præ-oral lobe not being very well marked. In the interior is a
globular digestive tract. The cilia form a broad central band leaving
free a narrow space at the apex of the præ-oral lobe, and also a
circumanal space. At the apex of the præ-oral lobe is placed a bunch
of long cilia, and a patch of cilia also marks out the anal area.
As the larva grows older it becomes elongated, and the anterior bunch
of cilia is absorbed. The alimentary canal divides itself into pharynx
and intestine. The former opens (?) by the mouth in the middle of the
central band of cilia, the latter in the anal patch. The setæ
indicating the segmentation are formed successively in the posterior
ring-like area free from cilia. The cilia disappear after the
formation of two segments.
In Lumbricus, the embryo of which ought perhaps to be grouped with the
atrochæ, the cilia (Kleinenberg) cover a ventral tract of epiblast
between the two mesoblastic cords, and are continued anteriorly to
form a circle round the mouth.
The monotrochal larvæ are provided only with the important præ-oral
ciliated ring before mentioned. In the majority of cases they are
transitional forms destined very shortly to become telotrochal, and in
such instances they usually have a more or less spherical body which
is nearly divided into two equal halves by a ciliated ring. In some
few instances, such as Polynoe, Dasychone, etc., the monotrochal
characters are not lost till the larval cilia are exuviated.
The telotrochal forms (of which examples are shewn in figs. 144, 150,
etc.) may (1) start as monotrochal; or (2) from the first have a
telotrochal character; or (3) be derived from atrochal forms. The last
mode of origin probably represents the ancestral one.
Their mode of development is well illustrated by the case of Terebella
nebulosa (_vide_ Milne-Edwards, No. 347). The embryo is at first a
nearly spherical ciliated mass. One end slightly elongates and becomes
free from cilia, and, acquiring dorsally two eye-spots, constitutes a
præ-oral lobe. The elongation continues at the opposite end, and near
this is formed a narrow area free from cilia. The larva now has the
same characters as the atrochal Eunicidan larva described above. It
consists of a non-ciliated præ-oral lobe, followed by a wide ciliated
band, behind which is a ring-like area free from cilia; and behind
this again a peri-anal patch of cilia. The ring-like area free from
cilia is, as in the Eunicidan larva, the region which becomes
segmented. It soon becomes longer, and is then divided into two
segments; a third and fourth etc. non-ciliated segment becomes
successively interposed immediately in front of the peri-anal patch;
and, after a certain number of segments have become formed, there
appear on some of the hinder of them short tubercles, provided with
single setæ (the notopodia), which are formed from before backwards,
like the segments.
The mouth, anus, and intestine become in the meantime clearly visible.
The mouth is on the posterior side of the ciliated band, and the anus
in the centre of the peri-anal patch.
The ciliated band in front now becomes contracted and provided with
long cilia. It passes below completely in front of the mouth, and
constitutes, in fact, a well-marked præ-oral ring, while the cilia
behind constitute an equally marked peri-anal ring. The larva has in
fact now acquired all the characters of a true telotrochal form.
Only a comparatively small number of Chætopod larvæ remain permanently
telotrochal. Of these Terebella nebulosa, already cited (though not
Terebella conchilega), is one; Polygordius, Saccocirrus and Capitella
are other examples of the same, though in the latter form the whole
ventral surface becomes ciliated.
The majority of the originally telotrochal forms become polytrochal.
In most cases the ciliated rings or half rings of the polytrochal
forms are placed at equal distances, one for each segment. They are
especially prominent in surface-swimming larvæ, and are in rare cases
preserved in the adult. In some instances (_e.g._ Nerine and Spio) the
ventral half rings, instead of being segmentally arranged, are
somewhat irregularly distributed amongst the segments, so that there
does not seem to be a necessary correspondence between the ciliated
rings and the segments. This is further shewn by the fact that the
ciliated rings are not precursors of the true segmentation, but are
developed after the establishment of the segments, and thus seem
rather to be secondarily adapted to the segments than primarily
indicative of them.
In most Polytrochæ the rings are incomplete, so that they fall under
the category of Nototrochæ or Gasterotrochæ.
The larva of Odontosyllis is an example of the former, and that of
Magelona of the latter. The larvæ of Nerine and Spio, already quoted
as examples of an unsegmented arrangement of the ventral ciliated half
rings, are both amphitrochal forms.
As an example of a polytrochal form with complete ciliated rings
Ophryotrocha puerilis may be cited. This form, discovered by Claparède
and Metschnikoff, develops a complete ciliated ring on each segment:
and the præ-oral ring, though at first single, becomes at a later
period divided into two. This form is further exceptional in that the
ciliated rings are persistent in the adult.
The unimportance of the character of the rings in the polytrochal
forms is shewn by such facts as the absence of these rings in
Terebella nebulosa and the presence of dorsal half rings in Terebella
conchilega.
The mesotrochal forms are the rarest of Chætopod larvæ, and would seem
to be confined to the Chætopteridæ.
Their most striking character is the presence of one or two complete
ciliated rings which girth the body between the mouth and anus. The
whole body is further covered with short cilia. The anus has a
distinct dorsal situation, while on its ventral side there projects
backwards a peculiar papilla.
The total absence of the typical præ-oral and of the peri-anal bands
separates the mesotrochal larvæ very sharply from all the previous
types.
A characteristic of many Chætopod larvæ is the presence of a bunch of
cilia or a single flagellum at the apex of the præ-oral lobe. The
presence of such a structure is characteristic of the larval forms of
many other groups, Turbellarians, Nemertines, Molluscs, etc.
In the preceding section the mode of multiplication of the segments
has already been sufficiently described[140].
[140] It has been insisted by Semper (No. 355) that certain of
the anterior segments, belonging to what he regards as the head
region in opposition to the trunk, become interpolated between
the trunk and the head. The general evidence, founded on
observations of budding, which he brings forward, cannot be
discussed here. But the special instance which he cites (founded
on Milne-Edwards's (No. 347) observations) of the interpolation
of the head segments, bearing the gills, in Terebella appears to
me quite unjustified from Milne-Edwards's own statements; and is
clearly shewn to be unfounded by the careful observations of
Claparède on Ter. conchilega, where the segments in question are
demonstrated to be present from the first.
[FIG. 151. LARVA OF PHYLLODOCE FROM THE VENTRAL SIDE. (From Alex.
Agassiz.)]
Apart from the formation of the segments the larval metamorphosis
consists in the atrophy of the provisional ciliated rings and other
provisional organs, and in the acquirement of the organs of the adult.
The great variations in the nature of the Chætopod appendages render
it impossible to treat this part of the developmental history of the
Chætopoda in a systematic way.
The mode of development of the appendages is not constant, so that it
is difficult to draw conclusions as to the primitive form from which
the existing types of appendages are derived.
In a large number of cases the primitive rudiments of the feet exhibit
no indication of a division into notopodium and neuropodium; while in
other instances (_e.g._ Terebella and Nerine, fig. 152) the notopodium
is first developed, and subsequently the neuropodium quite
independently.
In many cases the setæ appear before there are any other visible
rudiments of the feet (_e.g._ Lumbriconereis); while in other cases
the reverse holds good. The gills are usually the last parts to
appear.
[FIG. 152. LARVA OF NERINE, WITH PROVISIONAL SETÆ. (From Alex.
Agassiz.)]
Not only does the mode of development of the feet differ greatly in
different types, but also the period. The appearance of setæ may
afford the first external indication of segmentation, or the rudiments
of the feet may not appear till a large number of segments are
definitely established.
A very considerable number of Chætopod larvæ are provided with very
long provisional setæ (figs. 152 and 153). These setæ are usually
placed at the sides of the anterior part of the body, immediately
behind the head, and also sometimes on the posterior parts of the
body. In some instances (_e.g._ fig. 153) they form the only
appendages of the trunk. Alex. Agassiz has pointed out that setæ of
this kind, though not found in existing Chætopods, are characteristic
of the fossil forms. Setæ of this kind are found in chætopod-like
larvæ of some Brachiopods (Argiope, fig. 136).
[FIG. 153. EMBRYO CHÆTOPOD WITH PROVISIONAL SETÆ. (From Agassiz.)]
It is tempting to suppose that the long provisional bristles springing
from the oral region are the setiform appendages handed down from the
unsegmented ancestors of the existing Chætopod forms. Claparède has
divided Chætopod larvæ into two great groups of Metachætæ and
Perennichætæ, according as they possess or are without provisional
setæ.
With reference to the head and its appendages it has already been
stated that the head is primarily formed of the præ-oral lobe and of
the peristomial region.
The embryological facts are opposed to the view that the præ-oral
region either represents a segment or is composed of segments
equivalent to those of the trunk. The embryonic peristomial region
may, on the other hand, be regarded as in a certain sense the first
segment. Its exact relations to the succeeding segments become
frequently more or less modified in the adult. The præ-oral region is
in most larvæ bounded behind by the ciliated ring already described.
On the dorsal part of the præ-oral lobe in front of this ring are
placed the eyes, and from it there may spring a variable number of
processes which form antennæ or cephalic tentacles. The number and
position of these latter are very variable. They appear as simple
processes, sometimes arising in pairs, and at other times alternating
on the two sides. There is frequently a median unpaired tentacle.
The development of the median tentacle in Terebella, where there is in
the adult a great number of similar tentacles, is sufficiently
remarkable to deserve special notice; _vide_ Milne-Edwards, Claparède,
etc. It arises long before any of the other tentacles as a single
anterior prolongation of the præ-oral lobe containing a parenchymatous
cavity, which communicates freely with the general perivisceral
cavity. It soon becomes partially constricted off at its base from the
procephalic lobe, but continues to grow till it becomes fully half as
long as the remainder of the body. A very characteristic figure of the
larva at this stage is given by Claparède and Metschnikoff, Pl. XVII.,
Fig. 1 E. It now strikingly resembles the larval proboscis of
Balanoglossus, and it is not easy to avoid the conclusion that they
are homologous structures.
Another peculiar cephalic structure which deserves notice is the gill
apparatus of the Serpulidæ.
In Dasychone (Sabella) the gill apparatus arises (Claparède and
Metschnikoff, No. 336) as a pair of membranous wing-like organs on the
dorsal side of the præ-oral lobe immediately in front of the ciliated
ring. Each subsequently becomes divided into two rays, and new rays
then begin to sprout on the ventral side of the two pairs already
present. A cartilaginous axis soon becomes formed in these rays, and
after this is formed fresh rays sprout irregularly from the
cartilaginous skeleton.
[FIG. 154. LARVA OF SPIRORBIS. (From Alex. Agassiz.)
The first odd tentacle (_t_) is shewn on the right side.
Behind the præ-oral ciliated ring is the large collar.]
In Spirorbis spirillum as observed by Alex. Agassiz, the right
gill-tentacle (fig. 154, _t_) first appears, and then the left, and
subsequently the odd opercular tentacle which covers the right
original tentacle. The third and fourth tentacles are formed
successively on the two sides, and rapidly become branched in the
succeeding stages.
With reference to the sense organs it may be noted that the eyes, or
at any rate the cephalic pigment spots, are generally more numerous in
the embryo than in the adult, and that they are usually present in the
larvæ of the Sedentaria, though absent in the adults of these forms.
The Sedentaria thus pass through a larval stage in which they resemble
the Errantia.
Paired auditory vesicles of a provisional character have been found on
the ventral side of the body, in the fourth segment behind the mouth,
in the larva of Terebella conchilega (Claparède).
Mitraria. A peculiar larval Chætopod form known as Mitraria, the
metamorphosis of which was first worked out by Metschnikoff, deserves
a special notice.
This form (fig. 155 A) in spite of its remarkable appearance can
easily be reduced to the normal type of larva.
The mouth (_m_) and anus (_an_) (fig. 155 A) are closely approximated,
and situated within a vestibule the edge of which is lined by a simple
or lobed ciliated ring. The shape of the body is somewhat conical. The
cavity of the vestibule forms the base of the cone, and at the apex is
placed a ciliated patch (_sg_). A pair of lobes (_br_) bear
provisional setæ. The alimentary canal is formed of the three normal
parts, oesophagus, stomach, and intestine.
[FIG. 155. TWO STAGES IN THE DEVELOPMENT OF MITRARIA. (After
Metschnikoff.)
_m._ mouth; _an._ anus; _sg._ supra-oesophageal ganglion; _br._
provisional bristles; _pr.b._ præ-oral ciliated band.]
To compare this larva with an ordinary Chætopod larva one must suppose
that the alimentary canal is abnormally bent, so that the post-oral
ventral surface is reduced to the small space between the mouth and
the anus. The ciliated band surrounding the vestibule is merely the
usual præ-oral band, borne on the very much extended edge of the
præ-oral lobe. The apex of the larva is the front end of the præ-oral
lobe with the usual ciliated patch. The two lobes with provisional
bristles are really dorsal and not posterior.
The correctness of the above interpretation is clearly shewn by the
metamorphosis.
The first change consists in the pushing in of a fold of skin, between
the mouth and anus, towards the intestine, which at the same time
rapidly elongates, and forms the axis of a conical projection, which
thereupon becomes segmented and is thereby shewn to be the rudiment of
the trunk (fig. 155 B). On the elongation of the trunk in this way the
præ-oral lobe and its ciliated ring assume an appearance not very
dissimilar to the same structures in Polygordius. At the ciliated apex
of the præ-oral lobe a paired thickening of epiblast gives rise to the
supra-oesophageal ganglia (_sg_). In the further metamorphosis, the
præ-oral lobe and its ciliated ring gradually become reduced, and
finally atrophy in the normal way, while the trunk elongates and
acquires setæ. The dorsally situated processes with provisional setæ
last for some time, but finally disappear. The young worm then
develops a tube and shews itself as a normal tubicolous Chætopod.
_Formation of Organs._
Except in the case of a few organs our knowledge of the formation of
the organs in the Chætopoda is derived from investigations on the
Oligochæta.
The embryo of the Oligochæta has a more or less spherical form, but it
soon elongates, and becoming segmented acquires a distinct vermiform
character. The ventral surface is however for a considerable time
markedly convex as compared to the dorsal.
The ventrally placed mouth is surrounded by a well-marked lip, and in
front of it is placed a small præ-oral lobe.
[FIG. 156. SECTION THROUGH THE HEAD OF A YOUNG EMBRYO OF LUMBRICUS
TRAPEZOIDES. (After Kleinenberg.)
_c.g._ cephalic ganglion; _cc._ cephalic portion of the body cavity;
_x._ oesophagus.]
The epiblast. The epiblast cells at the commencement of the gastrula
stage become much flattened, and on the completion of the invagination
form an investment of flattened cells, only thickened in the
neighbourhood of the mesoblastic bands (fig. 141 B and C). In the
Polychæta at any rate the statements of several investigators would
seem to indicate that the cuticle is derived from the chorion. It is
difficult to accept this conclusion, but it deserves further
investigation.
Nervous system. The most important organ derived from the epiblast is
the nervous system; the origin of which from this layer was first
established by Kowalevsky (No. 342).
[FIG. 157. SECTION THROUGH PART OF THE VENTRAL WALL OF THE TRUNK OF
AN EMBRYO OF LUMBRICUS TRAPEZOIDES. (After Kleinenberg.)
_m._ longitudinal muscles; _so._ somatic mesoblast; _sp._ splanchnic
mesoblast; _hy._ hypoblast; _Vg._ ventral nerve cord; _vv._ ventral
vessel.]
It arises[141] (Kleinenberg, No. 341) from two at first quite distinct
structures, viz. (1) the supra-oesophageal rudiment and (2) the
rudiment of the ventral cord. The former of these takes its origin as
an unpaired dorsal thickening of the epiblast at the front end of the
head (fig. 156, _c.g._), which sends two prolongations downwards and
backwards to meet the ventral cord. The latter arises as two
independent thickenings of the epiblast, one on each side of the
ventral furrow (fig. 157, _Vg_). These soon unite underneath the
furrow, in the median line, and after being differentiated into
segmentally arranged ganglionic and interganglionic regions become
separated from the epiblast. Both the supra-oesophageal and ventral
cord become surrounded by a layer of somatic mesoblast. The junction
between the two parts of the central nervous system takes place
comparatively late.
[141] For further details, _vide_ general chapter on Nervous
System.
The mesoblast. It is to Kowalevsky (No. 342) and Kleinenberg (No. 341)
that we mainly owe our knowledge of the history of the mesoblast. The
fundamental processes which take place are (1) the splitting of the
mesoblast into splanchnic and somatic layers with the body cavity
between them, (2) the transverse division of the mesoblast of the
trunk into distinct somites.
The former process commences in the cephalic mesoblastic commissure,
where it results in the formation of a pair of cavities each with a
thin somatic and thick splanchnic layer (fig. 156, _cc_); and thence
extends gradually backwards into the trunk (fig. 141 C, _pp_). In the
trunk however the division into somites precedes the horizontal
splitting of the mesoblast. The former process commences when the
mesoblastic bands form widish columns quite separate from each other.
These columns become broken up successively from before backwards into
somewhat cubical bodies, in the centre of which a cavity soon appears.
The cavity in each somite is obviously bounded by four walls, (1) an
outer, the somatic, which is the thickest; (2) an inner, the
splanchnic; and (3, 4) an anterior and posterior. The adjoining
anterior and posterior walls of successive somites unite together to
form the transverse dissepiments of the adult, which subsequently
become very thin and are perforated in numerous places, thus placing
in communication the separate compartments of the body cavity. The
somites, though at first confined to a small area on the ventral side,
gradually extend so as to meet their fellows above and below and form
complete rings (fig. 157) of which the splanchnic layer (_sp_)
attaches itself to the enteric wall and the somatic (_so_) to the
epiblast. In Polygordius and probably also Saccocirrus and other forms
the cavities of the somites of the two sides do not coalesce; and the
walls which separate them constitute dorsal and ventral mesenteries.
The two cavities in the cephalic commissure unite dorsally, but
ventrally open into the first somite of the trunk.
The mesoblastic masses of the head are probably not to be regarded as
forming a pair of somites equivalent to those in the trunk, but as
forming the mesoblastic part of the præ-oral lobe, of which so much
has been said in the preceding pages. Kleinenberg's observations are
however of great importance as shewing that the cephalic cavities are
simply an anterior part of the true body cavity.
The splanchnic layer of the head cavity gives rise to the musculature
of the oesophagus.
The somatic layer of the trunk somites becomes converted into the
musculature of the body wall and the external peritoneal layer of body
cavity. The first part of the muscular system to be definitely formed
is the ventral band of longitudinal muscles which arises on each side
of the nervous system in contact with the epidermis (fig. 157, _m_).
How the circular muscles become subsequently formed outside these
muscles has not been made out.
The splanchnic layer of the trunk somites gives rise to the muscular
and connective-tissue wall of the mesenteron, and also to the walls of
the vascular trunks. The ventral vessel is first formed (Kowalevsky)
as a solid mass of cells which subsequently becomes hollowed out. The
dorsal vessel in Lumbricus and Criodrilus is stated by Kowalevsky and
Vejdovsky to be formed by the coalescence of two lateral vessels; a
peculiarity which is probably to be explained by the late extension of
the mesoblast into the dorsal region.
The layer from which the sacks for the setæ and the segmental organs
spring is still doubtful. The sacks for the setæ are believed by
Kowalevsky (No. 342) to be epiblastic invaginations, but are stated by
Hatschek (No. 339) to be mesoblastic products. For the development of
the segmental organs the reader is referred to the chapter on the
excretory system.
In marine Polychæta the generative organs are no doubt mesoblastic
products, as they usually spring from the peritoneal epithelium,
especially the parts of it covering the vascular trunks.
The Alimentary Canal. In Lumbricus the enteric cavity is formed during
the gastrula stage. In Criodrilus the hypoblast has at first no lumen,
but this becomes very soon established. In Euaxes on the other hand,
where there is a true epibolic gastrula, the mesenteron is at first
represented by a solid mass of yolk (_i.e._ hypoblast) cells. As the
central amongst these become absorbed a cavity is formed. The
protoplasm of the yolk cells which line this cavity unites into a
continuous polynuclear layer containing at intervals masses of yolk.
These masses become gradually absorbed, and the protoplasmic wall of
the mesenteron then breaks up into a cylindrical glandular epithelium
similar to that of the other types.
In Lumbricus and Criodrilus the blastopore remains as the mouth, but
in Euaxes a new mouth or rather stomodæum is formed by an epiblastic
invagination between the front end of the two mesoblastic bands. This
epiblastic invagination forms the permanent oesophagus; and in
Lumbricus trapezoides and Criodrilus, where the oral opening is at
first lined by hypoblast, the epiblast soon becomes inflected so as to
line the oesophageal region. The splanchnic mesoblast of the cephalic
region subsequently invests the oesophagus, and some of its cells
penetrating between the adjoining epiblast cells give rise to a thick
wall for this part of the alimentary tract; the original epiblast
cells being reduced to a thin membrane. This mesoblastic wall is
sharply separated from the muscular wall outside, which is also formed
of splanchnic mesoblast.
The anus is a late formation.
_Alternations of generations._
Amongst Chætopoda a considerable number of forms exhibit the
phenomenon of alternations of generations, which in the same general
way as in the case of the Coelenterata, is secondarily caused by
budding or fission.
The process of fission essentially consists in the division of a
parent form into two zooids by the formation of a zone of fission
between two old rings, which becomes differentiated (1) into an anal
zone in front which forms the anal region of the anterior zooid, and
(2) into a cephalic zone behind which forms the head and some of the
succeeding segments of the posterior zooid. The anal zone is capable,
by growth and successive segmentation, of giving rise to an indefinite
number of fresh segments.
In Protula Dysteri, as shewn by Huxley, there is a simple fission into
two in the way described. Sexual reproduction does not take place at
the same time as reproduction by fission, but both zooids produced are
quite similar and multiply sexually.
In the freshwater forms Nais and Chætogaster a more or less similar
phenomenon takes place. By a continual process of growth in the anal
zones, and the formation of fresh zones of fission whenever four or
five segments are added in front of an anal zone, complicated chains
of adhering zooids are produced, each with a small number of segments.
As long as the process of fission continues sexual products are not
developed, but eventually the chains break up, the individuals derived
from them cease to go on budding, and, after developing a considerably
greater number of segments than in the asexual state, reproduce
themselves sexually. The forms developed from the ovum then repeat
again the phenomenon of budding, etc., and so the cycle is
continued[142].
[142] Reproduction by budding and formation of the sexual
products to some extent overlap.
The phenomena so far can hardly be described as cases of alternation
of generations. The process is however in certain types further
differentiated. In Syllis (Quatrefages) fission takes place, the
parent form dividing into two, of which only the posterior after its
detachment develops sexual organs. The anterior asexual zooid
continues to produce fresh sexual zooids by fission. In Myrianida
also, where a chain of zooids is formed, the sexual elements seem to
be confined to the individuals produced by budding.
The cases of Syllis and Myrianida seem to be genuine examples of
alternations of generations, but a still better instance is afforded
by Autolytus (Krohn, No. 343, and Agassiz, No. 333).
In Autolytus cornutus the parent stock, produced directly from the
egg, acquires about 40-45 segments, and then gives rise by fission,
with the production of a zone of fission between about the 13th and
14th rings, to a fresh zooid behind. This after becoming fully
developed into either a male or a female is detached from the parent
stock, from which it very markedly differs. The males and females are
moreover very different from each other. In the female zooid the eggs
are carried into a kind of pouch where they undergo their development
and give rise to asexual parent stocks. After the young are hatched
the female dies. The asexual stock, after budding off one asexual
zooid, elongates again and buds off a second zooid. It never develops
generative organs.
The life history of some species of the genus Nereis presents certain
very striking peculiarities which have not yet been completely
elucidated.
As was first shewn by Malmgren asexual examples of various species of
Nereis may acquire the characters of Heteronereis and become sexually
mature.
The metamorphosis of Nereis Dumerilii has been investigated by
Claparède, who has arrived at certain very remarkable conclusions. He
finds that there are two distinct sexual generations of the Nereis
form of this species, and two distinct sexual generations of the
Heteronereis form.
One sexual Nereis, characterized by its small size, is dioecious, the
other discovered by Metschnikoff is hermaphrodite.
Of the Heteronereis sexual forms, both are dioecious, one is small,
and swims on the surface, the other is larger and lives at the bottom.
How these various generations are mutually related has not been made
out; but Claparède traced the passage of large asexual examples of the
Nereis form into the large Heteronereis form.
BIBLIOGRAPHY.
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(333) Alex. Agassiz. "On the embryology of Autolytus cornutus and
alternations of generations, etc." _Boston Journal of Nat. History_,
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(334) W. Busch. _Beobachtungen ü. Anat. u. Entwick. einiger
wirbelloser Seethiere_, 1851.
(335) Ed. Claparède. _Beobachtungen ü. Anat. u. Entwick. wirbelloser
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(337) E. Grube. _Untersuchungen üb. Entwicklung d. Anneliden._
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(338) B. Hatschek. "Beiträge z. Entwick. u. Morphol. d. Anneliden."
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(339) B. Hatschek. "Studien über Entwicklungsgeschichte der
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(340) Th. H. Huxley. "On hermaphrodite and fissiparous species of
tubicolar Annelidæ (Protula)." _Edinburgh New Phil. Journal_, Vol. I.
1855.
(341) N. Kleinenberg. "The development of the earthworm Lumbricus
trapezoides." _Quart. J. of Micr. Science_, Vol. XIX. 1879. _Sullo
sviluppo del Lumbricus trapezoides._ Napoli, 1878.
(342) A. Kowalevsky. "Embryologische Studien an Würmern u.
Arthropoden." _Mém. Acad. Pétersbourg_, Series VII. Vol. XVI. 1871.
(343) A. Krohn. "Ueber die Erscheinungen bei d. Fortpflanzung von
Syllis prolifera u. Autolytus prolifer." _Archiv f. Naturgesch._ 1852.
(344) R. Leuckart. "Ueb. d. Jugendzustände ein. Anneliden, etc."
_Archiv f. Naturgesch._ 1855.
(345) S. Lovén. "Beobachtungen ü. die Metamorphose von Anneliden."
Weigmann's _Archiv_, 1842.
(346) E. Metschnikoff. "Ueber die Metamorphose einiger Seethiere
(Mitraria)." _Zeit. f. wiss. Zool._ Vol. XXI. 1871.
(347) M. Milne-Edwards. "Recherches zoologiques, etc." _Ann. Scie.
Natur._ III. Série, Vol. III. 1845.
(348) J. Müller. "Ueb. d. Jugendzustände einiger Seethiere." _Monats.
d. k. Akad. Wiss._ Berlin, 1851.
(349) Max Müller. "Ueber d. weit. Entwick. von Mesotrocha sexoculata."
Müller's _Archiv_, 1855.
(350) Quatrefages. "Mémoire s. l'embryogénie des Annelides." _Ann.
Scie. Natur._ III. Série, Vol. X. 1848.
(351) M. Sars. "Zur Entwicklung d. Anneliden." _Archiv f.
Naturgeschichte_, Vol. XI. 1845.
(352) A. Schneider. "Ueber Bau u. Entwicklung von Polygordius."
Müller's _Archiv_, 1868.
(353) A. Schneider. "Entwicklung u. system. Stell. d. Bryozoen u.
Gephyreen (Mitraria)." _Archiv f. mikr. Anat._ Vol. V. 1869. (354) M.
Schultze. _Ueb. die Entwicklung von Arenicola piscatorum u. anderer
Kiemenwürmer._ Halle, 1856.
(355) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten
Thiere." _Arbeiten a. d. zool.-zoot. Instit._ Würzburg, Vol. III.
1876-7.
(356) C. Semper. "Beiträge z. Biologie d. Oligochæten." _Arbeiten a.
d. zool.-zoot. Instit._ Würzburg, Vol. IV. 1877-8.
(357) M. Stossich. "Beiträge zur Entwicklung d. Chætopoden." _Sitz. d.
k. k. Akad. Wiss. Wien_, B. LXXVII. 1878.
(358) R. v. Willemoes-Suhm. "Biologische Beobachtungen ü. niedrige
Meeresthiere." _Zeit. f. wiss. Zool._ Bd. XXI. 1871.
CHAPTER XIII.
DISCOPHORA[143].
[143] The Discophora are divided into the following groups:
I. Rhyncobdellidæ.
II. Gnathobdellidæ.
III. Branchiobdellidæ.
The eggs of the Discophora, each enclosed in a delicate membrane, are
enveloped in a kind of mucous case formed by a secretion of the
integument, which hardens into a capsule or cocoon. In each cocoon
there are a limited number of eggs surrounded by albumen. The cocoons
are attached to water plants, etc. In Clepsine the embryos leave the
cocoon very soon after they get rid of the egg membrane, but in
Nephelis they remain within the cocoon for a very much longer period
(27-28 days after hatching). The young of Clepsine, after their
liberation, attach themselves to the ventral surface of their parent.
Our knowledge of the development of the Discophora is in a very
unsatisfactory state; but sufficient is known to shew that it has very
many points in common with that of the Oligochæta, and that the
Discophora are therefore closely related to the Chætopoda. In Clepsine
there is an epibolic gastrula, and mesoblastic bands like those in
Euaxes are also formed. In Nephelis however the segmentation is very
abnormal, and the formation of the germinal layers cannot easily be
reduced to an invaginate gastrula type, though probably it is modified
from such a type. Mesoblastic bands similar to those in the Oligochæta
occur in this form also.
The embryology of Clepsine, which will serve as type for the Leeches
without jaws (Rhyncobdellidæ), has recently been studied by Whitman
(No. 365), and that of Nephelis, which will serve as type for the
Leeches with jaws (Gnathobdellidæ), has been studied by Bütschli (No.
359). The early history of both types is imperfectly known[144].
[144] Hoffmann's account (No. 36) is so different from that of
other observers that I have been unable to make any use of it.
_Formation of the layers._
Clepsine. It is necessary to give a full account of the segmentation
of Clepsine, as the formation of the germinal layers would be
otherwise unintelligible.
Segmentation commences with the division of the ovum into two unequal
spheres by a vertical cleavage passing from the animal to the
vegetative pole. By a second vertical cleavage the large segment is
divided into two unequal parts, and the small one into two equal
parts. Of the four segments so produced three are relatively small,
and one, placed at the posterior end, is large. Each of the four
segments next gives rise to a small cell at the animal pole. These
small cells form the commencement of the epiblast, and, according to
Whitman, the mouth is eventually placed in their centre. Such a
position for the mouth, at the animal pole, is extremely unusual, and
the statements on this head require further confirmation.
[FIG. 158. TWO VIEWS OF THE LARVA OF CLEPSINE. (After Whitman.)
_o._ oral extremity; _m._ mouth; _pr._ germinal streak.
A. This figure shews the blastoderm (shaded) with a thickened edge
formed by the primitive (_i.e._ mesoblastic) streaks with the four
so-called neuroblasts posteriorly. The vitelline spheres are left
without shading.
B. represents an embryo in which the blastoderm has enclosed the
yolk, and in which the division into segments has taken place. At
the hind end are shewn the so-called neuroblasts forming the
termination of the germinal streak.]
The posterior large segment now divides into two, one of which is
dorsal, and the other and larger ventral. The former I shall call with
Whitman the neuroblast, and the latter the mesoblast. The mesoblast
very shortly divides again. During the formation of the neuroblast and
mesoblast additional epiblastic small cells are added from the three
spheres which give rise to three of the primitive epiblast cells,
which may now be called the vitelline spheres.
The neuroblast next divides into ten cells, of which the two smaller
are soon broken up into epiblastic cells, while the remaining eight
arrange themselves in two groups of four each, one group on each side
at the posterior border of the epiblastic cap. The two mesoblasts also
take up a position on the right and left sides immediately ventral to
the four neuroblasts of each side. The neuroblasts and mesoblasts now
commence to proliferate at their anterior border, and produce on each
side a thickened band of cells underneath the edge of the cap of
epiblast cells. Each of these bands is formed of a superficial
quadruple[145] row of neuroblasts budded off from the four primary
neuroblasts, and a deeper row of mesoblasts. The compound streaks so
formed may be called the germinal streaks.
[145] According to Robin it is more usual for there to be only a
triple row of primary neuroblasts.
The general appearance of the embryo as seen from the dorsal surface,
after the appearance of the two germinal streaks, may be gathered from
fig. 158 A. The epiblastic cap in this figure is shaded. The
epiblastic cap, accompanied by the germinal streaks, now rapidly
extends and encloses the three vitelline spheres by a process
equivalent to that of an ordinary epibolic gastrula; but the front and
hind ends of the streaks remain practically stationary. Owing to this
mode of growth the edges of the epiblastic cap and the germinal
streaks meet in a linear fashion along the ventral surface of the
embryo (fig. 159, A and B). The germinal streaks first meet anteriorly
(B) and their junction is then gradually continued backwards. The
process is completed at about the time of hatching.
During the above changes the nuclei of the vitelline spheres pass to
the surface and rapidly divide. Eventually, together with part of the
protoplasm of the vitelline spheres, they appear to give rise to a
layer of hypoblastic cells. This layer encloses the remains of the
vitelline spheres, which become the yolk.
[FIG. 159. TWO EMBRYOS OF CLEPSINE IN WHICH THE GERMINAL STREAKS
HAVE PARTIALLY MET ALONG THE VENTRAL LINE. (After Robin.)
_gs._ germinal, _i.e._ mesoblastic streaks.
The area covered by epiblast is shaded. The so-called neuroblasts at
the end of the germinal streaks are shewn in B.]
At the front end of the germinal streaks, in a position corresponding
with that of the four original epiblast cells, two depressions appear
which coalesce to form the single oral invagination; in the centre of
which are formed the mouth and pharynx by a second epiblastic
invagination.
The most important point in connection with the above history is the
fate of what have been called the germinal streaks. According to
Whitman they are composed of two kinds of cells, viz. four rows of
smaller superficial cells, which he calls neuroblasts, and, in the
later stages at any rate, a row of deeper large cells, which he calls
mesoblasts. As to the eventual fate of these cells he states that the
neuroblasts uniting together in the median line form the rudiment of
the ventral ganglionic chain, while the mesoblasts equally coalesce
and give rise to the mesoblast. Such a mode of origin for a ventral
ganglionic chain is, so far as I know, without a parallel in the whole
animal kingdom; and whatever evidence Whitman may have that the cells
in question really do give rise to the nervous system he has not
thought fit to produce it in his paper. He figures a section with the
eight neuroblastic cells in the middle ventral line, and in the next
stage described the nervous system is divided up into ganglia! The
first stage, in which the so-called nervous system has the form of a
single row of eight cells, is quite unlike any rudiment of the nervous
system such as is usually met with in the Chætopoda, and not a single
stage between this and a ganglionated cord is described or figured.
Whitman, whose views seem to have been influenced by a peculiar, and
in my opinion erroneous, theory of Rauber's about the relation of the
neural groove of Vertebrata to the blastopore, does not seem to be
aware that his determination of the fate of his neuroblasts requires
any special support.
He quotes the formation of these parts in Euaxes (_vide_ preceding
Chapter, p. 324) as similar to that in Clepsine. In this comparison it
appears to me probable that he may be quite correct, but the result of
the comparison would be to shew that the neuroblasts and mesoblasts
composed together a mesoblastic band similar to that of the
Oligochæta. Till more evidence is brought forward by Whitman or some
other observer in support of the view that the so-called neuroblasts
have any share in forming the nervous system, they must in my opinion
be regarded as probably forming, in conjunction with the mesoblasts,
two simple mesoblastic bands. Kowalevsky has moreover briefly stated
that he has satisfied himself that the nervous system in Clepsine
originates from the epiblast--a statement which certainly could not be
brought into harmony with Whitman's account.
Nephelis. Nephelis will form my type of the Gnathobdellidæ. The
segmentation of this form has not yet been thoroughly investigated,
but Bütschli's (No. 359) observations are probably the most
trustworthy.
The ovum first divides into two, and then into four segments of which
two are slightly smaller than the others. Four small cells which form
the commencement of the epiblast are now formed. Three of them are
derived by budding from the two larger and one of the smaller of the
four cells, and the fourth from a subsequent division of one of the
larger cells[146]. The three cells which assisted in the formation of
the epiblast cells again give rise each to a small cell; and the small
cells so formed constitute a layer underneath the epiblast which is
the commencement of the hypoblast, while the cells from which they
originated form the vitelline spheres. Shortly after the formation of
the hypoblast, the large sphere which has hitherto been quiescent
divides into two, one of which then gives rise in succession to two
small epiblastic elements.
[146] Doubts have been cast by Whitman on the above account of
the origin of the four epiblast cells.
The two large spheres, resulting from the division of the originally
quiescent sphere, next divide again on the opposite side of the
embryo, and form a layer of epiblast there; so that there is now on
one side of the embryo (the ventral according to Robin) a layer of
epiblast formed of six cells, and on the opposite side a layer formed
of four cells. The two layers meet at the front border of the embryo
and between them are placed the three large vitelline spheres. The two
patches of epiblast cells now rapidly increase, and gradually spread
over the three large vitelline spheres. Except where they meet each
other at the front edge they leave uncovered a large part of the
margin of the vitelline spheres.
While these changes have been taking place on the exterior, the
hypoblast cells have increased in number (additional cells being
probably derived from the three large vitelline spheres) and fill up
in a column-like fashion a space which is bounded behind by the three
vitelline spheres, and in front by the epiblast of the anterior end of
the embryo. At the sides of the hypoblast the mesoblast has become
established, probably as two lateral bands. The origin of the cells
forming it has not yet been determined. The hypoblast cells in the
succeeding stage arrange themselves round a central archenteric
cavity, and at the same time rapidly increase in size and become
filled with a secondary deposit of food-yolk. Shortly afterwards a
mouth and thick-walled oesophagus are formed, probably from an
epiblastic invagination. The mesoblast now forms two curved lateral
bands at the two sides of the body, equivalent to the mesoblastic
bands of the Chætopoda. The three vitelline spheres, still largely
uncovered by the epiblast, lie at the posterior end of the body. The
embryo grows rapidly, especially anteriorly, and the three vitelline
spheres become covered by a layer of flattened epiblast cells. Around
the oesophagus a cavity traversed by muscular fibres is established.
Elsewhere there is no trace of such a cavity. The cephalic region
becomes ciliated, and the dorsal part of it, which represents a
rudimentary præ-oral lobe, is especially prominent. The cilia of the
oral region are continued into the lumen of the oesophagus, and at a
later period are prolonged, as in Lumbricus, along the median line of
the ventral surface.
The mesoblastic bands would seem from Bütschli's observations, which
receive confirmation from Kleinenberg's researches on Lumbricus, to be
prolonged dorsally to the oesophagus into the cephalic region.
Posteriorly they abut on the large vitelline spheres, which were
supposed by Kowalevsky to give origin to them, and to play the same
part as the large mesoblasts in Lumbricus. It has already been shewn
that the function of the large cells in Lumbricus has been
exaggerated, and Bütschli denies to them in Nephelis any share in the
production of the mesoblast. It seems in fact probable that they are
homologous with the three vitelline spheres of Clepsine; and that
their primitive function is to give origin to the hypoblast. They are
visible for a long time at the hind end of the embryo, but eventually
break up into smaller cells, the fate of which is unknown.
The embryo of Hirudo would appear from the researches of Robin to
develop in nearly the same way as that of Nephelis. The anterior part
is not however ciliated. The three large posterior cells disappear
relatively early.
_General history of the larva._
The larva of Clepsine, at the time when the mesoblastic bands have met
along the ventral line, is represented in fig. 158 B. It is seen to be
already segmented, the process having proceeded _pari passu_ with the
ventral coalescence of the mesoblastic bands. The segments are formed
from before backwards as in Chætopoda. The dorsal surface is flat and
short, and the ventral very convex. The embryo about this time leaves
its capsule, and attaches itself to its parent. It rapidly elongates,
and the dorsal surface, growing more rapidly than the ventral, becomes
at last the more convex. Eventually thirty-three post-oral segments
become formed; of which the eight last coalesce to form the posterior
sucker.
The general development of the body of Nephelis and Hirudo is nearly
the same as that of Clepsine. The embryo passes from a spherical to an
oval, and then to a vermiform shape. For full details the reader is
referred to Robin's memoir.
The presence of a well-marked protuberance above the oesophagus, which
forms the rudiment of a præ-oral lobe, has already been mentioned as
characteristic of the embryo of Nephelis; no such structure is found
in Clepsine.
_History of the germinal layers and development of organs._
The epiblast. The epiblast is formed of a single layer of cells and
early develops a delicate cuticle which is clearly formed quite
independently of the egg membrane. It becomes raised into a series of
transverse rings which bear no relation to the true somites of the
mesoblast.
The nervous system. The nervous system is probably derived from the
epiblast, but its origin still requires further investigation. The
ventral cord breaks up into a series of ganglia, which at first
correspond exactly with the somites of the mesoblast. Of these, four
or perhaps three eventually coalesce to form the sub-oesophageal
ganglion, and seven or eight become united in the posterior sucker.
It would appear from Bütschli's statements that the supra-oesophageal
ganglion arises, as in Oligochæta, independently of the ventral cord.
Mesoblast. It has already been indicated that the mesoblast probably
takes its origin both in Nephelis and Clepsine from the two
mesoblastic bands which unite in the median ventral line. The further
history of these bands is only imperfectly known. They become
segmented from before backwards. The somites formed by the
segmentation gradually grow upwards and meet in the dorsal line. Septa
are formed between the somites probably in the same way as in the
Oligochæta.
In Clepsine the mesoblastic bands are stated by Kowalevsky to become
split into somatic and splanchnic layers, between which are placed the
so-called lateral sinuses. These sinuses form, according to Whitman, a
single continuous tube investing the alimentary tract; a tube which
differs therefore to a very small extent from the normal body cavity
of the Chætopoda. The somatic layer of mesoblast no doubt gives rise
to the circular and longitudinal muscular layers of the embryo. The
former is stated to appear the earliest, while the latter, as in the
Oligochæta, first takes its origin on the ventral side.
A delicate musculature, formed mainly of transverse but also of
longitudinal fibres, would appear to be developed independently of the
mesoblastic bands in Nephelis and Hirudo (Rathke, Leuckart, Robin, and
Bütschli). It develops apparently from certain stellate cells which
are found between the walls of the alimentary tract and the skin, and
which probably correspond to the system of contractile fibres which
pass from the body wall to the alimentary tract through the
segmentation cavity in the larva of Chætopoda, various Vermes and
Mollusca[147].
[147] According to Robin this system of muscles becomes gradually
strengthened and converted into the permanent system. Rathke on
the other hand states that it is provisional, and that it is
replaced by the muscles developed from the mesoblastic somites.
It is possible to suppose that it may really become incorporated
in the latter system.
The mesoblast, so far as is known, gives rise, in addition to the
parts already mentioned, to the excretory organs, generative organs,
vascular system, etc.
_Excretory organs._ There are found in the embryo of Nephelis and
Hirudo certain remarkable provisional excretory organs the origin and
history of which is not yet fully made out. In Nephelis they appear as
one (according to Robin, No. 364), or (according to Bütschli, No. 359)
as two successive pairs of convoluted tubes on the dorsal side of the
embryo, which are stated by the latter author to develop from the
scattered mesoblast cells underneath the skin. At their fullest
development they extend, according to Robin, from close to the head to
near the ventral sucker. Each of them is U-shaped, with the open end
forwards, each limb of the U being formed by two tubes united in
front. No external opening has been clearly made out. Semper believed
that the tubes were continuous with the three posterior vitelline
cells, but this has been shewn not to be the case. Fürbringer[148] is
inclined from his own researches to believe that they open laterally.
They contain a clear fluid.
[148] _Morphologisches Jahrbuch_, Vol. IV. p. 676. He further
speaks of the tube as "feinverzweigt u. netzförmig verästelt,"
but whether from his own observations is not clear.
In Hirudo, Leuckart (No. 362) has described three similar pairs of
organs the structure of which he has fully elucidated. They are
situated in the posterior part of the body, and each of them commences
with an enlargement from which a convoluted tube is continued for some
distance backwards; it then turns forwards again and afterwards bends
upon itself to open to the exterior. The anterior part is broken up
into a kind of labyrinthic network.
The true segmental organs are found in a certain number of the
segments and are stated (Whitman) to develop from groups of mesoblast
cells. Their origin requires however further investigation.
A double row of colossal cells on each side of the body has been
described in Clepsine by Whitman as derived from the mesoblastic
plates. These cells (fig. 58 B), which he calls segment-cells, lie
opposite the walls of the septa. The inner row is stated to be
connected with the segmental organs. Their eventual history is
unknown, but they are conjectured by Whitman to be the mother cells of
the testes.
The alimentary tract. This is formed primitively of two parts--the
epiblastic stomodæum--forming mouth, pharynx, and oesophagus, and the
hypoblastic mesenteron. The anus is formed very late as a simple
perforation immediately dorsal to the posterior sucker.
In Clepsine, where there is an epibolic gastrula, the rudiment of the
mesenteron is at first formed of the three vitelline spheres, from the
surface of which a true hypoblastic layer enclosing a central yolk
mass becomes differentiated, as already described. The mesenteric sack
so formed is constricted by the growth of the mesoblastic septa into a
series of lobes, while the posterior part forms a narrow and at first
very short tube opening by the anus.
The lobed region forms the sacculated stomach of the adult. The
sacculations of the stomach by their mode of origin necessarily
correspond with the segments. In the adult however the anterior lobe
is really double and has two divisions for the two segments it fills,
while the posterior lobe, which, as is well known, extends backwards
parallel with the rectum, is composed of five segmental sacculations.
In connection with the stomodæum a protrusible pharynx is developed.
In Hirudo and Nephelis the mesenteron has from the first a sack-like
form. The cells which compose the sack give rise to a secondary
deposit of food-yolk. The further changes are practically the same as
in Clepsine. In Hirudo the posterior sacculation of the stomach is
primitively unpaired. The jaws are formed at about the same time as
the eyes as protuberances on the wall of the oral cavity.
BIBLIOGRAPHY.
(359) O. Bütschli. "Entwicklungsgeschichtliche Beiträge (Nephelis)."
_Zeit. f. wiss. Zool._ Vol. XXIX. 1877.
(360) E. Grube. _Untersuchungen üb. d. Entwicklung d. Anneliden._
Königsberg, 1844.
(361) C. K. Hoffmann. "Zur Entwicklungsgeschichte d. Clepsineen."
_Niederländ. Archiv f. Zool._ Vol. IV. 1877.
(362) R. Leuckart. _Die menschlichen Parasiten (Hirudo)_, Vol. I. p.
686, et seq.
(363) H. Rathke. _Beit. z. Entwicklungsgesch. d. Hirudineen._ Leipzig,
1862.
(364) Ch. Robin. _Mém. sur le Développement embryogenique des
Hirudinées._ Paris, 1875.
(365) C. O. Whitman. "Embryology of Clepsine." _Quart. J. of Micro.
Science_, Vol. XVIII. 1878.
[_Vide_ also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated
observations.]
CHAPTER XIV.
GEPHYREA[149].
[149] The following scheme shews the classification of the
Gephyrea adopted in the present chapter:
I. Gephyrea nuda. { (1) _Inermia._
{ (2) _Armata._
II. Gephyrea tubicola. (Phoronis).
It is convenient for the purposes of embryology to divide the Gephyrea
into two groups, viz. (1) Gephyrea nuda or true Gephyrea; and (2)
Gephyrea tubicola formed by the genus Phoronis.
GEPHYREA NUDA.
_Segmentation and formation of the layers._
An embolic or epibolic gastrula is characteristic of the Gephyrea, and
the blastopore appears, in some cases at any rate (Phascolosoma,
Thalassema), to become the mouth.
Bonellia. In Bonellia (Spengel, No. 370) the segmentation is unequal
but complete, and, as in many Molluscs etc., the ovum exhibits before
its commencement a distinction into a protoplasmic and a yoke pole.
The ovum first divides into four equal segments, each of them formed
of the same constituents as the original ovum. At the animal pole four
small cells, entirely formed of protoplasm, are next formed by an
equatorial furrow. They soon place themselves in the intervals between
the large spheres. Four small cells are again budded off from the
large spheres and the eight small cells then divide. By a further
continuation of the division of the existing small cells, and the
formation of fresh ones from the large spheres, a layer of small cells
is eventually formed, which completely envelops the four large spheres
except for a small blastopore at the vegetative pole of the ovum (fig.
160 A). The large spheres continue to give rise to smaller cells which
however no longer take a superficial position but lie within the layer
of small cells, and give rise to the hypoblast (fig. 160 B). The small
cells become the epiblast, and at the blastopore they curl inwards
(fig. 160 B) and give rise to a layer of cells, which appears to
extend as an unbroken sheet between the epiblast and hypoblast, and to
form the mesoblast. The blastopore now closes up, but its position in
relation to the parts of the embryo has not been made out.
[FIG. 160. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)
A. Stage when the four hypoblast cells are nearly enclosed.
B. Stage after the formation of the mesoblast has commenced by an
infolding of the lips of the blastopore.
_ep._ epiblast; _me._ mesoblast; _bl._ blastopore.]
In Phascolosoma (Selenka, No. 369) the ovum, enclosed in a porous zona
radiata, divides into two unequal spheres, of which the smaller next
divides into two and then into four. An invagination takes place which
is intermediate between the embolic and the epibolic types. The small
cells, the number of which is increased by additions from the large
sphere, divide, and grow round the large sphere. The latter in the
meantime also divides, and the cells produced from it form on the one
hand a small sack which opens by the blastopore, and on the other they
fill up the segmentation cavity, and become the mesoblast and blood
corpuscles. The blastopore becomes the permanent mouth.
_Larval forms and development of organs._
Amongst the Gephyrea armata the larva has as a rule (Thalassema,
Echiurus) the characters of a trochosphere, and closely approaches the
typical form characteristic of the larva of Polygordius, often known
as Lovén's larva. In Bonellia this larval form is less perfectly
preserved.
Echiurus. In Echiurus (Salensky, No. 368) the youngest known larva has
all the typical trochosphere characters (fig. 161). It is covered with
cilia and divided into a præ-oral lobe and post-oral region of nearly
equal dimensions. There is a double ciliated ring which separates the
two sections of the body as in the larva of Polygordius: the mouth
(_m_) opens between its two elements. The alimentary canal is divided
into a stomodæum with a ventral opening, a large stomach, and a short
intestine opening by a terminal anus (_an_). Connecting the oesophagus
with the apex of the præ-oral lobe is the usual contractile band, and
at the insertion of this band is a thickening of the epiblast which
probably represents the rudiment of the supra-oesophageal ganglion. A
ventral nerve cord is stated by Salensky to be present, but his
observations on this point are not quite satisfactory.
[FIG. 161. LARVA OF ECHIURUS. (After Salensky.)
_m._ mouth; _an._ anus; _sg._ supra-oesophageal ganglion (?).]
The metamorphosis is accompanied by the loss of swimming power, and
consists in the enlargement of the post-oral portion of the trunk, and
in the simultaneous reduction of the præ-oral lobe, which remains
however permanently as the cylindrical proboscis. A groove which
terminates posteriorly at the mouth is very early formed on its
ventral side. The ciliated rings gradually disappear during the
metamorphosis.
Of the further external changes the most important are (1) the early
appearance round the anal end of the body of a ring of bristles; and
(2) the appearance of a pair of ventral setæ in the anterior part of
the body. The anterior ring of bristles characteristic of the adult
Echiurus does not appear till a late period.
Of the internal changes the earliest is the formation of the anal
respiratory sacks. With the growth of the posterior part of the trunk
the intestine elongates, and becomes coiled.
Bonellia. The embryo of Bonellia, while still within the egg, retains
a spherical form and acquires an equatorial band of cilia, behind
which a second narrower band is soon established, while in front of
the first one a pair of eye-spots becomes formed (fig. 162 A). The
embryo on becoming hatched rapidly elongates, while at the same time
it becomes dorso-ventrally flattened and acquires a complete coating
of cilia (fig. 162 B). According to Spengel it resembles at this time
in its form and habits a rhabdocoelous Turbellarian. The anterior part
is however somewhat swollen and presents an indication of a præ-oral
lobe.
[FIG. 162. THREE STAGES IN THE DEVELOPMENT OF BONELLIA. (After
Spengel.)
A. Larva with two ciliated bands and two eye-spots.
B. Ripe larva from the dorsal surface.
C. Young female Bonellia from the side.
_al._ alimentary tract; _m._ mouth; _se._ provisional excretory
tube; _s._ ventral hook; _an.v._ anal vesicle.]
During the above changes important advances are made in the formation
of the organs from the embryonic layers.
The epiblast acquires a superficial cuticula, which is perhaps
directly derived from the vitelline membrane. The nervous system
is also formed, probably from the epiblast. The band-like
supra-oesophageal ganglion is the first part of the nervous system
formed, and appears to be undoubtedly derived from the epiblast. The
ventral cord arises somewhat later, but the first stages in its
development have not been satisfactorily traced. It is continuous with
the supra-oesophageal band which completely girths the oesophagus
without exhibiting any special dorsal enlargement. After the ventral
cord has become completely separated from the epiblast a central
fibrous mass becomes differentiated in it, while the lateral parts are
composed of ganglion cells. In the arrangement of its cells it
presents indications of being composed of two lateral halves. It is,
however, without ganglionic swellings.
The mesoblast, though at first very thin, soon exhibits a
differentiation into a splanchnic and somatic layer--though the two do
not become distinctly separated by a body cavity. The somatic layer
rapidly becomes thicker, and enlarges laterally to form two bands
united dorsally and ventrally by narrow, thinner bands. The outermost
parts of each of these bands become differentiated into an external
circular and an internal longitudinal layer of muscles. In the
præ-oral lobe the mesoblast assumes a somewhat vacuolated character.
The hypoblast cells form a complete layer round the four yolk cells
from which they arise (fig. 162 B, _al_), but at first no alimentary
lumen is developed. The oesophagus appears during this period as an,
at first solid, but subsequently hollow, outgrowth of the hypoblast
towards the epiblast.
The metamorphosis of the larva into the adult female Bonellia
commences with the conversion of many of the indifferent mesoblast
cells into blood corpuscles, and the introduction into the body cavity
of a large amount of fluid, which separates the splanchnic and somatic
layers of mesoblast. The fluid is believed by Spengel to be sea-water,
introduced by two anal pouches, the development of which is described
below.
The body cavity is lined by a peritoneum, and very soon distinct
vessels, formed by folds of the peritoneum, become established. Of
these there are three trunks, two lateral and a median in the præ-oral
lobe (proboscis), and in the body a ventral trunk above the nerve
cord, and an intestinal trunk opening anteriorly into the ventral one.
The vessels appear to communicate with the body cavity.
In the course of the above changes the two ciliated bands disappear,
the hinder one first. The cilia covering the general surface become
atrophied, with the exception of those on the ventral side of the
præ-oral lobe. The latter structure becomes more prominent; the
stellate mesoblast cells, which fill up its interior, become
contractile, and it gives rise to the proboscis (fig. 162 C).
At the point where the oesophageal protuberance joined the epiblast at
a previous stage the mouth becomes established (fig. 162 C, _m_), and
though it is formed subsequently to the atrophy of the anterior
ciliated band, yet there is evidence that it is potentially situated
behind this band. The lumen of the alimentary canal becomes
established by the absorption of the remains of the four central
cells. The anus is formed on the ventral side of the posterior end of
the body, and close to it the pouches already noticed grow out from
the hindermost part of the alimentary tract (fig. 162 C, _an.v_). They
are at first simple blind pouches, but subsequently open into the body
cavity[150]. They become the anal pouches of the adult. There is
present when the mouth is first formed a peculiar process of the
alimentary tract projecting into the præ-oral lobe, which appears to
atrophy shortly afterwards.
[150] The fact that these pouches are outgrowths of the
alimentary tract appears to preclude the possibility of their
being homologous with excretory tubes of the Platyelminthes and
Rotifera.
After the formation of the mouth, there are formed on the ventral side
of and slightly behind it (1) anteriorly a pair of tubes, which appear
to be provisional excretory organs and soon disappear (fig. 162 C,
_sc_); and (2) behind them a pair of bristles (_s_) which remain in
the adult. The formation of the permanent excretory (?) organ (oviduct
and uterus) has not been followed out. The ovary appears very early as
a differentiation of the epithelium lining the ventral vessel.
The larvæ, which become the minute parasitic males, undergo a very
different and far less complete metamorphosis than those which become
females. They attach themselves to the proboscis of an adult female,
and lose their ciliated bands. Germinal cells make their appearance in
the mesoblast, which form spherical masses, and, like the germinal
balls in the female ovary, consist of a central cell, and an
epithelium around it. The central cell becomes very large, while the
peripheral cells give rise to the spermatozoa. A body cavity becomes
developed in the larvæ, into which the spermatic balls are dehisced.
Neither mouth nor anus is formed. The further changes have not been
followed out.
The larval males make their way into the oesophagus of the female,
where they no doubt live for some time, and probably become mature,
though the seminal pouch of the adult is not found in many of the
males living in the oesophagus. When mature the males leave the
oesophagus, and pass into the uterus.
Phascolosoma. Cilia appear in Phascolosoma (Selenka, No. 369) while
the ovum is still segmenting. After segmentation they form a definite
band immediately _behind_ the mouth, which divides the larva into two
hemispheres--a præ-oral and a post-oral. A præ-oral band of cilia is
soon formed close to the post-oral band, and at the apex of the
præ-oral lobe a tuft of cilia also appears.
The larva has now the characters of a trochosphere, but differs from
the typical trochosphere in the post-oral part of the ciliated
equatorial ring being more important than the præ-oral, and in the
absence of an anus.
The metamorphosis commences very early. The trunk rapidly elongates,
and the præ-oral lobe becomes relatively less and less conspicuous.
The zona radiata becomes the larval cuticle.
Three pairs of bristles are formed on the trunk, of which the
posterior pair appears first, then the anterior, and finally the
middle pair: an order of succession which clearly proves they can have
no connection with a true segmentation.
The tentacles become developed _between_ the two parts of the ciliated
ring, and finally the præ-oral lobe, unlike what takes place in the
Gephyrea armata, nearly completely vanishes.
The anus appears fairly late on the dorsal surface, and the ventral
nerve cord is established as an unganglionated thickening of the
ventral epiblast.
GEPHYREA TUBICOLA.
The larva of Phoronis was known as Actinotrocha long before its
connection with Phoronis was established by Kowalevsky (No. 372).
There is a complete segmentation leading to the formation of a
blastosphere, which is followed by an invagination, the opening of
which is said by Kowalevsky to remain as the mouth[151]. It is at
first terminal, but on the development of a large præ-oral lobe it
assumes a ventral position. The anus is formed at a later period at
the posterior end of the body.
[151] Kowalevsky states that what I have called the mouth is the
anus, but his subsequent descriptions shew that he has transposed
the mouth and anus in the embryo, and that the opening, which he
asserts to be the anus, is in reality the mouth.
[FIG. 163. A SERIES OF STAGES IN THE DEVELOPMENT OF PHORONIS FROM
ACTINOTROCHA. (After Metschnikoff.)
A. Young larva.
B. Larva after the formation of post-oral ring of tentacles.
C. Larva with commencing invagination to form the body of Phoronis.
D. Invagination partially everted.
E. Invagination completely everted.
_m._ mouth; _an._ anus; _iv._ invagination to form the body of
Phoronis.]
The youngest free larva observed by Metschnikoff (No. 373) was less
developed than the oldest larva found by Kowalevsky. It probably
belongs to a different species. The body is uniformly ciliated (fig.
163 A). There is a large contractile præ-oral lobe, and the body ends
behind in two processes. The mouth (_m_) is ventral, and the anus
(_an_) dorsal, and not terminal as in Kowalevsky's larva.
The alimentary tract is divided into stomodæum, stomach and intestine.
The two processes at the hind end of the body are the rudiments of the
first-formed pair of the arms which are so characteristic of the fully
developed Actinotrocha. A second pair of arms next become established
on the dorsal side of the previously existing pair, and the region
where the anus is placed grows out as a special process. New pairs of
arms continue to be formed in succession dorsalwards and forwards, and
soon constitute _a complete oblique post-oral ring_ (fig. 163 B). They
are covered by long cilia. Round the anal process a very conspicuous
ciliated ring also becomes established.
At the period when five pairs of arms are present a delicate membrane
becomes visible on the ventral side of the intestine which joins the
somatic mesoblast anteriorly. This membrane is the rudiment of the
future ventral vessel. The somatic mesoblast is present even before
this period as a delicate layer of circular muscular fibres.
When six pairs of arms have become formed an involution (fig. 163 C,
_iv_) appears on the ventral side, immediately behind the ring of
arms. This involution consists both of the epiblast and somatic
mesoblast. It grows inwards towards the intestine, and, increasing
greatly in length, becomes at the same time much folded.
When it has reached its full development the critical period of the
metamorphosis of Actinotrocha into Phoronis is reached, and is
completed in about a quarter of an hour. The ventral involution
becomes evoluted (fig. 163 D), just as one might turn out the finger
of a glove which had been pulled inwards. When the involution has been
to a certain extent everted, the alimentary canal passes into it, and
at the same time the body of the larva becomes violently contracted.
By the time the evagination is completed it forms (fig. 162 E) a long
conical body, containing the greater part of the alimentary tract, and
_constituting the body of the young Phoronis_. The original anal
process remains on the dorsal side as a small papilla (fig. 162 E,
_an_).
While these changes have been taking place the præ-oral lobe has
become much contracted, and partly withdrawn into the stomodæum. At
the same time the arms have become bent forward, so as to form a ring
round the mouth. Their bases become much thickened. The metamorphosis
is completed by the entire withdrawal of the præ-oral lobe within the
oesophagus, and by the casting off of the ends of the arms, their
bases remaining as the circumoral ring of tentacles, which form
however a lophophore rather than a complete ring. The peri-anal ring
of cilia is also thrown off, and the anal process withdrawn into the
body of the young Phoronis. There are now three longitudinal vascular
trunks, united anteriorly by a circular vessel which is prolonged into
the tentacles.
_General Considerations._
The development of Phoronis is so different from that of the other
Gephyrea that further investigations are required to shew whether
Phoronis is a true Gephyrean. Apart from its peculiar metamorphosis
Actinotrocha is a very interesting larval form, in that it is without
a præ-oral ciliated ring, and that the tentacles of the adult are
derived from a true post-oral ring, prolonged into arm-like processes.
The other Gephyrea present in their development an obvious similarity
to the normal Chætopoda, but their development stops short of that of
the Chætopoda, in that they are clearly without any indications of a
true segmentation. In the face of what is known of their development
it is hardly credible that they can represent a _degenerate_ Chætopod
phylum in which segmentation has become lost. Further than this the
Gephyrea armata seem in one respect to be a very primitive type in
that they retain through life a well-developed præ-oral lobe, which
constitutes their proboscis. In almost all other forms, except
Balanoglossus, the larval præ-oral lobe becomes reduced to a
relatively insignificant anterior part of the head.
BIBLIOGRAPHY.
_Gephyrea nuda._
(366) A. Kowalevsky. _Sitz. d. zool. Abth. d. III. Versam. russ.
Naturj._ (Thalassema). _Zeit. f. wiss. Zool._ Vol. XXII. 1872, p. 284.
(367) A. Krohn. "Ueb. d. Larve d. Sipunculus nudus nebst Bemerkungen,"
etc. Müller's _Archiv_, 1857.
(368) M. Salensky. "Ueber die Metamorphose d. Echiurus."
_Morphologisches Jahrbuch_, Bd. II.
(369) E. Selenka. "Eifurchung u. Larvenbildung von Phascolosoma
elongatum." _Zeit. f. wiss. Zool._ 1875, Bd. XXV. p. 1.
(370) J. W. Spengel. "Beiträge z. Kenntniss d. Gephyreen (Bonellia)."
_Mittheil. a. d. zool. Station z. Neapel_, Vol. I. 1879.
_Gephyrea tubicola (Actinotrocha)._
(371) A. Krohn. "Ueb. Pilidium u. Actinotrocha." Müller's _Archiv_,
1858.
(372) A. Kowalevsky. "On anatomy and development of Phoronis,"
Pétersbourg, 1867. 2 Pl. Russian. _Vide_ Leuckart's _Bericht_, 1866-7.
(373) E. Metschnikoff. "Ueber d. Metamorphose einiger Seethiere
(Actinotrocha)." _Zeit. f. wiss. Zool._ Bd. XXI. 1871.
(374) J. Müller. "Bericht üb. ein. Thierformen d. Nordsee." Müller's
_Archiv_, 1846.
(375) An. Schneider. "Ueb. d. Metamorphose d. Actinotrocha
branchiata." Müller's _Arch._ 1862.
CHAPTER XV.
CHÆTOGNATHA, MYZOSTOMEA AND GASTROTRICHA.
The present chapter deals with three small isolated groups, which only
resemble each other in that the systematic position of all of them is
equally obscure.
_Chætognatha._
The discoveries of Kowalevsky (No. 378) confirmed by Bütschli (No.
376) with reference to the development of Sagitta, though they have
not brought us nearer to a knowledge of the systematic position of
this remarkable form, are nevertheless of great value for the more
general problems of embryology. The development commences after the
eggs are laid. The segmentation is uniform, and a blastosphere, formed
of a single layer of columnar cells, is the product of it. An
invagination takes place, the opening of which narrows to a blastopore
situated at the pole of the embryo opposite that at which the mouth
subsequently appears (fig. 164 A). The simple archenteron soon becomes
anteriorly divided into three lobes, which communicate freely with the
still single cavity behind (fig. 164 B). The two lateral lobes are
destined to form the body cavity, and the median lobe the alimentary
tract of the adult. An invagination soon arises at the opposite pole
of the embryo to the blastopore and forms the mouth and oesophagus
(fig. 164 B and C, _m_).
[FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C
after Bütschli and B after Kowalevsky.) The three embryos are
represented in the same positions.
A. The gastrula stage.
B. A succeeding stage in which the primitive archenteron is
commencing to be divided into three parts, the two lateral of
which are destined to form the body cavity.
C. A later stage in which the mouth involution (_m_) has become
continuous with the alimentary tract, and the blastopore has
become closed.
_m._ mouth; al. alimentary canal; _ae._ archenteron; _bl.p._
blastopore; _pv._ perivisceral cavity; _sp._ splanchnopleuric
mesoblast; _so._ somatopleuric mesoblast; _ge._ generative organs.]
At the gastrula stage there is formed a paired mass destined to give
rise to the generative organs. It arises as a prominence of six cells,
projecting from the hypoblast at the anterior pole of the archenteron,
and soon separates itself as a mass, or probably a pair of masses,
lying freely in the cavity of the archenteron (fig. 164 A, _ge_). When
the folding of the primitive cavity takes place the generative
rudiment is situated at the hind end of the median lobe of the
archenteron in the position represented in fig. 164 C, _ge_.
An elongation of the posterior end of the embryo now takes place, and
the embryo becomes coiled up in the egg, and when eventually hatched
sufficiently resembles the adult to be recognisable as a young
Sagitta.
Before hatching takes place various important changes become manifest.
The blastopore disappears after being carried to the ventral surface.
The middle section of the trilobed region of the archenteron becomes
separated from the unpaired posterior part, and forms a tube, blind
behind, but opening in front by the mouth (fig. 165 A, _al_). It
constitutes the permanent alimentary tract, and is formed of a
pharyngeal epiblastic invagination, and a posterior hypoblastic
section derived from the primitive archenteron. The anus is apparently
not formed till comparatively late. After the isolation of the
alimentary tract the remainder of the archenteron is formed of two
cavities in front, which open freely into a single cavity behind (fig.
165 A). _The whole of it constitutes the body cavity and its walls
the mesoblast._ The anterior paired part becomes partitioned off into
a head section and a trunk section (fig. 165 A and B). The former
constitutes a pair of distinct cavities (_c.pv_) in the head, and the
latter two cavities opening freely into the unpaired portion behind.
At the junction of the paired cavities with the unpaired cavity are
situated the generative organs (_ge_). The inner wall of each of the
paired cavities forms the splanchnopleuric mesoblast, and the outer
wall of the whole the somatic mesoblast. The inner walls of the
posterior cavities unite above and below the alimentary tract, and
form the dorsal and ventral mesenteries, which divide the body cavity
into two compartments in the adult. Before the hatching of the embryo
takes place this mesentery is continued backwards so as to divide the
primitively unpaired caudal part of the body cavity in the same way.
[FIG. 165. TWO VIEWS OF A LATE EMBRYO OF SAGITTA. A. from the dorsal
surface. B. from the side. (After Bütschli.)
_m._ mouth; _al._ alimentary canal; _v.g._ ventral ganglion
(thickening of epiblast); _ep._ epiblast; _c.pv._ cephalic section
of body cavity; _so._ somatopleure; _sp._ splanchnopleure; _ge._
generative organs.]
From the somatic mesoblast of the trunk is derived the single layer of
longitudinal muscles of Sagitta, and part of the epithelioid lining of
the body cavity. The anterior termination of the trunk division of the
body cavity is marked in the adult by the mesentery dividing into two
laminæ, which bend outwards to join the body wall.
The cephalic section of the body cavity seems to atrophy, and its
walls to become converted into the complicated system of muscles
present in the head of the adult Sagitta.
In the presence of a section of the body cavity in the head the embryo
of Sagitta resembles Lumbricus, Spiders, etc.
The generative rudiment of each side divides into an anterior and a
posterior part (fig. 165, _ge_). The former constitutes the ovary, and
is situated in front of the septum dividing the tail from the body;
and the latter, in the caudal region of the trunk, forms the testis.
The nervous system originates from the epiblast. There is a ventral
thickening (fig. 165 B, _v.g_) in the anterior region of the trunk,
and a dorsal one in the head. The two are at first continuous, and on
becoming separated from the epiblast remain united by thin cords.
The ventral ganglion is far more prominent during embryonic life than
in the adult. Its position and early prominence in the embryo perhaps
indicate that it is the homologue of the ventral cord of
Chætopoda[152].
[152] Langerhans has recently made some important investigations
on the nervous system of Sagitta, and identifies the ventral
ganglion with the parieto-splanchnio ganglia of Molluscs, while
he has found a pair of new ganglia, the development of which is
unknown, which he calls the suboesophageal or pedal ganglia. The
embryological facts do not appear to be in favour of these
interpretations.
BIBLIOGRAPHY.
(376) O. Bütschli. "Zur Entwicklungsgeschichte der Sagitta."
_Zeitschrift f. wiss. Zool._, Vol. XXIII. 1873.
(377) C. Gegenbaur. "Über die Entwicklung der Sagitta." _Abhand. d.
naturforschenden Gesellschaft in Halle_, 1857.
(378) A. Kowalevsky. "Embryologische Studien an Würmern u.
Arthropoden." _Mém. Acad. Pétersbourg_, VII. sér., Tom. XVI., No. 12.
1871.
MYZOSTOMEA.
The development of these peculiar parasites on Crinoids has been
investigated by Metschnikoff (No. 380), Semper (No. 381), and Graff
(No. 379).
The segmentation is unequal, and would appear to be followed by an
epibolic invagination. The outer layer of cells (epiblast) becomes
covered with cilia, and the inner is transformed into a non-cellular
(?) central yolk mass. At this stage the larva is hatched, and
commences to lead a free existence. In the next stage observed by
Metschnikoff, the mouth, oesophagus, stomach, and anus had become
developed; and two pairs of feet were present. In both of these feet
Chætopod-like setæ were present, which in the hinder pair were simple
fine bristles without a terminal hook. The papilliform portion of the
foot is at first undeveloped. The feet become successively added, like
Chætopod segments, and the stomach does not become dendriform till the
whole complement of feet (5 pairs) are present.
In the primitive covering of cilia, combined with a subsequent
indication of segments in the formation of the feet and setæ, the
larva of the Myzostomea shews an approximation to the Chætopoda, and
the group is probably to be regarded as an early Chætopod type
specially modified in connection with its parasitic habits.
BIBLIOGRAPHY.
(379) L. Graff. _Das Genus Myzostoma._ Leipzig, 1877.
(380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum."
_Zeit. f. wiss. Zool._, Vol. XVI. 1866.
(381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." _Zeit. f.
wiss. Zool._, Vol. IX. 1858.
GASTROTRICHA.
A few observations of Ludwig on the winter eggs of Ichthydium larus
shew that the segmentation is a total and apparently a regular one. It
leads to the formation of a solid morula. The embryo has a ventral
curvature, and the caudal forks are early formed as cuticular
structures. By the time the embryo leaves the egg, it has almost
reached the adult state. The ventral cilia arise some little time
prior to the hatching.
BIBLIOGRAPHY.
(382) H. Ludwig. "Ueber die Ordnung Gastrotricha _Metschn_." _Zeit. f.
wiss. Zool._, Vol. XXVI. 1876.
CHAPTER XVI.
NEMATELMINTHES AND ACANTHOCEPHALA.
NEMATELMINTHES[153].
[153] The following classification of the Nematoda is employed in
this chapter:
{ Ascaridæ.
{ Strongylidæ.
I. Nematoidea. { Trichinidæ.
{ Filaridæ
{ Mermithidæ.
{ Anguillulidæ.
II. Gordioidea.
III. Chætosomoidea.
Nematoidea. Although the ova of various Nematodes have formed some of
the earliest, as well as the most frequent objects of embryological
observation, their development is still but very imperfectly known.
Both viviparous and oviparous forms are common, and in the case of the
oviparous forms the eggs are usually enveloped in a hard shell. The
segmentation is total and nearly regular, though the two first
segments are often unequal. The relation of the segmentation spheres
to the germinal layers is however only satisfactorily established
(through the researches of Bütschli (No. 383)) in the case of
Cucullanus elegans, a form parasitic in the Perch[154].
[154] The ova of Anguillula aceti are stated by Hallez to undergo
a similar development to those of Cucullanus.
The early development of this embryo takes place within the body of
the parent, and the egg is enveloped in a delicate membrane. After the
completion of the early stages of segmentation the embryo acquires the
form of a thin flat plate composed of two layers of cells (fig. 166 A
and B). The two layers of this plate give rise respectively to the
epiblast and hypoblast, and at a certain stage the hypoblastic layer
ceases to grow, while the growth of the epiblastic layer continues. As
a consequence of this the sides of the plate begin to fold over
towards the side of the hypoblast (fig. 166 D.) This folding results
in the formation of a remarkably constituted gastrula, which has the
form of a hollow two-layered cylinder with an incompletely closed slit
on one side (fig. 166 E, _bl.p_). This slit has the value of a
blastopore. It becomes closed by the coalescence of the two edges, a
process which commences posteriorly, and then gradually extends
forwards. In front the blastopore never becomes completely closed, but
remains as the permanent mouth. The embryo after these changes has a
worm-like form, which becomes the more obvious as it grows in length
and becomes curved (fig. 166 F).
[FIG. 166. VARIOUS STAGES IN THE DEVELOPMENT OF CUCULLANUS ELEGANS.
(From Bütschli.)
A. Surface view of flattened embryo at an early stage in the
segmentation.
B. Side view of an embryo at a somewhat later stage, in optical
section.
C. Flattened embryo at the completion of segmentation.
D. Embryo at the commencement of the gastrula stage.
E. Embryo when the blastopore is reduced to a mere slit.
F. Vermiform embryo after the division of the alimentary tract into
oesophageal and glandular divisions.
_m._ mouth; _ep._ epiblast; _hy._ hypoblast; _me._ mesoblast; _oe._
oesophagus; _bl.p._ blastopore.]
The hypoblast of the embryo gives rise to the alimentary canal, and
soon becomes divided into an oesophageal section (fig. 166 F, _oe_)
formed of granular cells, and a posterior division formed of clear
cells. The mesoblast (fig. 166, _me_) takes its origin from certain
special hypoblast cells around the mouth, and thence grows backwards
towards the posterior end of the body.
The young Cucullanus becomes hatched while still in the generative
ducts of its parent, and is distinguished by the presence of a
remarkable thread-like tail. On the dorsal surface is a provisional
boring apparatus in the form of a conical papilla. A firm cuticle
enveloping the body is already present. In this condition it leaves
its parent and host, and leads for a time a free existence in the
water. Its metamorphosis is dealt with in another section.
The ova of the Oxyuridæ parasitic in Insects are stated by Galeb (No.
386) to take the form of a blastosphere at the close of segmentation.
An inner layer is then formed by delamination. What the inner layer
gives rise to is not clear, since the whole alimentary canal is stated
to be derived from two buds, which arise at opposite ends of the body,
and grow inwards till they meet.
The generative organs. The study of the development of the generative
organs of Nematodes has led to some interesting results. In the case
of both sexes the generative organs originate (Schneider, No. 390)
from a single cell. This cell elongates and its nuclei multiply. After
assuming a somewhat columnar form, it divides into (1) a superficial
investing layer, and (2) an axial portion.
In the female the superficial layer is only developed distinctly in
the median part of the column. In the course of the further
development the two ends of the column become the blind ends of the
ovary, and the axial tissue they contain forms the germinal tissue of
nucleated protoplasm. The superficial layer gives rise to the
epithelium of the uterus and oviduct. The germinal tissue, which is
originally continuous, is interrupted in the middle part (where the
superficial layer gives rise to the uterus and oviduct), and is
confined to the two blind extremities of the tube.
In the male the superficial layer, which gives rise to the epithelium
of the vas deferens, is only formed at the hinder end of the original
column. In other respects the development takes place as in the
female.
Gordioidea. The ovum of Gordius undergoes a regular segmentation.
According to Villot (No. 391) it forms at the close of segmentation a
morula, which becomes two-layered by delamination. The embryo is at
first spherical, but soon becomes elongated.
By an invagination at the anterior extremity the head is formed. It
consists of a basal portion, armed with three rings of stylets, and a
conical proboscis, armed with three large stylets. When the larva
becomes free the head becomes everted, though it remains retractile.
By the time the embryo is hatched a complete alimentary tract is
formed with an oral opening at the end of the proboscis, and a
subterminal ventral anal opening. It is divided into an oesophagus and
stomach, and a large gland opens into it at the base of the proboscis.
The body has a number of transverse folds, which give it a ringed
appearance.
_Metamorphosis and life history._
Nematoidea. Although a large number of Nematodes have a free existence
and simple life history, yet the greater number of known genera are
parasitic, and undergo a more or less complicated metamorphosis[155].
According to this metamorphosis they may be divided into two groups
(which by no means closely correspond with the natural divisions),
viz. those which have a single host, and those with two hosts. Each of
these main divisions may be subdivided again into two.
[155] The following facts are mainly derived from Leuckart's
exhaustive treatise (No. 388).
In the first group with one host the simplest cases are those in which
the adult sexual form of parasite lays its eggs in the alimentary
tract of its host, and the eggs are thence transported to the
exterior. The embryo still in the egg, if favoured by sufficient
warmth and moisture, completes its development up to a certain point,
and, if then swallowed by an individual of the species in which it is
parasitic in the adult condition, it is denuded of its shell by the
action of the gastric juice, and develops directly into the sexual
form.
Leuckart has experimentally established this metamorphosis in the case
of Trichocephalus affinis, Oxyurus ambigua, and Heterakis
vermicularis. The Oxyuridæ of Blatta and Hydrophilus have a similar
life history (Galeb, No. 386), and it is almost certain that the
metamorphosis of the human parasites, Ascaris lumbricoides and Oxyurus
vermicularis, is of this nature.
A slightly more complicated metamorphosis is common in the genera
Ascaris and Strongylus. In these cases the egg-shell is thin, and the
embryo becomes free externally, and enjoys for a shorter or longer
period a free existence in water or moist earth. During this period it
grows in size, and though not sexual usually closely resembles the
adult form of the permanently free genus Rhabditis. In some cases the
free larva becomes parasitic in a freshwater Mollusc, but without
thereby undergoing any change. It eventually enters the alimentary
tract of its proper host and there become sexual.
As examples of this form of development worked out by Leuckart may be
mentioned Dochmius trigonocephalus, parasitic in the dog, and Ascaris
acuminata, in the frog. The human parasite Dochmius duodenale
undergoes the same metamorphosis as Dochmius trigonocephalus.
A remarkable modification of this type of metamorphosis is found in
Ascaris (Rhabdonema) nigrovenosa, which in its most developed
condition is parasitic in the lungs of the frog (Metschnikoff,
Leuckart, No. 388). The embryos pass through their first developmental
phases in the body of the parent. They have the typical Rhabditis
form, and make their way after birth into the frog's rectum. From this
they pass to the exterior, and then living either in moist earth, or
the fæces of the frog, develop into a sexual form, but are very much
smaller than in the adult condition. The sexes are distinct, and the
males are distinguished from the females by their smaller size,
shorter and rounded tails, and thinner bodies. The females have paired
ovaries with a very small number of eggs, but the testis of the males
is unpaired. Impregnation takes place in the usual way, and in summer
time about four embryos are developed in each female, which soon burst
their egg-capsules, and then move freely in the uterus. Their active
movements soon burst the uterine walls, and they then come to lie
freely in the body cavity. The remaining viscera of the mother are
next reduced to a finely granular material, which serves for the
nutrition of the young forms which continue to live in the maternal
skin. The larvæ eventually become free, and though in many respects
different from the parent form which gave rise to them, have
nevertheless the Rhabditis form. They live in water or slime, and
sometimes become parasitic in water-snails; in neither case however do
they undergo important changes unless eventually swallowed by a frog.
They then pass down the trachea into the lungs and there rapidly
develop into the adult form. No separate males have been found in the
lungs of the frog, but it has been shewn by Schneider (No. 390) that
the so-called females are really hermaphrodites; the same gland giving
origin to both spermatozoa and ova, the former being developed before
the latter[156]. The remarkable feature of the above life history is
the fact that in the stage corresponding with the free larval stage of
the previous forms the larvæ of this species become sexual, and give
rise to a second free larval generation, which develops into the adult
form on again becoming parasitic in the original host. It constitutes
a somewhat exceptional case of heterogamy as defined in the
introduction.
[156] Leuckart does not appear to be satisfied as to the
hermaphroditism of these forms; and holds that it is quite
possible that the ova may develop parthenogenetically.
Amongst the Nematodes with but a single host a remarkable parasite in
wheat has its place. This form, known as Anguillula scandens, inhabits
in the adult condition the ears of wheat, in which it lays its eggs.
After hatching, the larvæ become encysted, but become free on the
death of the plant. They now inhabit moist earth, but eventually make
their way into the ears of the young wheat and become sexually mature.
The second group of parasitic Nematodes with two hosts may be divided
into two groups, according to whether the larva has a free existence
before passing into its first or intermediate host, or is taken into
it while still in the egg. In the majority of cases the larval forms
live in special connective-tissue capsules, or sometimes free in the
tissues of their intermediate hosts; but the adults, as in the cases
of other parasitic Nematodes, inhabit the alimentary tract.
The life history of Spiroptera obtusa may be cited as an example of a
Nematode with two hosts in which the embryo is transported into its
intermediate host while still within the egg. The adult of this form
is parasitic in the mouse, and the ova pass out of the alimentary
tract with the excreta, and may commonly be found in barns, etc. If
one of the ova is now eaten by the meal-worm (larva of Tenebrio), it
passes into the body cavity of this worm and undergoes further
development. After about five weeks it becomes encapsuled between the
'fat bodies' of the meal-worm. It then undergoes an ecdysis, and, if
the meal-worm with its parasites is now eaten by the mouse, the
parasites leave their capsule and develop into the sexual form.
As examples of life histories in which a free state intervenes before
the intermediate host, Cucullanus elegans and Dracunculus may be
selected. The adult Cucullanus elegans is parasitic in the alimentary
tract of the Perch and other freshwater fishes. It is a viviparous
form, and the young after birth pass out into the water. They next
become parasitic in Cyclops, passing in through the mouth, so into the
alimentary tract, and thence into the body cavity. They soon undergo
an ecdysis, in the course of which the oesophagus becomes divided into
a muscular pharynx and true glandular oesophagus. They then grow
rapidly in length, and at a second ecdysis acquire a peculiar
beaker-like mouth cavity approaching that of the adult. They do not
become encapsuled. No further development of the worm takes place so
long as it remains in the Cyclops, but, if the Cyclops is now
swallowed by a Perch, the worm undergoes a further ecdysis, and
rapidly attains to sexual maturity.
The observations of Fedschenko on Dracunculus medinensis[157], which
is parasitic in the subcutaneous connective tissue in Man, would seem
to shew that it undergoes a metamorphosis very similar to that of
Cucullanus. There is moreover a striking resemblance between the larvæ
of the two forms. The larvæ of Dracunculus become transported into
water, and then make their way into the body cavity of a Cyclops by
boring through the soft skin between the segments on the ventral
surface of the body. In the body cavity the larvæ undergo an ecdysis
and further development. But on reaching a certain stage of
development, though they remain a long time in the Cyclops, they grow
no further. The remaining history is unknown, but probably the next
host is man, in which the larva comes to maturity. In the adult
condition only females of Dracunculus are known, and it has been
suggested by various writers that the apparent females are in reality
hermaphrodites, like Ascaris nigrovenosa, in which the male organs
come to maturity before the female.
[157] _Vide_ Leuckart, _D. men. Par._, Vol. II. p. 704.
Another very remarkable human parasite belonging to the same group as
Dracunculus is the form known as Filaria sanguinis hominis, or Filaria
Bancrofti[158].
[158] _Vide_ D. P. Manson, "On the development of Filaria
sanguinis hominis." _Journal of the Linnean Society_, Vol. XIV.
No. 75.
The sexual form is parasitic in warm climates in the human tissues,
and produces multitudes of larvæ which pass into the blood, and are
sometimes voided with the urine. The larvæ in the blood do not undergo
a further development, and unless transported to an intermediate host
die before very long. Some, though as yet hardly sufficient, evidence
has been brought forward to shew that if the blood of an infected
patient is sucked by a mosquito the larvæ develop further in the
alimentary tract of the mosquito, pass through a more or less
quiescent stage, and eventually grow considerably in size, and on the
death of the mosquito pass into the water. From the water they are
probably transported directly or indirectly into the human intestines,
and then bore their way into the tissues in which they are parasitic,
and become sexually mature.
The well-known Trichina spiralis has a life history unlike that of
other known Nematodes, though there can be little doubt that this form
should be classified in respect to its life history with the
last-described forms. The peculiarity of the life history of Trichina
is that the embryos set free in the alimentary canal pass through the
walls into the muscular tissues and there encyst; but do not in a
general way pass out from the alimentary canal of one host and thence
into a fresh host to encyst. It occasionally however happens that this
migration does take place, and the life history of Trichina spiralis
then becomes almost identical with that of some of the forms of the
third type. Trichina is parasitic in man, and in swine, and also in
the rat, mouse, cat, fox and other forms which feed upon them.
Artificially it can be introduced into various herbivorous forms
(rabbit, guinea-pig, horse) and even birds.
The sexual form inhabits the alimentary canal. The female is
viviparous, and produces myriads of embryos, which pass into the
alimentary canal of their host, through the walls of which they make
their way, and travelling along lines of connective tissue pass into
the muscles. Here the embryos, which are born in a very imperfect
condition, rapidly develop, and eventually assume a quiescent
condition in a space inclosed by sarcolemma. Within the sarcolemma a
firm capsule is developed for each larva, which after some months
becomes calcified; and after the atrophy of the sarcolemma a
connective-tissue layer is formed around it. Within its capsule the
larva can live for many years, even ten or more, without undergoing
further development, but if at last the infected flesh is eaten by a
suitable form, _e.g._ the infected flesh of the pig by man, the
quiescent state of the larva is brought to a close, and sexual
maturity is attained in the alimentary tract of the new host.
Gordioidea. The free larva of Gordius already described usually
penetrates into the larva of Chironomus where it becomes encysted. On
the Chironomus being eaten by some fish (Villot, No. 39) (Phoxinus
lævis or Cobitis barbatula), it penetrates into the wall of the
intestine of its second host, becomes again encysted and remains
quiescent for some time. Eventually in the spring it leaves its
capsule, and enters the intestine, and passes to the exterior with the
fæces. It then undergoes a gradual metamorphosis, in the course of
which it loses its ringed structure and cephalic armature, grows in
length, acquires its ventral cord, and on the development of the
generative organs loses the greater part of its alimentary tract.
Young examples of Gordius have often been found in various terrestrial
carnivorous Insecta, but the meaning of this fact is not yet clear.
BIBLIOGRAPHY.
(383) O. Bütschli. "Entwicklungsgeschichte d. Cucullanus elegans."
_Zeit. f. wiss. Zool._, B. XXVI. 1876.
(384) T. S. Cobbold. _Entozoa._ Groombridge and Son, 1864.
(385) T. S. Cobbold. Parasites: _A Treatise on the Entozoa of Man and
Animals._ Churchill, 1879.
(386) O. Galeb. "Organisation et développement des Oxyuridés," &c.
_Archives de Zool. expér. et génér._, Vol. VII. 1878.
(387) R. Leuckart. _Untersuchungen üb. Trichina spiralis._ 2nd ed.
Leipzig, 1866.
(388) R. Leuckart. _Die menschlichen Parasiten_, Bd. II. 1876. (389)
H. A. Pagenstecher. _Die Trichinen nach Versuchen dargestellt._
Leipzig, 1865.
(390) A. Schneider. _Monographie d. Nematoden._ Berlin, 1866.
(391) A. Villot. "Monographie des Dragoneaux" (Gordioidea). _Archives
de Zool. expér. et génér._, Vol. III. 1874.
ACANTHOCEPHALA.
The Acanthocephala appear to be always viviparous. At the time of
impregnation the ovum is a naked cell, and undergoes in this condition
the earlier phases of segmentation.
The segmentation is unequal (Leuckart, No. 393), but whether there is
an epibolic gastrula has not clearly been made out.
Before segmentation is completed there are formed round the ovum thick
protecting membranes, which are usually three in number, the middle
one being the strongest. After segmentation the central cells of the
ovum fuse together to give rise to a granular mass, while the
peripheral cells at a slightly later period form a more transparent
syncytium. At the anterior end of the embryo there appears a
superficial cuticle bearing in front a ring of hooks.
The embryo is now carried out with the excreta from the intestine of
the vertebrate host in which its parent lives. It is then swallowed by
some invertebrate host[159].
[159] Echin. proteus, which is parasitic in the adult state in
many freshwater fish, passes through its larval condition in the
body cavity of Gammarus pulex. Ech. angustatus, parasitic in the
Perch, is found in the larval condition in the body cavity of
Asellus aquatious. Ech. gigas, parasitic in swine, is stated by
Schneider (No. 394) to pass through its larval stages in maggots.
In the intestine of the invertebrate host the larva is freed from its
membranes, and is found to have a somewhat elongated conical form,
terminating anteriorly in an obliquely placed disc, turned slightly
towards the ventral surface and armed with hooks. Between this disc
and the granular mass, already described as formed from the central
cells of the embryo, is a rather conspicuous solid body. Leuckart
supposes that this body may represent a rudimentary functionless
pharynx, while the granular mass in his opinion is an equally
rudimentary and functionless intestine. The body wall is formed of a
semifluid internal layer surrounding the rudimentary intestine, if
such it be, and of a firmer outer wall immediately within the cuticle.
The adult Echinorhyncus is formed by a remarkable process of
development within the body of the larva, and the skin is the only
part of the larva which is carried over to the adult.
In Echinorhyncus proteus the larva remains mobile during the formation
of the adult, but in other forms the metamorphosis takes place during
a quiescent condition of the larva.
The organs of the adult are differentiated from a mass of cells which
appears to be a product of the central embryonic granular mass, and is
called by Leuckart the embryonic nucleus. The embryonic nucleus
becomes divided into four linearly arranged groups of cells, of which
the hindermost but one is the largest, and very early differentiates
itself into (1) a peripheral layer, and (2) a central mass formed of
two distinct bodies. The peripheral layer of this segment grows
forwards and backwards, and embraces the other segments, with the
exception of the front end of the first one which is left uncovered.
The envelope so formed gives rise to the splanchnic and somatic
mesoblast of the adult worm. Of the four groups of cells within it the
anterior gives rise to the proboscis, the next to the nerve ganglion,
the third, formed of two bodies, to the paired generatives, and the
fourth to the generative ducts. The whole of the above complex rapidly
elongates, and as it does so the enveloping membrane becomes split
into two layers; of which the outer forms the muscular wall of the
body (somatic mesoblast), and the inner the muscular sheath of the
proboscis and the so-called generative ligament enveloping the
generative organs. The inner layer may be called the splanchnic
mesoblast in spite of the absence of an intestine. The cavity between
the two mesoblastic layers forms the body cavity.
The various parts of the adult continue to differentiate themselves as
the whole increases in size. The generative masses very early shew
traces of becoming differentiated into testes or ovaries. In the male
the two generative masses remain spherical, but in the female become
elongated: the rudiment of the generative ducts becomes divided into
three sections in both sexes. The most remarkable changes are,
however, those undergone by the rudiment of the proboscis.
In its interior there is formed a cavity, but the wall bounding the
front end of the cavity soon disappears. By the time that this has
taken place the body of the adult completely fills up the larval skin,
to which it very soon attaches itself. The hollow rudiment of the
proboscis then becomes everted, and forms a papilla at the end of the
body, immediately adjoining the larval skin. This papilla, with the
larval skin covering it, constitutes the permanent proboscis. The
original larval cuticle is either now or at an earlier period thrown
off and a fresh cuticle developed. The hooks of the proboscis are
formed from cells of the above papilla, which grow through the larval
skin as conical prominences, on the apex of which a chitinous hook is
modelled. The remainder of the larval skin forms the skin of the
adult, and at a later period develops in its deeper layer the peculiar
plexus of vessels so characteristic of the Acanthocephala. The
anterior oval appendages of the adult cutis, known as the lemnisci,
are outgrowths from the larval skin.
The Echinorhyncus has with the completion of these changes practically
acquired its adult structure; but in the female the ovaries undergo at
this period remarkable changes, in that they break up into a number of
spherical masses, which lie in the lumen of the generative ligaments,
and also make their way into the body cavity.
The young Echinorhyncus requires to be transported to its permanent
host, which feeds on its larval host, before attaining to sexual
maturity.
BIBLIOGRAPHY.
(392) R. Greeff. "Untersuchungen ü. d. Bau u. Entwicklung des Echin.
miliarius." _Archiv f. Naturgesch._ 1864.
(393) R. Leuckart. _Die menschlichen Parasiten._ Vol. II. p. 801 et
seq. 1876.
(394) An. Schneider. "Ueb. d. Bau d. Acanthocephalen." _Archiv f.
Anat. u. Phys._ 1868.
(395) G. R. Wagener. _Beiträge z. Entwicklungsgeschichte d.
Eingeweidewürmer._ Haarlem, 1865.
CHAPTER XVII.
TRACHEATA.
PROTOTRACHEATA.
The remarkable researches of Moseley (No. 396) on Peripatus capensis
have brought clearly to light the affinities of this form with the
tracheate Arthropoda; and its numerous primitive characters, such as
the generally distributed tracheal apertures, the imperfectly
segmented limbs, the diverging ventral nerve cords with imperfectly
marked ganglia, and the nephridia (segmental organs[160]), would
render its embryology of peculiar interest. Unfortunately Moseley was
unable, from want of material, to make so complete a study of its
development as of its anatomy. The youngest embryo observed was in
part distinctly segmented, and coiled up within the egg (fig. 168 A).
The procephalic lobes resemble those of the Arthropoda generally, and
are unlike the præ-oral lobe of Chætopods or Discophora. They are not
marked off by a transverse constriction from the succeeding segments.
The three embryonic layers are differentiated, and the interior is
filled with a brownish mass--the remnant of the yolk--which is
probably enclosed in a distinct intestinal wall, and is lobed in
correspondence with the segmentation of the body. The mouth
invagination is not present, and but two pairs of slight prominences
mark the rudiments of the two anterior post-oral appendages.
[160] F. M. Balfour, "On certain points in the Anatomy of
Peripatus capensis." _Quart. Journ. of Micros. Science_, Vol.
XIX. 1879.
[FIG. 167. ADULT EXAMPLE OF PERIPATUS CAPENSIS, natural size.
(From Moseley.)]
[FIG. 168. TWO STAGES IN THE DEVELOPMENT OF PERIPATUS CAPENSIS.
(After Moseley.)
A. Youngest stage hitherto observed before the appearance of the
legs.
B. Later stage after the legs and antennæ have become developed.
Both figures represent the larva as it appears within the egg.
1 and 2. First and second post-oral appendages.]
The single pair of antennæ is formed in the next stage, and is
followed by the remaining post-oral appendages, which arise in
succession from before backwards somewhat later than the segments to
which they appertain.
The posterior part of the embryo becomes uncoiled, and the whole
embryo bent double in the egg (fig. 168 B).
[FIG. 169. EMBRYO OF PERIPATUS CAPENSIS. Slightly older than A in fig.
168; unrolled. (After Moseley.)
_a._ antennæ; _o._ mouth; _i._ intestine; _c._ procephalic lobe. 1, 2,
3, etc., post-oral appendages.]
The mouth appears as a slit-like opening between and below the
procephalic lobes. On each side and somewhat behind it there grows out
an appendage--the first post-oral pair (fig. 169, 1)--while in front
and behind it are formed the upper and lower lips. These two
appendages next turn inwards towards the mouth, and their bases become
gradually closed over by two processes of the procephalic region (fig.
170, _m_). The whole of these structures assist in forming a kind of
secondary mouth cavity, which is at a later period further completed
by the processes of the procephalic region meeting above the mouth,
covering over the labrum, and growing backwards to near the origin of
the second pair of post-oral appendages.
[FIG. 170. VENTRAL VIEW OF THE HEAD OF AN EMBRYO OF PERIPATUS
CAPENSIS AT A LATE STAGE OF DEVELOPMENT.
_l._ thickening of epiblast of procephalic lobe to form
supra-oesophageal ganglion; _m._ process from procephalic lobe
growing over the first post-oral appendage; _o._ mouth; _e._ eye; 1
and 2, first and second pair of post-oral appendages.]
The antennæ early become jointed, and fresh joints continue to be
added throughout embryonic life; in the adult there are at present
fully thirty joints. It appears to me probable (though Mr Moseley
takes the contrary view) from the late development of the paired
processes of the procephalic lobes, which give rise to the circular
lip of the adult, that they are not true appendages. The next pair
therefore to the antennæ is the first post-oral pair. It is the only
pair connected with the mouth. At their extremities there is formed a
pair of claws similar to those of the ambulatory legs (fig. 171). The
next and largest pair of appendages in the embryo are the oral
papillæ. They are chiefly remarkable for containing the ducts of the
slime glands which open at their bases. They are without claws. The
succeeding appendages become eventually imperfectly five-jointed; two
claws are formed as cuticular investments of papillæ in pockets of the
skin at the ends of their terminal joints.
[FIG. 171. HEAD OF AN EMBRYO PERIPATUS. (From Moseley.)
The figure shews the jaws (mandibles), and close to them epiblastic
involutions, which grow into the supra-oesophageal ganglia. The
antennæ, oral cavity, and oral papillæ are also shewn.]
I have been able to make a few observations on the internal structure
of the embryos from specimens supplied to me by Moseley. These are so
far confined to a few stages, one slightly earlier, the others
slightly later, than the embryo represented in fig. 168 B. The
epiblast is formed of a layer of columnar cells, two deep on the
ventral surface, except along the median line where there is a
well-marked groove and the epiblast is much thinner (fig. 172).
The ventral cords of the trunk are formed as two independent
epiblastic thickenings. In my earlier stage these are barely separated
from the epiblast, but in the later ones are quite independent (fig.
172, _v.n_), and partly surrounded by mesoblast.
The supra-oesophageal ganglia are formed as thickenings of the
epiblast of the ventral side of the procephalic lobes in front of the
stomodæum. They are shewn at _l_ in fig. 170. The thickenings of the
two sides are at first independent. At a somewhat later period an
invagination of the epiblast grows into each of these lobes. The
openings of these invaginations extend from the oral cavity forwards;
and they are shewn in fig. 171[161]. Their openings become closed, and
the walls of the invaginations constitute a large part of the
embryonic supra-oesophageal ganglia.
[161] This figure is taken from Moseley. The epiblastic
invaginations are represented in it very accurately, and though
not mentioned in the text of the paper, Moseley informs me that
he has long been aware of the homology of these folds with those
in various other Tracheata.
Similar epiblastic invaginations assist in forming the supra-oesophageal
ganglia of other Tracheata. They are described in the sequel for
Insects, Spiders and Scorpions. The position of the supra-oesophageal
ganglia on the ventral side of the procephalic lobes is the same as
that in other Tracheata.
[FIG. 172. SECTION THROUGH THE TRUNK OF AN EMBRYO OF PERIPATUS. The
embryo from which the section is taken was somewhat younger than
fig. 171.
_sp.m._ splanchnic mesoblast. _s.m._ somatic mesoblast. _mc._ median
section of body cavity. _lc._ lateral section of body cavity. _v.n._
ventral nerve cord. _me._ mesenteron.]
The mesoblast is formed, in the earliest of my embryos, of scattered
cells in the fairly wide space between the mesenteron and the
epiblast. There are two distinct bands of mesoblast on the outer sides
of the nervous cords. In the later stage the mesoblast is divided into
distinct somatic and splanchnic layers, both very thin; but the two
layers are connected by transverse strands (fig. 172). There are two
special longitudinal septa dividing the body cavity into three
compartments, a median (mc), containing the mesenteron, and two
lateral (_lc_) containing the nerve cords. This division of the body
cavity persists, as I have elsewhere shewn, in the adult. A similar
division is found in some Chætopoda, _e.g._ Polygordius.