but the cuticle covering it
becomes very thin, and it forms a simple epithelial ridge placed parallel
to the inner blade. The cuticle covering the epithelial ridge of the inner
blade is, on the contrary, prolonged behind the blade itself as a thick
rod, which, penetrating backwards along a deep pocket of the buccal
epithelium, behind the main part of the buccal cavity for the whole length
of the pharynx, forms a very powerful lever, on which a great part of the
muscles connected with the jaws find their insertion. The relations of the
epithelial pocket bearing this lever are somewhat peculiar.
Footnote 570: Some material for this memoir was left by Prof.
Balfour, which will be published separately.
The part of the epithelial ridge bearing the proximal part of this lever is
bounded on both its outer and inner aspect by a deep groove. The wall of
the outer groove is formed by the epithelial ridge of the outer blade, and
that of the inner by a special epithelial ridge at the side of the tongue.
Close to the hinder border of the buccal cavity (as shewn in Pl. 49,
fig. 12, on the right hand side), the outer walls of these two grooves meet
over the lever, so as completely to enclose it in an epithelial tube, and
almost immediately behind this point the epithelial tube is detached from
the oral epithelium, and appears in section as a tube with a chitinous rod
in its interior, lying freely in the body-cavity (shewn in Pl. 49, figs.
13-16, _le_). This apparent tube is the section of the deep pit already
spoken of. It may be traced back even beyond the end of the pharynx, and
serves along its whole length for the attachment of muscles.
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 Salivary Glands._--These two bodies were originally described by
Grube, by whom their nature was not made out, and subsequently by Moseley,
who regarded them as fat bodies. They are placed in the lateral
compartments of the body-cavity immediately dorsal to the ventral nerve
cords, and extend for a very variable distance, sometimes not more than
half the length of the body, and in other instances extending for nearly
its whole length. Their average length is perhaps about two-thirds that of
the body. Their middle portion is thickest, and they thin off very much
behind and to a slight extent in front. Immediately behind the mouth and in
front of the first pair of legs, they bend inwards and downwards, and fall
(fig. 7) one on each side into the hind end of the narrow section of the
oral diverticulum just spoken of as the common duct for the two salivary
glands. The glandular part of these organs is that extending back from the
point where they bend inwards. This part (fig. 16) is formed of very
elongated cells supported by a delicate membrana propria. The section of
this part is somewhat triangular, and the cells are so long as to leave a
comparatively small lumen. The nuclei of the cells are placed close to the
supporting membrane, and the remainder of the cells are filled with very
closely packed secretory globules, which have a high index of refraction.
It was the presence of these globules which probably led Moseley to regard
the salivary glands as fat bodies. The part of each gland which bends
inwards must be regarded as the duct.
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.
_The OEsophagus._--Behind the pharynx there follows a narrow oesophagus
(fig. 7, _oe_) shewn in section in fig. 16. It has somewhat folded and
fairly thick walls, and lies freely in the central division of the
body-cavity without any mesenteric support. Its walls are formed of five
layers, viz. from without inwards.
(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 oesophageal
walls. The oesophagus is vertically oval in front, but more nearly circular
behind. Characteristic of the oesophagus 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 oesophagus 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 connective tissue and muscular coats are extremely thin. There is
present everywhere a peritoneal covering, and in front a fairly well-marked
though very thin layer of muscles formed of an external circular and an
internal longitudinal layer. In the middle and posterior parts, however, I
was unable to recognize these two layers in section; although in surface
view Grube found an inner layer of circular fibres and an outer layer
formed of bands of longitudinal fibres, which he regards as muscular.
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.
The rectum extends from the end of the stomach to the anus. The region of
junction between the stomach and the rectum is somewhat folded. The usual
arrangement of the parts is shewn in fig. 6, where the hind end of the
stomach is seen to be bent upon itself in a U-shaped fashion, and the
rectum extending forwards under this bent portion and joining the front end
of the dorsal limb of the U. The structure of the walls of the rectum is
entirely different to that of the stomach, and the transition between the
two is perfectly sudden. Within the peritoneal investment comes a
well-developed muscular layer with a somewhat unusual arrangement of its
layers, there being an external circular layer and an internal layer formed
of isolated longitudinal bands. The epithelium is fairly columnar, formed
of granular cells with large nuclei, and is lined by a prolongation of the
external cuticle. It is raised into numerous longitudinal folds, which are
visible from the surface, and give a very characteristic appearance to this
part of the alimentary tract. The muscular layers do not penetrate into the
epithelial folds, which are supported by a connective tissue layer.
NERVOUS SYSTEM.
The central nervous system consists of a pair of supra-oesophageal ganglia
united in the middle line, and of a pair of widely divaricated ventral
cords, continuous in front with the supra-oesophageal 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 oesophageal
commissures, where the nerves to the oral papillæ are given off (Pl. 51,
fig. 22, _or.g._), and the region of junction between the oesophageal
commissures with the supra-oesophageal 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.
They are connected with each other, rather like the pedal nerves of Chiton
and the lower Prosobranchiata, by a number of commissures. These
commissures exhibit a fairly regular arrangement from the region included
between the first and the last pair of true feet. There are nine or ten of
them between each pair of feet (Pl. 52, fig. 26). They pass along the
ventral wall of the body, perforating the ventral mass of longitudinal
muscles. On their way they give off nerves which innervate the skin.
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.
The ventral cords give off a series of nerves from their outer borders,
which present throughout the trunk a fairly regular arrangement. From each
ganglion two large nerves (figs. 8, 22, 26) are given off, which, diverging
somewhat from each other, pass into the feet, and, giving off branches on
their way, may be traced for a considerable distance within the feet along
their anterior and posterior borders.
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 oesophageal 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 oesophageal commissures, close to their junction with the
supra-oesophageal ganglia, a nerve arises on each side which passes to the
jaws, and a little in front of this, apparently from the supra-oesophageal
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.
_The Supra-oesophageal Ganglia._--The supra-oesophageal ganglia (figs. 8
and 22) are large, somewhat oval masses, broader in front than behind,
completely fused in the middle, but free at their extremities. Each of them
is prolonged anteriorly into an antennary nerve, and is continuous behind
with one of the oesophageal commissures. On the ventral surface of each,
rather behind the level of the eye, is placed a very peculiar protuberance
(fig. 22, _d_), of which I shall say more in dealing with the histology of
the nervous system.
A number of nerves arise from the supra-oesophageal 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-oesophageal 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-oesophageal 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.
The two grooves are continuous with the two dorsal angles of the pharynx;
and embedded in the muscles of the pharynx, in juxtaposition with the
epithelium, these two nerves may easily be traced in sections. They pass
backwards the whole length of the pharynx till the latter joins the
oesophagus. Here they at once approach and shortly meet in the median
dorsal line (fig. 16). They can only be traced for a very short distance
beyond their meeting point. These nerves are, without doubt, the homologues
of the sympathetic system of Chætopods, occupying as they do the exact
position which Semper has shewn to be characteristic of the sympathetic
nerves in that group, and arising from an almost identical part of the
brain[571].
Footnote 571: Vide Spengel, "Oligognathus Bonelliæ." _Naples
Mittheilungen_, Bd. III. pl. IV. fig. 52.
_Histology of the Nervous System._
_Ventral Cords._--The histology of the ventral cords and oesophageal
commissures is very simple and uniform. They consist of a cord almost
wholly formed of nerve-fibres, placed dorsally, and a ventral layer of
ganglion cells (figs. 16 and 20).
The fibrous portion of the cord has the usual structure, being formed
mainly of longitudinal fibres, each probably being a bundle of fibres of
various sizes, enveloped in a sponge-work of connective tissue. The larger
bundles of fibres are placed near the inner borders of the cords. In this
part of the cord there are placed a very small number of ganglion cells.
The layer of ganglion cells is somewhat crescent-shaped in section, and, as
shewn in figs. 16 and 20, envelopes the whole ventral aspect of the fibrous
parts of the cord, and even creeps up slightly on to the dorsal side. It is
thicker on the inner than on the outer side, and increases considerably in
bulk at each ganglionic enlargement. The cells of which it is composed are
for the most part of a nearly uniform size, but at the border of the
fibrous matter a fair sprinkling of larger cells is found.
The tracheal vessels supplying the nervous system are placed amongst the
larger cells, at the boundary between the ganglionic and fibrous regions of
the cords.
With reference to the peripheral nerve-stems there is not much to be said.
They have for the most part a similar structure to the fibrous parts of the
main cord, but are provided with a somewhat larger number of cells.
_Sheath of the Ventral Cords._--The ventral cords are enveloped by a double
sheath, the two layers of which are often in contact, while in other cases
they may be somewhat widely separated from each other. The inner layer is
extremely thin and always very closely envelopes the nerve-cords. The outer
layer is thick and fibrous, and contains a fair sprinkling of nuclei.
_Supra-oesophageal Ganglia._--In the present state of our knowledge a very
detailed description of the histology of the supra-oesophageal 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 oesophageal commissures. These cells
have, moreover, an arrangement very similar to that in the ventral cords,
so that a section through the supra-oesophageal 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.
Where the two ganglia separate in front the ganglion cells spread up the
inner side, and arch over so as to cover part of the dorsal side. Thus, in
the anterior part, where the two ganglia are separate, there is a complete
covering of ganglionic substance, except for a narrow strip, where the
dorso-lateral lobe of white matter is exposed on the surface (fig. 19A).
From the point where the two ganglia meet in front the nerve-cells extend
backwards as a median strip on the dorsal surface (fig. 19D and E). This
strip, becoming gradually smaller behind, reaches nearly, though not quite,
the posterior limit of the junction of the ganglia. Behind it there is,
however, a region where the whole dorsal surface of the ganglia is without
any covering of nerve-cells.
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-oesophageal 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.
There is a central lobe of white matter (fig. 19E), which is continuous
from ganglion to ganglion, where the two are united. It is smaller behind
than in front. On its ventral side it exhibits fairly well-marked
transverse commissural fibres, connecting the two halves of the ganglion.
Laterally and somewhat ventrally it is prolonged into a horn (fig. 19D, E,
_b_), which I propose calling the ventro-lateral horn. In front it is
placed in a distinct protuberance of the brain, which is placed ventrally
to and nearly in the same vertical plane as the optic nerve. This
protuberance is best shewn in the view of the brain from below given in
Pl. 51, fig. 22. This part of the horn is characterized by the presence of
large vertically-directed bundles of nerve-fibres, shewn in transverse
section in fig. 19 D. Posteriorly the diameter of this horn is larger than
in front (fig. 19E, F, G), but does not give rise to a protuberance on the
surface of the brain owing to the smaller development of the median lobe
behind.
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.
In the front part of the brain the median lobe of white matter extends
dorsalwards to the dorsal strip of ganglion cells, but behind the region of
the transverse prolongation of these cells, into the white matter already
described (p. 890), there is a more or less distinctly defined lobe of
white matter on the dorsal surface, which I propose calling the
postero-dorsal lobe of white matter. It is shewn in the transverse sections
(fig. 19F and G, _c_). It gradually thins away and disappears behind. It is
mainly characterized by the presence on the ventral border of definite
transverse commissural fibres.
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.
In fig. 32 a region of the epidermis is figured, in which the cells are
exceptionally columnar. The cuticle has, moreover, in the process of
cutting the section, been somewhat raised and carried away from the
subjacent cells. The cells of the epidermis are provided with large oval
nuclei, which contain a well-developed reticulum, giving with low powers a
very granular appearance to the nuclei. The protoplasm of the cells is also
somewhat granular, and the granules are frequently so disposed as to
produce a very well-marked appearance of striation on the inner end of the
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. An attempt is made to shew this in fig. 32.
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.
In certain regions of the body the oval aggregations of cells are extremely
numerous; more especially is this the case in the antennæ, lips, and oral
papillæ. On the ventral surface of the peripheral rings of the thicker
sections of the feet they are also very thick set (fig. 20, P). They here
form a kind of pad, and have a more elongated form than in other regions.
In the antennæ they are thickly set side by side on the rings of skin which
give such an Arthropod appearance to these organs in Peripatus.
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.
The apertures of the tracheal system are placed in the depressions between
the papillæ or ridges of the skin. Each of them leads into a tube, which I
shall call the tracheal pit (fig. 30), the walls of which 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. The pits vary somewhat in depth; the pit figured was about 0.09 mm.
It perforates the dermis and terminates in the subjacent muscular layer.
The investigation of the inner end of the pit gave me some little trouble.
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.
The tracheæ themselves are extremely minute, unbranched (so far as I could
follow them) tubes. Each opening by a separate aperture into the base of
the tracheal pit, and measuring about 0.002 mm. in diameter. They exhibit a
faint transverse striation, which I take to be the indication of a spiral
fibre. [Moseley (_Phil. Trans._, 1874, Pl. 73, fig. 1) states that the
tracheæ branch, but only exceptionally.]
_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.
(2) Inserted a little dorsal to the transverse band just described is a
second band which immediately crosses the first, and then passes on the
outer side of the nervous cord and salivary gland, where such is present,
and is inserted ventrally in the space between the ventro-lateral and
lateral longitudinal band.
Where the feet are given off the second transverse band becomes continuous
with the main retractor muscular fibres in the foot, which are inserted
both on to the dorsal side and ventral side.
_Muscular system of the feet._--This consists of the retractors of the feet
connected with the outer transverse muscle and the circular layer of
muscles. In addition to these muscles there are intrinsic transverse
muscles which cross the cavity of the feet in various directions (Pl. 51,
fig. 20). There is no special circular layer of fibres.
_Histology of the muscle._--The main muscles of the body are unstriated and
divided into fibres, each invested by a delicate membrane. Between the
membrane and muscle are scattered nuclei, which are never found inside the
muscle fibres. The muscles attached to the jaws form an exception in that
they are distinctly transversely striated.
THE BODY-CAVITY AND VASCULAR SYSTEM.
The body-cavity, as already indicated, is formed of three compartments--one
central and two lateral. The former is by far the largest, and contains the
alimentary tract, the generative organs, and the mucous glands. It is lined
by a delicate endothelial layer, and is not divided into compartments nor
traversed by muscular fibres.
The lateral divisions are much smaller than the central, and are shut off
from it by the inner transverse band of muscles. They are almost entirely
filled with the nerve-cord and salivary gland in front and with the
nerve-cord alone behind, and their lumen is broken up by muscular bands.
They further contain the segmental organs which open into them. They are
prolonged into the feet, as is the embryonic body-cavity of most
Arthropoda.
The vascular system is usually stated to consist of a dorsal heart. I find
between the dorsal bands of longitudinal fibres a vessel in a space shut
off from the body-cavity by a continuation of the endothelial lining of the
latter (fig. 16). The vessel has definite walls and an endothelial lining,
but I could not make out whether the walls were muscular. The ventral part
of it is surrounded by a peculiar cellular tissue, probably, as suggested
by Moseley, equivalent to the fat bodies of insects. It is continued from
close to the hind end of the body to the head, and is at its maximum
behind. In addition to this vessel there is present a very delicate ventral
vessel, by no means easy to see, situated between the cutis and the outer
layer of circular muscles.
SEGMENTAL ORGANS.
A series of glandular organs are found in Peripatus which have their
external openings situated on the ventral surface of a certain number of
the legs, and which, to the best of my belief, end internally by opening
into the lateral compartments of the body-cavity. These organs are probably
of an excretory nature, and I consider them homologous with the nephridia
or segmental organs of the Chætopoda.
In _Peripatus capensis_ they are present in all the legs. In all of them
(except the first three) the following parts may be recognized:
(1) A vesicular portion opening to the exterior by a narrow passage.
(2) A coiled portion, which is again subdivided into several sections.
(3) A terminal section ending by a somewhat enlarged opening into the
lateral compartment of the body-cavity.
The last twelve pairs of these organs are all constructed in a very similar
manner, while the two pairs situated in the fourth and fifth pairs of legs
are considerably larger than those behind, and are in some respects very
differently constituted.
It will be convenient to commence with one of the hinder nephridia. Such a
nephridium from the ninth pair of legs is represented in fig. 28. The
external opening is placed at the outer end of a transverse groove placed
at the base of one of the feet, while the main portion of the organ lies in
the body-cavity in the base of the leg, and extends into the trunk to about
the level of the outer edge of the nerve-cord of its side. The external
opening (_os_) leads into a narrow tube (_sd_), which gradually dilates
into a large sack (_s_).
The narrow part is lined by small epithelial cells, which are directly
continuous with and perfectly similar to those of the epidermis (fig. 20).
It is provided with a superficial coating of longitudinal muscular fibres,
which thins out where it passes over the sack, along which it only extends
for a short distance.
The sack itself, which forms a kind of bladder or collecting vesicle for
the organ, is provided with an extremely thin wall, lined with very large
flattened cells. These cells are formed of granular protoplasm, and each of
them is provided with a large nucleus, which causes a considerable
projection into the lumen of the sack (figs. 20, 29, _s_). The epithelial
wall of the sack is supported by a membrana propria, over which a delicate
layer of the peritoneal epithelium is reflected.
The coiled tube forming the second section of the nephridium varies in
length, and by the character of the epithelium lining it may be divided
into four regions. It commences with a region lined by a fairly columnar
epithelium with smallish nuclei (fig. 28, _sc_1). The boundaries of the
cells of this epithelium are usually very indistinct, and the protoplasm
contains numerous minute granules, which are usually arranged in such a
manner as to give to optical or real sections of the wall of this part of
the tube a transversely striated appearance. These granules are very
probably minute balls of excretory matter.
The nuclei of the cells are placed near their free extremities, contrary to
what might have been anticipated, and the inner ends of the cells project
for very different lengths into the interior, so causing the inner boundary
of the epithelium of this part of the tube to have a very ragged
appearance. This portion of the coiled tube is continuous at its outer end
with the thin-walled vesicle. At its inner end it is continuous with region
No. 2 of the coiled tube (fig. 28, _sc_2), which is lined by small
closely-packed columnar cells. This portion is followed by region No. 3,
which has a very characteristic structure (fig. 28 _sc_3). The cells lining
this part are very large and flat, and contain large disc-shaped nuclei,
which are usually provided with large nucleoli, and often exhibit a
beautiful reticulum. They may frequently be observed in a state of
division. The protoplasm of this region is provided with similar granules
to that in the first region, and the boundaries of the cells are usually
very indistinct. The fourth region is very short (fig. 28, _sc_4), and is
formed of small columnar cells. It gradually narrows till it opens suddenly
into the terminal section (_sot_), which ends by opening into the
body-cavity, and constitutes the most distinct portion of the whole organ.
Its walls are formed of columnar cells almost filled by oval nuclei, which
absorb colouring matters with very great avidity, and thus renders this
part extremely conspicuous. The nuclei are arranged in several rows.
The study of the internal opening of this part gave me some trouble. No
specimens ever shew it as rounded off in the characteristic fashion of
tubes ending in a cul-de-sac. It is usually somewhat ragged and apparently
open. In the best preserved specimens it expands into a short funnel-shaped
mouth, the free edge of which is turned back. Sections confirm the results
of dissections. Those passing longitudinally through the opening prove its
edges are turned back, forming a kind of rudimentary funnel. This is
represented in fig. 29, from the last leg of a female. I have observed
remains of what I consider to be cilia in this section of the organ. The
fourth region of the organ is always placed close to the thin-walled
collecting vesicle (figs. 28 and 29). In the whole of the coiled tube just
described the epithelium is supported by a membrana propria, which in its
turn is invested by a delicate layer of peritoneal epithelium.
The fourth and fifth pairs are very considerably larger than those behind,
and are in other respects peculiar. The great mass of each organ is placed
behind the leg, on which the external opening is placed, immediately
outside one of the lateral nerve-cords. Its position is shewn in fig. 8.
The external opening, instead of being placed near the base of the leg, is
placed on the ventral side of the third ring (counting from the outer end)
of the thicker portion of the leg. It leads (fig. 27) into a portion which
clearly corresponds with the collecting vesicle of the hinder nephridia.
This part is not, however, dilated into a vesicle in the same sort of way,
and the cells which form the lining epithelium have not the same
characteristic structure, but are much smaller. Close to the point where
the vesicle joins the coiled section of the nephridium the former has a
peculiar nick or bend in it. At this nick it is firmly attached to the
ventral side of the foot by muscles and tracheæ, and when cut away from its
attachment the muscles and tracheæ cannot easily be detached from it. The
main part of the coils are formed by region No. 1, and the epithelial cells
lining this part present very characteristically the striated appearance
which has already been spoken of. The large-celled region of the coiled
tube (fig. 27) is also of considerable dimensions, and the terminal portion
is wedged in between this and the commencing part of the coiled tube. The
terminal portion with its internal opening is in its histological
characters exactly similar to the homologous region in the hinder
nephridia.
The three pairs of nephridia in the three foremost pairs of legs are very
rudimentary, consisting, so far as I have been able to make out, solely of
the collecting vesicle and the duct leading from them to the exterior. The
external opening is placed on the ventral side of the base of the feet, in
the same situation as that of the posterior nephridia, but the histological
characters of the vesicle are similar to those of the fourth and fifth
pairs.
GENERATIVE ORGANS.
[The sexes are distinct, and the average size of the females appears to be
greater than that of the males.
The only outward characteristic by which the males can be distinguished
from the females is the presence in the former of a small white papilla on
the ventral side of the 17th pair of legs (Pl. 47, fig. 4). At the
extremity of this papilla the modified crural gland of the last leg opens
by a slit-like aperture.
The generative orifice in both sexes is placed on the ventral surface of
the body, close to the anus, and between the two anal papillæ, which are
much more marked in small specimens than in large ones, and in two cases
(of females) were observed to bear rudimentary claws.
1. _The Male Organs._ Pl. 53, fig. 43.
The male organs consist of a pair of testes (_te_), a pair of prostrates
(_pr_) and vasa deferentia (_vd_) and accessory glandular tubules (_f_).
All the above parts lie in the central compartment of the body-cavity. In
addition, the accessory glandular bodies or crural glands of the last
(17th) pair of legs are enlarged and prolonged into an elongated tube
placed in the lateral compartment of the body-cavity (_ag_).
The arrangement of these parts represented in the figure appears
essentially that which Moseley has already described for this species. The
dilatations on the vasa deferentia, which he calls vesiculæ seminales, is
not so marked; nor can the peculiar spiral twisting of this part of the vas
deferens which he figures (No. 13) be made out in this specimen. The testes
are placed at different levels in the median compartment of the
body-cavity, and both lie on the same side of the intestine (right side).
The arrangement of the terminal portions of the vas deferens is precisely
that described by Moseley. The right vas deferens passes under both
nerve-cords to join the left, and from the enlarged tube (_p_), which,
passing beneath the nerve-cord of its side, runs to the external orifice.
The enlarged terminal portion possesses thick muscular walls, and possibly
constitutes a spermatophore maker, as has been shewn to be the case in P.
N. Zealandiæ, by Moseley.
In some specimens a different arrangement obtains, in that the left vas
deferens passes under both nerve-cords to join the right.
In addition to the above structures, which are all described by Moseley,
there are a pair of small glandular tubes (_f_), which open with the
unpaired terminal portion of the vas deferens at the generative orifice.
2. _Female Organs._ Pl. 52, fig. 33.
The female organs consist of a median unpaired ovary and a pair of
oviducts, which are dilated for a great part of their course to perform a
uterine function, and which open behind into a common vestibule
communicating directly with the exterior.
_Ovary._--In the specimen figured the following is the arrangement:
The ovary lies rather to the dorsal side in the central compartment of the
body-cavity, and is attached to one of the longitudinal septa separating
this from the lateral compartment. It lies between the penultimate and
antepenultimate pair of legs.
The oviducts cross before opening to the exterior. The right oviduct passes
under the rectum, and the left over the rectum. They meet by opening into a
common vestibule, which in its turn opens to the exterior immediately
ventral to the anus. It has not been ascertained how far this arrangement,
which differs from that observed by Moseley, is a normal one. The young
undergo nearly the whole of their development within the uterus. They
possess at birth the full number of appendages, and differ from the parent
only in size and colour.]
NOTES ON ADDITIONAL GLANDULAR BODIES IN THE LEGS
[CRURAL GLANDS].
1. They are present in all except the first.
2. They open externally to the nephridia (Pl. 51, fig. 20), except in the
fourth and fifth pairs of legs, in which they are internal.
3. A muscular layer covers the whole gland, consisting, I believe, of an
oblique circular layer.
4. The accessory gland in the male (fig. 43, _ag_) is probably a
modification of one of these organs.
[The structure and relations of these glands may be best understood by
reference to Pl. 51, fig. 20. Each consists of a dilated vesicular portion
(_fgl_) placed in the lateral compartment of the body-cavity in the foot,
and of a narrow duct leading to the exterior, and opening on the ventral
surface amongst the papillæ of the second row (counting from the internal
of the three foot pads--fig. 20, P).
The vesicular portion is lined by columnar cells, with very large oval
nuclei, while the duct is lined by cells similar to the epidermic cells,
with which they are continuous at the opening.
In the last (17th) leg of the males of this species, this gland (vide
above, note 4) possesses a slit-like opening placed at the apex of a
well-developed white papilla (Pl. 47, fig. 4). It is enormously enlarged,
and is prolonged forward as a long tubular gland, the structure of which
resembles that of the vesicles of the crural glands in the other legs. This
gland lies in the lateral compartment of the body-cavity, and extends
forward to the level of the 9th leg (Pl. 48, fig. 8, and Pl. 53, fig. 43).
It is described by Professor Balfour as the accessory gland of the male,
and is seen in section lying immediately dorsal to the nerve-cord in
fig. 20, _ag_.]
PART III.
THE DEVELOPMENT OF PERIPATUS CAPENSIS.
[The remarkable discoveries about the early development of Peripatus, which
Balfour made in June last, shortly before starting for Switzerland, have
already been the subject of a short communication to the Royal Society
(_Proc. Roy. Soc._ No. 222, 1882). They relate (1) to the blastopore, (2)
to the origin of the mesoblast.
Balfour left no manuscript account or notes of his discovery in connection
with the drawings which he prepared in order to illustrate it, but he spoke
about it to Professor Ray Lankester and also to us, and he further gave a
short account of the matter in a private letter to Professor Kleinenberg.
In this letter, which by the courtesy of Professor Kleinenberg we have been
permitted to see, he describes the blastopore as an elongated slit-like
structure extending along nearly the whole ventral surface; and further
states, as the result of his examination of the few and ill-preserved
embryos in his possession, that the mesoblast appears to originate as
paired outgrowths from the lips of the blastopore.
The drawings left by Balfour in connection with the discoveries are four in
number: one of the entire embryo, shewing the slit-like blastopore and the
mesoblastic somites, the other three depicting the transverse sections of
the same embryo.
The first drawing (fig. 37), viz. that of the whole embryo, shews an embryo
of an oval shape, possessing six somites, whilst along the middle of its
ventral surface there are two slit-like openings, lying parallel to the
long axis of the body, and placed one behind the other. The mesoblastic
somites are arranged bilaterally in pairs, six on either side of these
slits. The following note in his handwriting is attached to this drawing:
"Young larva of _Peripatus capensis_.--I could not make out for certain
which was the anterior end. Length 1.34 millimetres."
Balfour's three remaining drawings (figs. 40-42) are, as already stated,
representations of transverse sections of the embryo figured by him as a
whole. They tend to shew, as he stated in the letter referred to above,
that the mesoblast originates as paired outgrowths from the hypoblast, and
that these outgrowths are formed near the junction of the hypoblast with
the epiblast at the lips of the blastopore.
In fig. 42 the walls of the mesoblastic somites appear continuous with
those of the mesenteron near the blastopore.
In fig. 40, which is from a section a little in front of fig. 42, the walls
of the mesoblastic somites are independent of those of the mesenteron.
Fig. 41 is from a section made in front of the region of the blastopore.
In all the sections the epiblast lying over the somites is thickened, while
elsewhere it is formed of only one layer of cells; and this thickening
subsequently appears to give rise to the nervous system. Balfour in his
earlier investigations on the present subject found in more advanced stages
of the embryo the nerve-cords still scarcely separated from the
epiblast[572].
Footnote 572: _Comparative Embryology_; original edition,
Vol. I. p. 318. [This edition, Vol. II. p. 385.]
We have since found, in Balfour's material, embryos of a slightly different
age to that just described. Of these, three (figs. 34, 35, 36) are younger,
while one (fig. 38) is older than Balfour's embryo.
Stage A.--The youngest (fig. 34) is of a slightly oval form, and its
greatest length is .48 mm. It possesses a blastopore, which is elongated in
the direction of the long axis of the embryo, and is slightly narrower in
its middle than at either end. From one end of the blastopore there is
continued an opaque band. This we consider to be the posterior end of the
blastopore of the embryo. The blastopore leads into the archenteron.
Stage B.--In the next stage (fig. 35) the embryo is elongate-oval in form.
Its length is .7 mm. The blastopore is elongated and slightly narrowed in
the middle. At the posterior end of the embryo there is a mass of opaque
tissue. On each side of the blastopore are three mesoblastic somites. The
length of the blastopore is .45 mm.
Stage C.--In the next stage (fig. 36) the features are much the same as in
the preceding. The length of the whole embryo is .9 mm.
The following were the measurements of an embryo of this stage with five
somites, but slightly younger than that from which fig. 36 was drawn.
Length of embryo .74 mm.
" blastopore .46 "
Distance between hind end of blastopore and hind end
of body .22 "
Distance between front end of body and front end of
blastopore .06 "
The somites have increased to five, and there are indications of a sixth
being budded off from the posterior mass of opaque tissue. The median parts
of the lips of the blastopore have come together preparatory to the
complete fusion by which the blastopore becomes divided into two parts.
Stage D.--The next stage is Balfour's stage, and has been already
described.
The length is 1.34.
It will be observed, on comparing it with the preceding embryos, that while
the anterior pair of somites in figs. 35 and 36 lie at a considerable
distance from what we have called the anterior end of the embryo (_a_), in
the embryo now under consideration they are placed at the anterior end of
the body, one on each side of the middle line. We cannot speak positively
as to how they come there, whether by a pushing forward of the anterior
somites of the previous stage, or by the formation of new somites
anteriorly to those of the previous stage.
In the next stage it is obvious that this anterior pair of somites has been
converted into the præoral lobes.
The anterior of the two openings to which the blastopore gives rise is
placed between the second pair of somites; we shall call it the embryonic
mouth. The posterior opening formed from the blastopore is elongated, being
dilated in front and continued back as a narrow slit (?) to very near the
hind end of the embryo, where it presents a second slight dilatation. The
anterior dilatation of the posterior open region of the blastopore we shall
call the embryonic anus.
Lately, but too late to be figured with this memoir, we have been fortunate
enough to find an embryo of apparently precisely the same stage as fig. 37.
We are able, therefore, to give a few more details about the stage.
The measurements of this embryo were:
Length of whole embryo 1.32 mm.
Distance from front end of body to front end of mouth .32 "
Distance from embryonic mouth to hind end of embryonic
anus .52 "
Distance from hind end of embryonic anus to hind end
of body .45 "
Length of embryonic anus .2 "
" part of blastopore behind embryonic anus .2 "
Greatest width of embryo .64 "
Stage E.--In the next stage (figs. 38 and 39) the flexure of the hind end
of the body has considerably increased. The anterior opening of the
blastopore, the embryonic mouth, has increased remarkably in size. It is
circular, and is placed between the second pair of mesoblastic somites. The
anterior dilatation of the posterior opening of the blastopore, the
embryonic anus, has, like the anterior opening, become much enlarged. It is
circular, and is placed on the concavity of the ventral flexure. From its
hind end there is continued to the hind end of the body a groove (shewn in
fig. 39 as a dotted line), which we take to be the remains of the posterior
slit-like part of the posterior opening of the blastopore of the preceding
stage. The posterior dilatation has disappeared. The embryo has apparently
about thirteen somites, which are still quite distinct from one another,
and apparently do not communicate at this stage with the mesenteron.
The epiblast lying immediately over the somites is, as in the, earlier
stages, thickened, and the thickenings of the two sides join each other in
front of the embryonic mouth, where the anterior pair of mesoblastic
somites (the præoral lobes) are almost in contact.
The median ventral epiblast, _i.e._ the epiblast in the area, bounded by
the embryonic mouth and anus before and behind and by the developing
nerve-cords laterally, is extremely thin, and consists of one layer of very
flat cells. Over the dorsal surface of the body the epiblast cells are
cubical, and arranged in one layer.
Measurements of Embryo of Stage E.
Length of embryo 1.12 mm.
Greatest width .64 "
Distance from front end of embryonic mouth to hind
end of embryonic anus .48 "
Greatest length of embryonic mouth .16 "
Length between hind end of embryonic mouth and
front end of embryonic anus .29 "
These measurements were made with a micrometer eyepiece, with the embryo
lying on its back in the position of fig. 38, so that they simply indicate
the length of the straight line connecting the respective points.
This is the last embryo of our series of young stages. The next and oldest
embryo was 3.2 mm. in length. It had ringed antennæ, seventeen (?) pairs of
legs, and was completely doubled upon itself, as in Moseley's figure.
The pits into the cerebral ganglia and a mouth and anus were present. There
can be no doubt that the mouth and anus of this embryo become the mouth and
anus of the adult.
The important question as to the connection between the adult mouth and
anus, and the embryonic mouth and anus of the Stage E, must, considering
the great gap between Stage E and the next oldest embryo, be left open.
Meanwhile, we may point out that the embryonic mouth of Stage E has exactly
the same position as that of the adult; but that the anus is considerably
in front of the hind end of the body in Stage E, while it is terminal in
the adult.
If the embryonic mouth and anus do become the adult mouth and anus, there
would appear to be an entire absence of stomodæum and proctodæum in
_Peripatus_, unless the buccal cavity represents the stomodæum. The latter
is formed, as has been shewn by Moseley, by a series of outgrowths round
the simple mouth-opening of the embryo, which enclosing the jaws give rise
to the tumid lips of the adult.
For our determination of the posterior and anterior ends of each of these
embryos, Stage A to E, we depend upon the opaque tissue seen in each case
at one end of the blastopore.
In Stage A it has the form of a band, extending backwards from the
blastopore.
In Stages B-D, it has the form of an opaque mass of tissue occupying the
whole hind end of the embryo, and extending a short distance on either side
of the posterior end of the blastopore.
This opacity is due in each case to a proliferation of cells of the
hypoblast, and, perhaps, of the epiblast (?).
There can be no doubt that the mesoblast so formed gives rise to the great
majority of the mesoblastic somites.
This posterior opacity is marked in Stage C by a slight longitudinal groove
extending backwards from the hind end of the blastopore. This is difficult
to see in surface views, and has not been represented in the figure, but is
easily seen in sections.
But in Stage D this groove has become very strongly marked in surface
views, and looks like a part of the original blastopore of Stage C.
Sections shew that it does not lead into the archenteron, but only into the
mass of mesoblast which forms the posterior opacity. It presents an
extraordinary resemblance to the primitive streak of vertebrates, and the
ventral groove of insect embryos.
We think that there can be but little doubt that it is a part of the
original blastopore, which, on account of its late appearance (this being
due to the late development of the posterior part of the body to which it
belongs), does not acquire the normal relations of a blastopore, but
presents only those rudimentary features (deep groove connected with origin
of mesoblast) which the whole blastopore of other tracheates presents.
We think it probable that the larval anus eventually shifts to the hind end
of the body, and gives rise to the adult anus. We reserve the account of
the internal structure of these embryos (Stages A-E) and of the later
stages for a subsequent memoir.
We may briefly summarise the more important facts of the early development
of _Peripatus capensis_, detailed in the preceding account.
1. The greater part of the mesoblast is developed from the walls of the
archenteron.
2. The embryonic mouth and anus are derived from the respective ends of the
original blastopore, the middle part of the blastopore closing up.
3. The embryonic mouth almost certainly becomes the adult mouth, _i.e._ the
aperture leading from the buccal cavity into the pharynx, the two being in
the same position. The embryonic anus is in front of the position of the
adult anus, but in all probability shifts back, and persists as the adult
anus.
4. The anterior pair of mesoblastic somites gives rise to the swellings of
the præoral lobes, and to the mesoblast of the head[573].
Footnote 573: We have seen nothing in any of our sections
which we can identify as of so-called mesenchymatous origin.
There is no need for us to enlarge upon the importance of these facts.
Their close bearing upon some of the most important problems of morphology
will be apparent to all, and we may with advantage quote here some passages
from Balfour's _Comparative Embryology_, which shew that he himself long
ago had anticipated and in a sense predicted their discovery.
"Although the mesoblastic groove of insects is not a gastrula, it is quite
possible that it is the rudiment of a blastopore, the gastrula
corresponding to which has now vanished from development." (_Comparative
Embryology_, Vol. I. p. 378, the original edition[574].)
Footnote 574: This edition, Vol. II. p. 457.
"TRACHEATA.--Insecta. It (the mesoblast) grows inwards from the lips of the
germinal groove, which probably represents the remains of a blastopore."
(_Comparative Embryology_, Vol. II. p. 291, the original edition[575].)
Footnote 575: This edition, Vol. III. p. 352.
"It is, therefore, highly probable that the paired ingrowths of the
mesoblast from the lips of the blastopore may have been, in the first
instance, derived from a pair of archenteric diverticula." (_Comparative
Embryology_, Vol. II. p. 294, the original edition[576].)
Footnote 576: This edition, Vol. III. p. 356.
The facts now recorded were discovered in June last, only a short time
before Balfour started for Switzerland; we know but little of the new ideas
which they called up in his mind. We can only point to passages in his
published works which seem to indicate the direction which his speculations
would have taken.
After speculating as to the probability of a genetic connection between the
circumoral nervous system of the Coelenterata, and the nervous system of
Echinodermata, Platyhelminthes, Chætopoda, Mollusca, &c., he goes on to
say:
"A circumoral nerve-ring, if longitudinally extended, might give rise
to a pair of nerve-cords united in front and behind--exactly such a
nervous system, in fact, as is present in many Nemertines (the Enopla
and Pelagonemertes), in _Peripatus_ and in primitive molluscan types
(Chiton, Fissurella, &c.). From the lateral parts of this ring it
would be easy to derive the ventral cord of the Chætopoda and
Arthropoda. It is especially deserving of notice, in connection with
the nervous system of the above mentioned Nemertines and Peripatus,
that the commissure connecting the two nerve-cords behind is placed on
the dorsal side of the intestines. As is at once obvious, by referring
to the diagram (fig. 231 B), this is the position this commissure
ought, undoubtedly, to occupy if derived from part of a nerve-ring
which originally followed more or less closely the ciliated edge of
the body of the supposed radiate ancestor." (_Comparative Embryology_,
Vol. II. pp. 311, 312, the original edition[577].)
Footnote 577: This edition, Vol. III. pp. 378, 379.
The facts of development here recorded give a strong additional support to
this latter view, and seem to render possible a considerable extension of
it along the same lines.]
LIST OF MEMOIRS ON PERIPATUS.
1. M. Lansdown Guilding. "An Account of a New Genus of Mollusca,"
_Zoological Journal_, Vol. II. p. 443, 1826.
2. M. Andouin and Milne-Edwards. "Classific. des Annélides et description
de celles qui habitent les côtes de France," p. 411, _Ann. Scien. Nat._
ser. I. Vol. XXX. 1833.
3. M. Gervais. "Études p. servir à l'histoire naturelle des Myriapodes,"
_Ann. Scien. Nat._ ser. II. Vol. VII. 1837, p. 38.
4. Wiegmann. Wiegmann's _Archiv_, 1837.
5. H. Milne-Edwards. "Note sur le _Peripate juluforme_," _Ann. Scien. Nat._
ser. II. Vol. XVIII. 1842.
6. Blanchard. "Sur l'organisation des Vers," chap. IV. pp. 137-141, _Ann.
Scien. Nat._ ser. III. Vol. VIII. 1847.
7. Quatrefages. "Anat. des Hermelles, note on," p. 57, _Ann. Scien. Nat._
ser. III. Vol. X. 1848.
8. Quatrefages. _Hist. Nat. des Annelés_, 1865, Appendix, pp. 675-6.
9. De Blainville. _Suppl. au Dict. des Sc. Nat._ Vol. I.
10. Ed. Grube. "Untersuchungen üb. d. Bau von _Peripatus Edwardsii_,"
_Archiv für Anat. und Physiol._ 1853.
11. Saenger. "Moskauer Naturforscher Sammlung," _Abth. Zool._ 1869.
12. H. N. Moseley. "On the Structure and Development of _Peripatus
capensis_," _Proc. Roy. Soc._ No. 153, 1874.
13. H. N. Moseley. "On the Structure and Development of _Peripatus
capensis_," _Phil. Trans._ Vol. CLXIV. 1874.
14. H. N. Moseley. "Remarks on Observations by Captain Hutton, Director of
the Otago Museum, on _Peripatus novæ zealandiæ_," _Ann. and Mag. of Nat.
History_, Jan. 1877.
15. Captain Hutton. "Observations on _Peripatus novæ zealandiæ_," _Ann. and
Mag. of Nat. History_, Nov. 1876.
16. F. M. Balfour. "On Certain Points in the Anatomy of _Peripatus
capensis_," _Quart. Journ. of Micr. Science_, Vol. XIX. 1879.
17. A. Ernst. _Nature_, March 10th, 1881.
EXPLANATION OF PLATES 46-53[578].
Footnote 578: The explanations of the figures printed within
inverted commas are by Professor Balfour, the rest are by the
Editors.
COMPLETE LIST OF REFERENCE LETTERS.
A. Anus. _a._ Dorso-lateral horn of white matter in brain. _a.g._ Accessory
gland of male (modified accessory leg gland). _at._ Antenna. _at.n._
Antennary nerve. _b._ Ventro-lateral horn of white matter of brain. _b.c._
Body-cavity. _bl._ Blastopore. C. Cutis. _c._ Postero-dorsal lobe of white
matter of brain. _c.g._ Supra-oesophageal ganglia. _cl._ Claw. _c.m._
Circular layer of muscles. _co._ Commissures between the ventral
nerve-cords. _co.2._ Second commissure between the ventral nerve-cords.
_co_{1}. 2. Mass of cells developed on second commissure. _cor._ Cornea.
_c.s.d._ Common duct for the two salivary glands. _cu._ Cuticle. _d._
Ventral protuberance of brain. _d.l.m._ Dorsal longitudinal muscle of
pharynx. _d.n._ Median dorsal nerve to integument from supra-oesophageal
ganglia. _d.o._ Muscular bands passing from the ventro-lateral wall of the
pharynx at the region of its opening into the buccal cavity. E. Eye. E.
Central lobe of white matter of brain. _e.n._ Nerves passing outwards from
the ventral cords. _ep._ Epidermis. _ep.c._ Epidermis cells. F.1, F.2,
_&c._ First and second pair of feet, &c. _f._ Small accessory glandular
tubes of the male generative apparatus. F._g._ Ganglionic enlargement on
ventral nerve-cord, from which a pair of nerves to foot pass off. _f.gl._
Accessory foot-gland. F._n._ Nerves to feet. _g.co._ Commissures between
the ventral nerve-cords containing ganglion cells. _g.o._ Generative
orifice. H. Heart. _h._ Cells in lateral division of body-cavity. _hy._
Hypoblast. _i.j._ Inner jaw. _j._ Jaw. _j.n._ Nerves to jaws. L. Lips. _l._
Lens. _l.b.c._ Lateral compartment of body-cavity. _le._ Jaw lever
(cuticular prolongation of inner jaw lying in a backwardly projecting
diverticulum of the buccal cavity). _l.m._ Bands of longitudinal muscles.
M. Buccal cavity. M{1}. Median backward diverticulum of mouth or common
salivary duct which receives the salivary ducts. _me._ Mesenteron. _mes._
Mesoblastic somite. _m.l._ Muscles of jaw lever. _m.s._ Sheets of muscle
passing round the side walls of pharynx to dorsal body-wall. _od._ Oviduct.
_oe._ OEsophagus. _oes.co._ OEsophageal commissures. _o.f.g._ Orifice of
duct of foot-gland. _o.j._ Outer jaw. _op._ Optic ganglion. _op.n._ Optic
nerve. _or.g._ Ganglionic enlargements for oral papillæ. _or.n._ Nerves to
oral papillæ. _or.p._ Oral papillæ. _o.s._ Orifice of duct of segmental
organ. _ov._ Ovary. P. Pads on ventral side of foot. _p._ Common duct into
which the vasa deferentia open. _p.c._ Posterior lobe of brain. _p.d.c._
Posterior commissure passing dorsal to rectum. _p.f._ Internal opening of
nephridium into body-cavity. _ph._ Pharynx. _pi._ Pigment in outer ends of
epidermic cells. _pi.r._ Retinal pigment. _p.n._ Nerves to feet. _p.p._
Primary papilla. _pr._ Prostate. R. Rectum. _Re._ Retinal rods. R. _m._
Muscle of claw. _s._ Vesicle of nephridium. _s_{1}. Part of 4th or 5th
nephridium which corresponds to vesicle of other nephridia. _s.c._1. Region
No. 1 of coiled tube of nephridium. _s.c._2. Region No. 2 of ditto.
_s.c._3. Region No. 3 of ditto. _s.c._4. Region No. 4 of ditto. _s.d._
Salivary duct. _s.g._ Salivary gland. _sl.d._ Reservoir of slime gland.
_sl.g._ Tubules of slime gland. _s.o._1, 2, 3, _&c._ Nephridia of 1st, 2nd,
&c., feet. _s.o.f._ Terminal portion of nephridium. _s.p._ Secondary
papilla. _st._ Stomach. _st.e._ Epithelium of stomach. _sy._ Sympathetic
nerve running in muscles of tongue and pharynx. _sy´_. Origin of pharyngeal
sympathetic nerves. T. Tongue. _t._ Teeth on tongue. _te._ Testis. _tr._
Tracheæ. _tr.c._ Cells found along the course of the tracheæ. _tr.o._
Tracheal stigma. _tr.p._ Tracheal pit. _ut._ Uterus. _v.c._ Ventral nerve
cord. _v.d._ Vas deferens. _v.g._ Imperfect ganglia of ventral cord.
PLATE 46.
Fig. 1. _Peripatus capensis_, x 4; viewed from the dorsal surface. (From a
drawing by Miss Balfour.)
PLATE 47.
Fig. 2. A left leg of _Peripatus capensis_, viewed from the ventral
surface; x 30. (From a drawing by Miss Balfour.)
Fig. 3. A right leg of _Peripatus capensis_, viewed from the front side.
(From a drawing by Miss Balfour.)
Fig. 4. The last left (17th) leg of a male _Peripatus capensis_, viewed
from the ventral side to shew the papilla at the apex of which the
accessory gland of the male, or enlarged crural gland, opens to the
exterior. (From a drawing by Miss Balfour.) Prof. Balfour left a rough
drawing (not reproduced) shewing the papilla, to which is appended the
following note. "Figure shewing the accessory genital gland of male, which
opens on the last pair of legs by a papilla on the ventral side. The
papilla has got a slit-like aperture at its extremity."
Fig. 5. Ventral view of head and oral region of _Peripatus capensis_. (From
a drawing by Miss Balfour.)
PLATE 48.
Figs. 6 and 7 are from one drawing.
Fig. 6. _Peripatus capensis_ dissected so as to shew the alimentary canal,
slime glands, and salivary glands; x 3. (From a drawing by Miss Balfour.)
Fig. 7. The anterior end of Fig. 6 enlarged; x 6. (From a drawing by Miss
Balfour.) 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, _j._, which have been turned outwards, and the tongue,
T., bearing a median row of chitinous teeth, which branches behind into
two. The junction of the salivary ducts, _s.d._, and the opening of the
median duct so formed into the buccal cavity is also shewn. The muscular
pharynx, extending back into the space between the 1st and 2nd 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.
Fig. 8. _Peripatus capensis_, x 4; male. (From a drawing by Miss Balfour.)
Dissected so as to shew the nervous system, slime glands, ducts of the
latter passing into the oral papilla, accessory glands opening on the last
pair of legs (enlarged crural glands), and segmental organs, viewed from
dorsal surface. The first three pairs of segmental organs consist only of
the vesicle and duct leading to the exterior. The fourth and fifth pairs
are larger than the succeeding, and open externally to the crural glands.
The ventral nerve-cords unite behind dorsal to the rectum.
A. Anus. _a.g._ Accessory generative gland, or enlarged crural gland of the
17th leg. _at._ Antenna. _c.g._ Supra-oesophageal ganglia with eyes. _co._
Commissures between the ventral nerve-cords. _d.n._ Large median nerve to
dorsal integument from hinder part of brain. F.1, 2, &c. Feet. _g.o._
Generative orifice. _oe._ OEsophagus. _oes.co._ OEsophageal commissures.
_or.p._ Oral papilla. _p.d.c._ Posterior dorsal commissure between the
ventral nerve-cords. _ph._ Pharynx. _p.n._ Nerves to feet, one pair from
each ganglionic enlargement. _sl.d._ Reservoir of slime gland. _sl.g._
Tubules of slime gland. _s.o._1, 2, 3, _&c._ Segmental organs. _v.c._
Ventral nerve-cords. _v.g._ Imperfect ganglia of ventral cords.
Figs. 9 and 10. Left jaw of _Peripatus capensis_ (male), shewing reserve
jaws. (From a drawing by Miss Balfour.)
Fig. 9. Inner jaw.
Fig. 10. Outer jaw.
PLATE 49.
Figs. 11-16. A series of six transverse sections through the head of
_Peripatus capensis._
Fig. 11. The section is taken immediately behind the junction of the
supra-oesophageal ganglia, _c.g._, and passes through the buccal cavity,
M., and jaws, _o.j._ and _i.j._
Fig. 12. The section is taken through the hinder part of the buccal cavity
at the level of the opening of the mouth into the pharynx and behind the
jaws. The cuticular rod-like continuation (_le._) of the inner jaw lying in
a backwardly directed pit of the buccal cavity is shewn; on the right hand
side the section passes through the opening of this pit.
Fig. 13. The section passes through the front part of the pharynx, and
shews the opening into the latter of the median backward diverticulum of
the mouth (M{1}), which receives the salivary ducts. It also shews the
commencement of the ventral nerve-cords, and the backwardly projecting
lobes of the brain.
Fig. 14. The section passes through the anterior part of the pharynx at the
level of the second commissure (_co._ 2), between the ventral nerve-trunks,
and shews the mass of cells developed on this commissure, which is in
contact with the epithelium of the backward continuation of the buccal
cavity (M{1}).
Fig. 15. Section through the point of junction of the salivary ducts with
the median oral diverticulum.
Fig. 16. Section behind the pharynx through the oesophagus.
_b.c._ Body-cavity. C. Cutis. _c.b.c._ Central compartment of body-cavity.
_c.g._ Supra-oesophageal ganglia. _c.m._ Layer of circular muscles. _co._
Commissure between ventral nerve-cords. _co._ 2. Second commissure between
the ventral nerve-cords. _co{1}._ 2. Mass of cells developed on second
commissure (probably sensory). _c.s.d._ Common duct for the two salivary
glands. _d.l.m._ Dorsal longitudinal muscles of pharynx. _d.o._ Muscles
serving to dilate the opening of the pharynx. _Ep._ Epidermis. _e.n._ Nerve
passing outwards from ventral nerve-cord. H. Heart. _i.j._ Inner jaw.
_j.p._ Jaw papillæ. _L._ Lips of buccal cavity. _l.b.c._ Lateral
compartment of body-cavity. _le._ Rod-like cuticular continuation of inner
jaw, lying in a pit of the buccal cavity. _l.m._ Bands of longitudinal
muscles. M. Buccal cavity. M{1}. Median backward continuation of buccal
cavity. _m.l._ Muscles of jaw lever. _m.s._ Muscular sheets passing from
side walls of pharynx to dorsal body-wall. _oe._ OEsophagus. _oes.co._
OEsophageal commissures. _o.j._ Outer jaw. _ph._ Pharynx. _s.d._ Salivary
duct. _s.g._ Salivary gland. _sl.d._ Reservoir of slime gland. _sy._
Sympathetic nerves running in muscles of tongue or pharynx. _sy{1}._ Origin
of sympathetic nerves to pharynx. T. Tongue. _v.c._ Ventral nerve-cords.
Figs. 17, 18. Two longitudinal horizontal sections through the head of
_Peripatus capensis_. Fig. 17 is the most ventral. They are both taken
ventral to the cerebral ganglia. In Fig. 17 dorsal tracheal pits are shewn
with tracheæ passing off from them. (Zeiss a a, Hartnack's camera.) C.
Cutis. _c.s.d._ Common salivary duct. _ep._ Epidermis. _i.j._ Inner jaw. M.
Buccal cavity. M{1}. Median backward diverticulum of mouth. _o.j._ Outer
jaw. _s.d._ Salivary ducts. T. Tongue. _t._ Teeth on tongue. _tr._ Tracheæ.
_tr.p._ Tracheal pits.
PLATE 50.
Fig. 19. "A, B, C, D, E, F, G." Seven transverse sections illustrating the
structure of the supra-oesophageal ganglia. (Zeiss A, Hartnack's camera.)
_a._ Dorso-lateral horn of white matter. _b._ Ventro-lateral horn of white
matter. _c._ Postero-dorsal lobe of white matter. _d._ Ventral protuberance
of brain. _e._ Central lobe of white matter. _o.p._ Optic ganglion.
"A. Section through anterior portions of ganglia close to the origin of the
antennary nerve. B. Section a little in front of the point where the two
ganglia unite. C. Section close to anterior junction of two ganglia. D.
Section through origin of optic nerve on the right side. E. Section shewing
origin of the optic nerve on the left side. F. Section through the
dorso-median lobe of white matter. G. Section near the termination of the
dorsal tongue of ganglion cells."
PLATE 51.
Fig. 20. Portion of a transverse section through the hinder part of
_Peripatus capensis_ (male). The section passes through a leg, and shews
the opening of the segmental organ (_o.s._), and of a crural gland,
_o.f.g._, and the forward continuation of the enlarged crural gland of the
17th leg (_f.gl._). (Zeiss a a, Hartnack's camera.) _a.g._ accessory gland
of male (modified crural gland of last leg). C. Cutis. _cl._ Claw. _cu._
Cuticle. _ep._ Epidermis. _f.gl._ Crural gland. _h._ Cells in lateral
compartment of body-cavity. _o.f.g._ Orifice of accessory foot gland.
_o.s._ Opening of segmental organ. P. Three spinous pads on ventral surface
of foot. _pr._ Prostate. R.M. Retractor muscle of claw. _s._ Vesicle of
nephridium. _s.c.i._ Region No. 1 of coiled part of nephridium. _sl.g._
Tubule of slime gland. _s.o.t._ Terminal portion of nephridium. _st._
Stomach. _st.e._ Epithelium of stomach. _v.c._ Ventral nerve-cord. _v.d._
Vas deferens.
Fig. 21. "Longitudinal vertical section through the supra-oesophageal
ganglion and oesophageal commissures of _Peripatus capensis_. (Zeiss a a,
Hartnack.)" _at._ Antenna. _e._ Central lobe of white matter. _j._ Part of
jaw. _s.g._ Salivary gland.
Fig. 22: drawn by Miss Balfour. Brain and anterior part of the ventral
nerve-cords of _Peripatus capensis_ enlarged and viewed from the ventral
surface. The paired appendages (_d_) of the ventral surface of the brain
are seen, and the pair of sympathetic nerves (_sy_{1}) arising from the
ventral surface of the hinder part.
From the commencement of the oesophageal commissures (_oes.co._) pass off
on each side a pair of nerves to the jaws (_j.n._).
The three anterior commissures between the ventral nerve-cords are placed
close together; immediately behind them the nerve-cords are swollen, to
form the ganglionic enlargements from which pass off to the oral papillæ a
pair of large nerves on each side (_or.n._).
Behind this the cords present a series of enlargements, one pair for each
pair of feet, from which a pair of large nerves pass off on each side to
the feet (_p.n_). _at.n._ Antennary nerves. _co._ Commissures between
ventral cords. _d._ Ventral appendages of brain. E. Eye. _e.n._ Nerves
passing outwards from ventral cord. _F.g._ Ganglionic enlargements from
which nerves to feet pass off. _j.n._ Nerves to jaws. _or.g._ Ganglionic
enlargement from which nerves to oral papillæ pass off. _or.n._ Nerves to
oral papillæ. _p.c._ Posterior lobe of brain. _p.n._ Nerves to feet. _s.y._
Sympathetic nerves.
Fig. 23. "Longitudinal horizontal section through the head of _Peripatus
capensis_, shewing the structure of the brain, the antennary and optic
nerves, &c. (Zeiss a a, Hartnack's camera.)" _at._ Antenna. _at.n._
Antennary nerve. _cor._ Cornea. _e._ Central mass of white matter. _l._
Lens. _op.n._ Optic nerve. _ph._ Pharynx. _p.p._ Primary papilla covered
with secondary papillæ and terminating in a long spine. _sy._ Pharyngeal
sympathetic nerves.
Fig. 24. "Eye of _Peripatus capensis_, as shewn in a longitudinal
horizontal section through the head. The figure is so far diagrammatic that
the lens is represented as filling up the whole space between the rods and
the cornea. In the actual section there is a considerable space between the
parts, but this space is probably artificial, being in part caused by the
shrinkage of the lens and in part by the action of the razor. (Zeiss C,
Hartnack's camera.)" (It appears that the ganglionic region of the eye is
covered by a thin capsule, which is omitted in the figure.)
_cor._ Cornea. _l._ Lens. _op._ Optic ganglion. _op.n._ Optic nerve.
_pi.r._ Pigment. _Re._ rods. _s.p._ Secondary papillæ.
Fig. 25. Longitudinal horizontal section through the dorsal skin, shewing
the peculiar arrangement of the circular muscular fibres. (Zeiss A,
Hartnack's camera.)
PLATE 52.
Fig. 26. Portion of ventral cord of _Peripatus capensis_ enlarged, shewing
two ganglionic enlargements and the origin of the nerves and commissures.
(From a drawing by Miss Balfour.)
_co._ Commissures. E._n._ Nerves passing out from ventral cords. F._n._
Nerves to feet. _g.co._ Commissures between the ventral cords containing
ganglion cells. _v.g._ Ganglionic enlargements.
Fig. 27. Segmental organ from the 5th pair of legs of _Peripatus capensis_.
This nephridium resembles those of the 4th legs, and differs from all the
others in its large size and in the absence of any dilatation giving rise
to a collecting vesicle on its external portion (enlarged). The terminal
portion has the same histological characters as in the case of the hinder
segmental organs. (From a drawing by Miss Balfour.)
Fig. 28. Segmental organ or nephridium from the 9th pair of legs of
_Peripatus capensis_, shewing the external opening, the vesicle, the coiled
portion and the terminal portion with internal opening (enlarged). (From a
drawing by Miss Balfour.)
_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_{1}. Portion of segmental organ of 4th and 5th legs,
corresponding to vesicle of the other nephridia. _s.c._1. First or external
portion of coiled tube of nephridium, lined by columnar epithelium with
small nuclei; the cells project for very different distances, giving the
inner boundary of this region a ragged appearance. _s.c._2. Region No. 2 of
coiled tube of nephridium, lined by small closely-packed columnar cells.
_s.c._3. Region No. 3 of coiled tube of segmental organ, lined by large
flat cells with large disc-shaped nuclei. _s.c._4. Region No. 4 of coiled
tube of nephridium; this region is very short and lined by small columnar
cells. _s.o.t._ Terminal portion of nephridium.
Fig. 29. "Portion of nephridium of the hindermost leg of _Peripatus
capensis_, seen in longitudinal and vertical section. The figure is given
to shew the peritoneal funnel of the nephridium. Portions of the collecting
sack (_s._) and other parts are also represented. (Zeiss B, Hartnack's
camera.)"
_p.f._ Peritoneal funnel. _s._ Vesicle. _s.c.1_, _s.c.2_, _s.c.3._ Portions
of coiled tube.
Fig. 30. "Section through a tracheal pit and diverging bundles of tracheal
tubes" taken transversely to the long axis of the body. (Zeiss E, oc. 2.)
(From a rough drawing by Prof. Balfour.)
_tr._ _Tracheæ_, shewing rudimentary spiral fibre. _tr.c._ Cells resembling
those lining the tracheal pits, which occur at intervals along the course
of the tracheæ. _tr.s._ Tracheal stigma. _tr.p._ Tracheal pit.
Fig. 31. "Sense organs and nerves attached from antenna of _Peripatus
capensis_ (Zeiss, immersion 2, oc. 2.)" (From a rough drawing by Prof.
Balfour.) The figure shews the arrangement of the epidermis cells round the
base of the spine. The spine is seen to be continuous with the inner layer
of the cuticle.
Fig. 32. Section through the skin of _Peripatus capensis_; it shews the
secondary papillæ covered with minute spinous tubercles and the relation of
the epidermis to them. (The cuticle in the process of cutting has been torn
away from the subjacent cells.) The cells of the epidermis are provided
with large oval nuclei, and there is a deposit of pigment in the outer ends
of the cells. The granules in the protoplasm of the inner ends of the cells
are arranged in lines, so as to give a streaked appearance. (Zeiss E, oc.
2.) (From a rough drawing by Prof. Balfour.)
_c._ Dermis. _cu._ Cuticle. _ep.c._ Epidermis cells. _pi._ Deposit of
pigment in outer ends of epidermis cells. _s.p._ Secondary papillæ.
Fig. 33. Female generative organs of _Peripatus capensis_, × 5. (From a
rough drawing by Prof. Balfour.) The following note was appended to this
drawing: "Ovary rather to dorsal side, lying in a central compartment of
body-cavity and attached to one of the longitudinal septa, dividing this
from the lateral compartment between the penultimate pair of legs and that
next in front. The oviducts cross before opening to the exterior, the right
oviduct passing under the rectum and the left over it. They meet by opening
into a common vestibule, which in its turn opens below the anus. On each
side of it are a pair of short papillæ (aborted feet?)."
F. 16, 17. Last two pairs of legs. _od._ Oviduct. _ov._ Ovary. _ut._
Uterus. _v.c._ Nerve-cord.
PLATE 53.
Figs. 34-39. Five young embryos of _Peripatus capensis_; ventral view. All,
excepting Fig. 37, from drawings by Miss Balfour. In figures 34 to 38_a_
denotes what is probably the anterior extremity.
Fig. 34, Stage A. Youngest embryo found, with slightly elongated
blastopore.
Fig. 35, Stage B. Embryo with three mesoblastic somites and elongated
blastopore. The external boundaries of the somites are not distinct.
Fig. 36, Stage C. Embryo with five somites. The blastopore is closing in
its middle portion.
Fig. 37, Stage D. The blastopore has completely closed in its middle
portion, and given rise to two openings, the future mouth and anus. (From a
rough drawing left by Professor Balfour.) (Zeiss A, Camera Oberhaus. on
level of stage.)
The following note was appended to this drawing in his handwriting: "Young
larva of _Peripatus capensis_. I could not tell for certain which was the
anterior end. Length, 1.34 mm."
Fig. 38, Stage E. Embryo with about thirteen mesoblastic somites in which
the flexure of the hind part of the body has commenced. The remains of the
original blastopore are present as the mouth, placed between the second
pair of mesoblastic somites, and the anus placed on the concavity of the
commencing flexure of the hind part of the body.
Fig. 39. Side view of same embryo.
Figs. 40-42. Drawings by Professor Balfour of three transverse sections
through the embryo from which fig. 36 was taken. (Zeiss c, Camera.) Figs.
40 and 42 pass through the region of the blastopore.
_bl._ Blastopore. _ep._ Epiblast. _hy._ Hypoblast. _me._ Mesenteron. _mes._
Mesoblastic somite.
Fig. 43. Male generative organs of _Peripatus capensis_, viewed from the
dorsal surface. (From a drawing by Miss Balfour.)
_a.g._ Enlarged crural glands of last pair of legs. F.16, 17. Last pairs of
legs. _f._ Small accessory glandular tubes. _p._ Common duct into which
vasa deferentia open. _p.r._ Prostate. _te._ Testes. _v.c._ Nerve-cord.
_v.d._ Vas deferens.
CAMBRIDGE: PRINTED BY C. J. CLAY, M.A., AND SON, AT THE UNIVERSITY PRESS.
TRANSCRIBER'S NOTES:
Underscores surround text in italics, _like this_.
Hyphens were spaced in ranges of small numbers to ease readability, e.g.,
"1/2000-1/3000 of an inch" was changed to "1/2000 - 1/3000 of an inch".
Raised dots in numbers were converted to decimals. Superscript letters
are enclosed in braces, e.g. P{1}.
The Greek letter, Lambda, is spelled out.
Use of periods and commas in the abbreviations within and referring to
illustrations is inconsistent. Often punctuation marks do not match the
illustrations to which they refer. Periods were retained; commas
were added to separate figure numbers from abbreviations within the figure.
Spacing within the abbreviations was standardized.
Footnotes were renumbered sequentially, indented, and moved to follow the
paragraph in which the anchor occurs. There is no anchor for footnote 496;
anchor was placed at the spot the transcriber deemed it likely belonged.
Changes for consistency within the text of the book:
body cavity to body-cavity
body wall to body-wall
choroid-slit to choroid slit
develope(s) to develop(s)
dog fish to dog-fish
Elasmobranchs to Elasmobranchii
Entwickelung to Entwicklung
head-fold to head fold
inter-renal to interrenal
juxta-position to juxtaposition
lenslike to lens-like
re-agent(s) to reagent(s)
omphalo-meseraic to omphalomeseraic
pleuroperitoneal to pleuro-peritoneal
proto-vertebra(æ) to protovertebra(æ)
re-appear to reappear
semi-lunar to semilunar
side-fold to side fold
spongework to sponge-work
subgerminal to sub-germinal
sub-intestinal to subintestinal
sub-kingdom to subkingdom
sub-notochordal to subnotochordal
suboesophageal to sub-oesophageal
supraoesophageal to supra-oesophageal
urino-genital to urinogenital
Urogenital-system to Urogenitalsystem, except where cited as a title of
a work.
Verwandschaft to Verwandtschaft
widespread to wide-spread
wood-cut(s) to woodcut(s)
zool. zoot. to zool.-zoot.
italics removed from eight instances of 'vide'
italics, where missing, were added to 'loc. cit', 'i.e.' and 'e.g.'
Other changes:
'reremainder' to 'remainder' ... as compared with the remainder of ...
'on' to 'or' ... one or two words ...
'splachnopleure' to 'splanchnopleure' ... where the somatopleure and
splanchnopleure unite,...
'Sitzen.' to 'Sitzun.' (Footnote - abbreviation for 'Sitzungsberichte')
'diffiulty' to 'difficulty' ... as a serious difficulty....
'it' to 'is' ... is still very difficult to observe....
'primive' to 'primitive' ... That such a condition could be a primitive
one seemed scarcely possible....
'opthalmicus' to 'ophthalmicus'
... to form ramus ophthalmicus superficialis ...
... as the _ramus ophthalmicus superficialis of the fifth nerve_ ...
'Ureierernester' to 'Ureiernester'
... nests of ova (Ureiernester),...
'vascula' to 'vascular' ... the subjacent vascular stroma ...
'Metozoa' to 'Metazoa' ... Coelenterata and the Metazoa....
duplicate word 'of' removed ...'(2) Of a layer of protoplasm' changed to
'(2) a layer of protoplasm' ...
'protodæum' to 'proctodæum' ... The proctodæum (_pr._) has also grown ...
'is it' to 'it is' ... where it is attach to the side of the body ...
'is is' to 'is in' ... but is in some respects peculiar....
'continous' to 'continuous' ... forms a continuous whole,...
'Zussammenhang' to 'Zusammenhang' ... Ueber d. Zusammenhang d. ...
'Tranverse' to 'Transverse' ... Transverse sections of the head ...
'odontophor' to 'odontophore' ... of the odontophore of a mollusc....
'lens' to 'legs' ... the last (17th) pair of legs are enlarged ...
'Platyelminthes' to 'Platyhelminthes' ...the nervous system of
Echinodermata, Platyhelminthes,...