their pectinated gill plate (epipodite) and four blades; and in the
fourth stage (fig. 235 A) the second pair of maxillæ (_Mx´´_) arises,
as a pair of curved plates, similar to the first pair of maxillæ at
their first appearance. The forked tail is indicated during the fourth
stage by two bristles. During the fifth stage (fig. 235 B) the number
of joints of the first pair of antennæ becomes increased, and the
posterior maxillæ develop a blade and become four-jointed ambulatory
appendages terminating in a hook. The caudal fork becomes more
distinct.
In the sixth stage (fig. 236) the second and hindermost pair of feet
becomes formed (_f´´´_) and the maxillæ of the second pair lose their
ambulatory function, and begin to be converted into definite
masticatory appendages by the reduced jointing of their palp, and the
increase of their cutting blades. By the seventh stage all the
appendages have practically attained their permanent form; the second
pair of maxillæ has acquired small branchial plates, and the two
following feet have become jointed. In the eighth and ninth stages the
generative organs attain their mature form.
[FIG. 236. SIXTH STAGE IN THE DEVELOPMENT OF CYPRIS. (From Claus.)
_Mx´._ first maxilla; _Mx´´._ _f´._ second maxilla; _f´´._ and
_f´´´._ first and second pair of feet; _Fu._ caudal fork; _L._
liver; _S.D._ shell-gland.]
The larva of Cythere at the time of birth has rudiments of all the
limbs, but the mandibular palp still functions as a limb, and the
three feet (2nd pair of maxillæ and two following appendages) are very
rudimentary.
The larvæ of Cypridina are hatched in a condition which to all intents
and purposes resembles the adult.
_Phylogeny of the Crustacea._
The classical work of Fritz Müller (No. 452) on the phylogeny of the
Crustacea has given a great impetus to the study of their larval
forms, and the interpretations of these forms which he has offered
have been the subject of a very large amount of criticism and
discussion. A great step forward in this discussion has been recently
made by Claus in his memoir (No. 448).
The most fundamental question concerns the meaning of the Nauplius. Is
the Nauplius the ancestral form of the Crustacea, as is believed by
Fritz Müller and Claus, or are its peculiarities and constant
occurrence due to some other cause? The most plausible explanation on
the second hypothesis would seem to be the following. The segments
with their appendages of Arthropoda and Annelida are normally formed
from before backwards, therefore every member of these two groups with
more than three segments must necessarily pass through a stage with
_only three segments_, and the fact that in a particular group this
stage is often reached on the larva being hatched is in itself no
proof that the ancestor of the group had only three segments with
their appendages. This explanation appears to me, so far as it goes,
quite valid; but though it relieves us from the necessity of supposing
that the primitive Crustacea had only three pairs of appendages, it
does not explain several other peculiarities of the Nauplius[203]. The
more important of these are the following.
[203] For the characters of Nauplius _vide_ p. 460.
1. That the mandibles have the form of biramous swimming feet and are
not provided with a cutting blade.
2. That the second pair of antennæ are biramous swimming feet with a
hook used in mastication, and are innervated (?) from the
suboesophageal ganglion.
3. The absence of segmentation in the Nauplius body. An absence which
is the more striking in that before the Nauplius stage is fully
reached the body of the embryo is frequently divided into three
segments, _e.g._ Copepoda and Cirripedia.
4. The absence of a heart.
5. The presence of a median single eye as the sole organ of vision.
Of these points the first, second, and fifth appear only to be capable
of being explained phylogenetically, while with reference to the
absence of a heart it appears very improbable that the ancestral
Crustacea were without a central organ of circulation. If the above
positions are accepted the conclusion would seem to follow that in a
certain sense the Nauplius is an ancestral form--but that, while it no
doubt had its three anterior pairs of appendages similar to those of
existing Nauplii, it may perhaps have been provided with a segmented
body behind provided with simple biramous appendages. A heart and
cephalo-thoracic shield may also have been present, though the
existence of the latter is perhaps doubtful. There was no doubt a
median single eye, but it is difficult to decide whether or no paired
compound eyes were also present. The tail ended in a fork between the
prongs of which the anus opened; and the mouth was protected by a
large upper lip. In fact, it may very probably turn out that the most
primitive Crustacea more resembled an Apus larva at the moult
immediately before the appendages lose their Nauplius characters (fig.
208 B), or a Cyclops larva just before the Cyclops stage (fig. 229),
than the earliest Nauplius of either of these forms.
If the Nauplius ancestor thus reconstructed is admitted to have
existed, the next question in the phylogeny of the Crustacea concerns
the relations of the various phyla to the Nauplius. Are the different
phyla descended from the Nauplius direct, or have they branched at a
later period from some central stem? It is perhaps hardly possible as
yet to give a full and satisfactory answer to this question, which
requires to be dealt with for each separate phylum; but it may
probably be safely maintained that the existing Phyllopods are members
of a group which was previously much larger, and the most central of
all the Crustacean groups; and which more nearly retains in the
characters of the second pair of antennæ etc. the Nauplius
peculiarities. This view is shared both by Claus and Dohrn, and
appears to be in accordance with all the evidence we have both
palæontological and morphological. Claus indeed carries this view
still further, and believes that the later Nauplius stages of the
different Entomostracan groups and the Malacostraca (Penæus larva)
exhibit undoubted Phyllopod affinities. He therefore postulates the
earlier existence of a Protophyllopod form, which would correspond
very closely with the Nauplius as reconstructed above, from which he
believes all the Crustacean groups to have diverged.
It is beyond the scope of this work to attempt to grapple with all the
difficulties which arise in connection with the origin and
relationships of the various phyla, but I confine myself to a few
suggestions arising out of the developmental histories recorded above.
Malacostraca. In attempting to reconstitute from the evidence in our
possession the ancestral history of the Malacostraca we may omit from
consideration the larval history of all those types which leave the
egg in nearly the adult form, and confine our attention to those types
in which the larval history is most completely preserved.
There are three forms which are of special value in this respect, viz.
Euphausia, Penæus and Squilla. From the history of these which has
already been given it appears that in the case of the Decapoda four
stages (Claus) may be traced in the best preserved larval histories.
1. A Nauplius stage with the usual Nauplius characters.
2. A Protozoæa stage in which the maxillæ and first pair of
maxillipeds are formed behind the Nauplius appendages; but in which
the tail is still unsegmented. This stage is comparatively rarely
preserved and usually not very well marked.
3. A Zoæa stage the chief features of which have already been fully
characterised (_vide_ p. 465). Three more or less distinct types of
Zoæa are distinguished by Claus. (_a_) That of Penæus, in which the
appendages up to the third pair of maxillipeds are formed, and the
thorax and abdomen are segmented, the former being however very short.
The heart is oval, with one pair of ostia. From this type the Zoæa
forms of the other Decapoda are believed by Claus to be derived. (_b_)
That of Euphausia, with but one pair of maxillipeds and those short
and Phyllopod-like. The heart oval with one pair of ostia. (_c_) That
of Squilla, with an elongated many-chambered heart, two pairs of
maxillipeds and the abdominal appendages in full activity.
4. A Mysis stage, which is only found in the macrurous Decapod larvæ.
The embryological questions requiring to be settled concern the value
of the above stages. Do they represent stages in the actual evolution
of the present types, or have their characters been secondarily
acquired in larval life?
With reference to the first stage this question has already been
discussed, and the conclusion arrived at, that the Nauplius does in a
much modified form represent an ancestral type. As to the fourth stage
there can be no doubt that it is also ancestral, considering that it
is almost the repetition of an actually existing form.
The second stage can clearly only be regarded as an embryonic
preparation for the third; and the great difficulty concerns the third
stage.
The natural view is that this stage like the others has an ancestral
value, and this view was originally put forward by Fritz Müller and
has been argued for also by Dohrn. On the other hand the opposite side
has been taken by Claus, who has dealt with the question very ably and
at great length, and has clearly shewn that some of Fritz Müller's
positions are untenable. Though Claus' opinion is entitled to very
great weight, an answer can perhaps be given to some of his
objections. The view adopted in this section can best be explained by
setting forth the chief points which Claus urges against Fritz
Müller's view.
The primary question which needs to be settled is whether the
Malacostraca have diverged very early from the Nauplius root, or later
in the history of the Crustacea from the Phyllopod stem. On this
question Claus[204] brings arguments, which appear to me very
conclusive, to shew that the Malacostraca are derived from a late
Protophyllopod type, and Claus' view on this point is shared also by
Dohrn. The Phyllopoda present so many characters (not possessed by the
Nauplius) in common with the Malacostraca or their larval forms, that
it is incredible that the whole of these should have originated
independently in the two groups. The more important of these
characters are the following.
[204] Claus speaks of the various Crustacean phyla as having
sprung from a Protophyllopod form, and it might be supposed that
he considered that they all diverged from the same form. It is
clear however from the context that he regards the Protophyllopod
type from which the Malacostraca originated as far more like
existing Phyllopods than that from which the Entomostracan groups
have sprung. It is not quite easy to get a consistent view of his
position on the question, since he states (p. 77) that the
Malacostraca and the Copepods diverged from a similar form, which
is represented in their respective developments by the Protozoæa
and earliest Cyclops stage. Yet if I understand him rightly, he
does not consider the Protozoæa stage to be the Protophyllopod
stage from which the Malacostraca have diverged, but states on p.
71 that it was not an ancestral form at all.
1. The compound eyes, so often stalked in both groups.
2. The absence of a palp on the mandible, a very marked character of
the Zoæa as well as of the Phyllopoda.
3. The presence of a pair of frontal sense knobs.
4. The Phyllopod character of many of the appendages. Cf. first pair
of maxillipeds of the Euphausia Zoæa.
5. The presence of gill pouches (epipodites) on many of the
appendages[205].
[205] Claus appears to consider it doubtful whether the
Malacostracan gills can be compared with the Phyllopod gill
pouches.
In addition to these points, to which others might be added, Claus
attempts to shew that Nebalia must be regarded as a type intermediate
between the Phyllopods and Malacostraca. This view seems fairly
established, and if true is conclusive in favour of the Phyllopod
origin of the Malacostraca. If the Protophyllopod origin of the
Malacostraca is admitted, it seems clear that the ancestral forms of
the Malacostraca must have developed their segments regularly from
before backwards, and been provided with nearly similar appendages on
all the segments. This however is far from the case in existing
Malacostraca, and Fritz Müller commences his summary of the characters
of the Zoæa in the following words[206]. "The middle body with its
appendages, those five pairs of feet to which these animals owe their
name, is either entirely wanting or scarcely indicated." This he
regards as an ancestral character of the Malacostraca, and is of
opinion that their thorax is to be regarded as a later acquirement
than the head or abdomen. Claus' answer on this point is that in the
most primitive Zoææ, viz. those already spoken of as types, the
thoracic and abdominal segments actually develop in regular succession
from before backwards, and he therefore concludes that the late
development of the thorax in the majority of Zoæa forms is secondary
and not an ancestral Phyllopod peculiarity.
[206] _Facts for Darwin_, p. 49.
This is the main argument used by Claus against the Zoæa having any
ancestral meaning. His view as to the meaning of the Zoæa may be
gathered from the following passage. After assuming that none of the
existing Zoæa types could have been adult animals, he says--"Much more
probably the process of alteration of the metamorphosis, which the
Malacostracan phylum underwent in the course of time and in
conjunction with the divergence of the later Malacostracan groups, led
secondarily to the three different Zoæa configurations to which
probably later modifications were added, as for instance in the young
form of the Cumaceæ. We might with the same justice conclude that
adult Insects existed as caterpillars or pupæ as that the primitive
form of the Malacostraca was a Protozoæa or Zoæa."
Granting Claus' two main positions, viz. that the Malacostraca are
derived from Protophyllopods, and that the segments were in the
primary ancestral forms developed from before backwards, it does not
appear impossible that a secondary and later ancestral form may have
existed with a reduced thorax. This reduction may only have been
partial, so that the Zoæa ancestor would have had the following form.
A large cephalo-thorax and well-developed tail (?) with swimming
appendages. The appendages up to the second pair of maxillipeds fully
developed, but the thorax very imperfect and provided only with
delicate foliaceous appendages not projecting beyond the edge of the
cephalo-thoracic shield.
Another hypothesis for which there is perhaps still more to be said is
that there was a true ancestral Zoæa stage in which the thoracic
appendages were completely aborted. Claus maintains that the Zoæa form
with aborted thorax is only a larval form; but he would probably admit
that its larval characters were acquired to enable the larva to swim
better. If this much be admitted it is not easy to see why an actual
member of the ancestral series of Crustacea should not have developed
the Zoæa peculiarities when the mud-dwelling habits of the Phyllopod
ancestors were abandoned, and a swimming mode of life adopted. This
view, which involves the supposition that the five (or six including
the third maxillipeds) thoracic appendages were lost in the adult (for
they may be supposed to have been retained in the larva) for a series
of generations, and reappeared again in the adult condition, at a
later period, may at first sight appear very improbable, but there
are, especially in the larval history of the Stomatopoda, some actual
facts which receive their most plausible explanation on this
hypothesis.
These facts consist in cases of the actual loss of appendages during
development, and their subsequent reappearance. The two most striking
cases are the following.
1. In the Erichthus form of the Squilla larva the appendages
corresponding to the third pair of maxillipeds and first two pairs of
ambulatory appendages of the Decapoda are developed in the Protozoæa
stage, but completely aborted in the Zoæa stage, and subsequently
redeveloped.
2. In the case of the larva of Sergestes in the passage from the
Acanthosoma (Mysis) stage to the Mastigopus stage the two hindermost
thoracic appendages become atrophied and redevelop again later.
Both of these cases clearly fit in very well with the view that there
was an actual period in the history of the Malacostraca in which the
ancestors of the present forms were without the appendages which are
aborted and redeveloped again in these larval forms. Claus' hypothesis
affords no explanation of these remarkable cases.
It is however always possible to maintain that the loss and
reappearance of the appendages in these cases may have no ancestral
meaning; and the abortion of the first pair of maxillipeds and
reduction of some of the other appendages in the case of the Loricata
is in favour of this explanation. Similar examples of the abortion and
reappearance of appendages, which cannot be explained in the way
attempted above, are afforded by the Mites and also by the Insects,
_e.g._ Bees.
On the other hand there is almost a conclusive indication that the
loss of the appendages in Sergestes has really the meaning assigned to
it, in that in the allied genius Leucifer the two appendages in
question are actually absent in the adult, so that the stage with
these appendages absent is permanently retained in an adult form. In
the absence of the mandibular palp in all the Zoæa forms, its actual
atrophy in the Penæus Zoæa, and its universal reappearance in adult
Malacostraca, are cases which tell in favour of the above explanation.
The mandibular palp is permanently absent in Phyllopods, which clearly
shews that its absence in the Zoæa stage is due to the retention of an
ancestral peculiarity, and that its reappearance in the adult forms
was a late occurrence in the Malacostracan history.
The chief obvious difficulty of this view is the redevelopment of the
thoracic feet after their disappearance for a certain number of
generations. The possibility of such an occurrence appears to me
however clearly demonstrated by the case of the mandibular palp, which
has undoubtedly been reacquired by the Malacostraca, and by the case
of the two last thoracic appendages of Sergestes just mentioned. The
above difficulty may be diminished if we suppose that the larvæ of the
Zoæa ancestors always developed the appendages in question. Such
appendages might first only partially atrophy in a particular Zoæa
form and then gradually come to be functional again; so that, as a
form with functional thoracic limbs came to be developed out of the
Zoæa, we should find in the larval history of this form that the limbs
were developed in the pre-zoæal larval stages, partially atrophied in
the Zoæa stage, and redeveloped in the adult. From this condition it
would not be difficult to pass to a further one in which the
development of the thoracic limbs became deferred till after the Zoæa
stage.
The general arguments in favour of a Zoæa ancestor with partially or
completely aborted thoracic appendages having actually existed in the
past appear to me very powerful. In all the Malacostracan groups in
which the larva leaves the egg in an imperfect form a true Zoæa stage
is found. That the forms of these Zoææ should differ considerably is
only what might be expected, considering that they lead a free
existence and are liable to be acted upon by natural selection, and it
is probable that none of those at present existing closely resemble
the ancestral form. The spines from their carapace, which vary so
much, were probably originally developed, as suggested by Fritz
Müller, as a means of defence. The simplicity of the heart--so
different from that of Phyllopods--in most forms of Zoæa is a
difficulty, but the reduction in the length of the heart may very
probably be a secondary modification; the primitive condition being
retained in the Squilla Zoæa. In any case this difficulty is not
greater on the hypothesis of the Zoæa being an ancestral form, than on
that of its being a purely larval one.
The points of agreement in the number and character of the appendages,
form of the abdomen, etc. between the various types of Zoæa appear to
me too striking to be explained in the manner attempted by Claus. It
seems improbable that a peculiarity of form acquired by the larva of
some ancestral Malacostracan should have been retained so permanently
in so many groups[207]--more permanently indeed than undoubtedly
ancestral forms like that of Mysis--and it would be still more
remarkable that a Zoæa form should have been two or more times
independently developed.
[207] A secondary larval form is less likely to be repeated in
development than an ancestral adult stage, because there is
always a strong tendency for the former, which is a secondarily
intercalated link in the chain, to drop out by the occurrence of
a reversion to the original type of development.
There are perhaps not sufficient materials to reconstruct the
characters of the Zoæa ancestor, but it probably was provided with the
anterior appendages up to the second pair of maxillipeds, and (?) with
abdominal swimming feet. The heart may very likely have been
many-chambered. Whether gill pouches were present on the maxillipeds
and abdominal feet does not appear to me capable of being decided. The
carapace and general shape were probably the same as in existing
Zoæas. It must be left an open question whether the six hindermost
thoracic appendages were absent or only very much reduced in size.
On the whole then it may be regarded as probable that the Malacostraca
are descended from Protophyllopod forms, in which, on the adoption of
swimming habits, six appendages of the middle region of the body were
reduced or aborted, and a Zoæa form acquired, and that subsequently
the lost appendages were redeveloped in the descendants of these
forms, and have finally become the most typical appendages of the
group.
The relationship of the various Malacostracan groups is too difficult
a subject to be discussed here, but it seems to me most likely that in
addition to the groups with a Zoæa stage the Edriophthalmata and
Cumaceæ are also post-zoæal forms which have lost the Zoæa stage.
Nebalia is however very probably to be regarded as a præ-zoæal form
which has survived to the present day; and one might easily fancy that
its eight thin thoracic segments with their small Phyllopod-like feet
might become nearly aborted.
Copepoda. The Copepoda certainly appear to have diverged very early
from the main stem, as is shewn by their simple biramous feet and the
retention of the median eye as the sole organ of vision. It may be
argued that they have lost the eye by retrogressive changes, and in
favour of this view cases of the Pontellidæ and of Argulus may be
cited. It is however more than doubtful whether the lateral eyes of
the Pontellidæ are related to the compound Phyllopod eye, and the
affinities of Argulus are still uncertain. It would moreover be a
great paradox if in a large group of Crustacea the lateral eyes had
been retained in a parasitic form only (Argulus), but lost in all the
free forms.
Cirripedia. The Cirripedia are believed by Claus to belong to the same
phylum as the Copepoda. This view does not appear to be completely
borne out by their larval history. The Nauplius differs very markedly
from that of the Copepoda, and this is still more true of the Cypris
stage. The Copepod-like appendages of this stage are chiefly relied
upon to support the above view, but this form of appendages was
probably very primitive and general, and the number (without taking
into consideration the doubtful case of Cryptophialus) does not
correspond to that in Copepoda. On the other hand the paired eyes and
the bivalve shell form great difficulties in the way of Claus' view.
It is clear that the Cypris stage represents more or less closely an
ancestral form of the Cirripedia, and that both the large bivalve
shell and the compound eyes were ancestral characters. These
characters would seem incompatible with Copepod affinities, but point
to the independent derivation of the Cirripedia from some early
bivalve Phyllopod form.
[FIG. 237. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. (From
Parker; after Reichenbach.)
A. Section through part of the ovum during segmentation. _n._
nuclei; _w.y._ white yolk; _y.p._ yolk pyramids; _c._ central
yolk mass.
B and C. Longitudinal sections during the gastrula stage. _a._
archenteron; _b._ blastopore; _ms._ mesoblast; _ec._ epiblast;
_en._ hypoblast distinguished from epiblast by shading.
D. Highly magnified view of the anterior lip of blastopore to shew
the origin of the primary mesoblast from the wall of the
archenteron. _p.ms._ primary mesoblast; _ec._ epiblast; _en._
hypoblast.
E. Two hypoblast cells to shew the amoeba-like absorption of yolk
spheres. _y._ yolk; _n._ nucleus; _p._ pseudopodial process.
F. Hypoblast cells giving rise endogenously to the secondary
mesoblast (_s.ms._). _n._ nuclei.]
Ostracoda. The independent origin of the Ostracoda from the main
Crustacean stem seems probable. Claus points out that the Ostracoda
present by no means a simple organisation, and concludes that they
were not descended from a form with a more complex organisation and a
larger number of appendages. Some simplifications have however
undoubtedly taken place, as the loss of the heart, and of the compound
eyes in many forms. These simplifications are probably to be explained
(as is done by Claus) as adaptations due to the small size of body and
its enclosure in a thick bivalve shell. Although Claus is strongly
opposed to the view that the number of the appendages has been
reduced, yet the very fact of the (in some respects) complex
organisation of this group might seem to indicate that it cannot have
diverged from the Phyllopod stem at so early a stage as (on Claus'
view of the Nauplius) would seem to be implied by the very small
number of appendages which is characteristic of it, and it therefore
appears most probable that the present number may be smaller than that
of the ancestral forms.
_The formation of the germinal layers._
The formation of the germinal layers has been more fully studied in
various Malacostraca, more especially in the Decapoda, than in other
groups.
Decapoda. To Bobretzky (No. 472) is due the credit of having been the
pioneer in this line of investigation; and his researches have been
followed up and enlarged by Haeckel, Reichenbach (No. 488), and Mayer
(No. 482). The segmentation is centrolecithal and regular (fig. 237
A). At its close the blastoderm is formed of a single uniform layer of
lens-shaped cells enclosing a central sphere of yolk, in which as a
rule all trace of the division into columns, present during the
earlier stages of segmentation, has disappeared; though in Palæmon the
columns remain for a long period distinct. The cells of the blastoderm
are at first uniform, but in Astacus, Eupagurus, and most Decapoda,
soon become more columnar for a small area, and form a circular patch.
The whole patch either becomes at once invaginated (Eupagurus,
Palæmon, fig. 239 A) or else the edge of it is invaginated as a
roughly speaking circular groove deeper anteriorly than posteriorly,
within which the remainder of the patch forms a kind of central plug,
which does not become invaginated till a somewhat later period
(Astacus, fig. 237 B and C). After the invagination of the above patch
the remainder of the blastoderm cells form the epiblast.
The invaginated sack appears to be the archenteron and its mouth the
blastopore. The mouth finally becomes closed[208], and the sack itself
then forms the mesenteron.
[208] Bobretzky first stated that the invagination remained open,
but subsequently corrected himself. _Zeit. f. Wiss. Zool._, Bd.
XXIV. p. 186.
[FIG. 238. TWO LONGITUDINAL SECTIONS OF THE EMBRYO OF ASTACUS. (From
Parker; after Bobretzky.)
A. Nauplius stage. B. Stage after the hypoblast cells have absorbed
the food-yolk. The ventral surface is turned upwards. _fg._
stomodæum; _hg._ proctodæum; _an._ anus; _m._ mouth; _mg._
mesenteron; _abd._ abdomen; _h._ heart.]
In Astacus the archenteron gradually grows forwards, its opening is at
first wide, but becomes continuously narrowed and is finally
obliterated. Very shortly after this occurrence there is formed,
slightly in front of the point where the last trace of the blastopore
was observable, a fresh epiblastic invagination, which gives rise to
the proctodæum, and the opening of which remains as the definite anus.
The proctodæum (fig. 238 A, _hg)_ is very soon placed in communication
with the mesenteron (_mg_). The stomodæum (_fg_) is formed during the
same stage as the proctodæum. It gives rise to the oesophagus and
stomach. The hypoblast cells which form the wall of the archenteron
grow with remarkable rapidity at the expense of the yolk; the
spherules of which they absorb and digest in an amoeba-like fashion by
means of their pseudopodia. They become longer and longer, and
finally, after absorbing the whole yolk, acquire a form almost exactly
similar to that of the yolk pyramids during segmentation (fig. 238 B).
They enclose the cavity of the mesenteron, and their nuclei and
protoplasm are situated externally. The cells of the mesenteron close
to its junction with the proctodæum differ from those elsewhere in
being nearly flat.
In Palæmon (Bobretzky) the primitive invagination (fig. 239 A) has far
smaller dimensions than in Astacus, and appears before the blastoderm
cells have separated from the yolk pyramids. The cells which are
situated at the bottom of it pass into the yolk, increase in number,
and absorb the whole yolk, forming a solid mass of hypoblast in which
the outlines of the individual cells would seem at first not to be
distinct. The blastopore in the meantime becomes closed. Some of the
nuclei now pass to the periphery of the yolk mass; the cells
appertaining to them gradually become distinct and assume a pyramidal
form (fig. 239 B, _hy_), the inner ends of the cells losing themselves
in a central mass of yolk, in the interior of which nuclei are at
first present but soon disappear. The mesenteron thus becomes
constituted of a layer of pyramidal cells which merge into a central
mass of yolk. Some of the hypoblast cells adjoining the junction of
the proctodæum and mesenteron become flattened, and in the
neighbourhood of these cells a lumen first appears. The stomodæum and
proctodæum are formed as in Astacus. Fig. 239 B shews the relative
positions of the proctodæum, stomodæum, and mesenteron. Although the
process of formation of the hypoblast and mesenteron is essentially
the same in Astacus and Palæmon, yet the differences between these two
forms are very interesting, in that the yolk is _external_ to the
mesenteron in Astacus, but _enclosed within it_ in Palæmon. This
difference in the position of the yolk is rendered possible by the
fact that the invaginated hypoblast cells in Palæmon do not, at first,
form a continuous layer enclosing a central cavity, while they do so
in Astacus.
[FIG. 239. TWO STAGES IN THE DEVELOPMENT OF PALÆMON SEEN IN SECTION.
(After Bobretzky.)
A. Gastrula stage.
B. Longitudinal section through a late stage. _hy._ hypoblast;
_sg._ supra-oesophageal ganglion; _vg._ ventral nerve
cord; hd. proctodæum; _st._ stomodæum.]
The mesoblast appears to be formed of cells budded off from the
anterior wall of the archenteron (Astacus, fig. 237 D), or from its
lateral walls generally (Palæmon). They make their first appearance
soon after the invagination of the hypoblast has commenced. The
mesoblast cells are at first spherical, and gradually spread,
especially in an anterior direction, from their point of origin.
According to Reichenbach there are formed in Astacus at the Nauplius
stage a number of peculiar cells which he speaks of as 'secondary
mesoblast cells.' His account is not very clear or satisfactory, but
it appears that they originate (fig. 237 F) in the hypoblast cells by
a kind of endogenous growth, and though they have at first certain
peculiar characters they soon become indistinguishable from the
remaining mesoblast cells.
Towards the end of the Nauplius period the secondary mesoblast cells
aggregate themselves into a rod close to the epiblast in the median
ventral line, and even bifurcate round the mouth and extend forwards
to the extremity of the procephalic lobes. This rod of cells very soon
vanishes, and the secondary mesoblast cells become indistinguishable
from the primary. Reichenbach believes, on not very clear evidence,
that these cells have to do with the formation of the blood.
_General form of the body._ The ventral thickening of epiblast or
ventral plate, continuous with the invaginated patch already
mentioned, forms the first indication of the embryo. It is at first
oval, but soon becomes elongated and extended anteriorly into two
lateral lobes--the procephalic lobes. Its bilateral symmetry is
further indicated by a median longitudinal furrow. The posterior end
of the ventral plate next becomes raised into a distinct lobe--the
abdomen--which in Astacus at first lies _in front_ of the still open
blastopore. This lobe rapidly grows in size, and at its extremity is
placed the narrow anal opening. It soon forms a well-marked abdomen
bent forwards over the region in front (figs. 239 B, and 240 A and B).
Its early development as a distinct outgrowth causes it to be without
yolk; and so to contrast very forcibly with the anterior thoracic and
cephalic regions of the body. In most cases this process corresponds
to the future abdomen, but in some cases (Loricata) it appears to
include part of the thorax. Before it has reached a considerable
development, three pairs of appendages spring from the region of the
head, viz. two pairs of antennæ and the mandibles, and inaugurate a
so-called Nauplius stage (fig. 240 A). These three appendages are
formed nearly simultaneously, but the hindermost appears to become
visible slightly before the two others (Bobretzky). The mouth lies
slightly behind the anterior pair of antennæ, but distinctly in front
of the posterior pair. The other appendages, the number of which at
the time of hatching varies greatly in the different Decapods (_vide_
section on larval development), sprout in succession from before
backwards (fig. 240 B). The food-yolk in the head and thoracic region
gradually becomes reduced in quantity with the growth of the embryo,
and by the time of hatching the disparity in size between the thorax
and abdomen has ceased to exist.
Isopoda. The early embryonic phases of the Isopoda have been studied
by means of sections by Bobretzky (No. 498) and Bullar (No. 499) and
have been found to present considerable variations. When laid the egg
is enclosed in a chorion, but shortly after the commencement of
segmentation (Ed. van Beneden and Bullar) a second membrane appears,
which is probably of the nature of a larval membrane.
[FIG. 240. TWO STAGES IN THE DEVELOPMENT OF PALÆMON.
A. Nauplius stage.
B. Stage with eight pairs of appendages. _op._ eyes; _at1._ and
_at2._ first and second antennæ; _md._ mandibles; _mx1_, _mx2._
first and second maxillæ; _mxp3._ third maxillipeds; _lb._ upper
lip.]
In all the forms the segmentation is followed by the formation of a
blastoderm, completely enclosing the yolk, and thickened along an area
which will become the ventral surface of the embryo. In this area the
blastoderm is formed of at least two layers of cells--an external
columnar epiblast, and an internal layer of scattered cells which form
the mesoblast and probably in part also the hypoblast (Oniscus,
_Bobretzky_; Cymothoa, _Bullar_).
In Asellus aquaticus there is a centrolecithal segmentation, ending in
the formation of a blastoderm, which appears first on the ventral
surface and subsequently extends to the dorsal.
In Oniscus murarius, and Cymothoa the segmentation is partial [for its
peculiarities and relationship _vide_ p. 120] and a disc, formed of a
single layer of cells, appears at a pole of the egg which corresponds
to the future ventral surface (Bobretzky). This layer gradually grows
round the yolk partly by division of its cells, though a formation of
fresh cells from the yolk may also take place. Before it has extended
far round the yolk, the central part of it becomes two or more layers
deep, and the cells of the deeper layers rapidly increase in number,
and are destined to give rise to the mesoblast and probably also to
part or the whole of the hypoblast. In Cymothoa this layer does not at
first undergo any important change, but in Oniscus it becomes very
thick, and its innermost cells (Bobretzky) become imbedded in the
yolk, which they rapidly absorb; and increasing in number first of all
form a layer in the periphery of the yolk, and finally fill up the
whole of the interior of the yolk (fig. 241 A), absorbing it in the
process.
It appears possible that these cells do not, as Bobretzky believes,
originate from the blastoderm, but from nuclei in the yolk which have
escaped his observation. This mode of origin would be similar to that
by which yolk cells originate in the eggs of the Insecta, etc. If
Bobretzky's account is correct we must look to Palæmon, as he himself
suggests, to find an explanation of the passage of the hypoblast cells
into the yolk. The thickening of the primitive germinal disc would,
according to this view, be equivalent to the invagination of the
archenteron in Astacus, Palæmon, etc.
[FIG. 241. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS
MURARIUS. (After Bobretzky.)
_st._ stomodæum; _pr._ proctodæum; _hy._ hypoblast formed of large
nucleated cells imbedded in the yolk; _m._ mesoblast; _vg._ ventral
nerve cord; _sg._ supra-oesophageal ganglion; _li._ liver; _do._
dorsal organ; _zp._ rudiment of masticatory apparatus; _ol._ upper
lip.]
Whatever may be the origin of the cells in the yolk they no doubt
correspond to the hypoblast of other types. In Cymothoa nothing
similar to them has been met with, but the hypoblast has a somewhat
different origin being apparently formed from some of the indifferent
cells below the epiblast, which collect as a solid mass on the ventral
surface, and then divide into two masses which become hollow and give
rise to the liver cæca. Their fate, as well as that of the hypoblast
in Oniscus, is dealt with in connection with the alimentary tract. The
completion of the enclosure of the yolk by the blastoderm takes place
on the dorsal surface. In all the Isopods which have been carefully
studied, there appears before any other organ a provisional structure
formed from the epiblast and known as the dorsal organ. An account of
it is given in connection with the development of the organs. The
general external changes undergone by the larva in its development are
as follows. The ventral thickened area of the blastoderm (ventral
plate) shapes itself and girths nearly the whole circumference of the
ovum in Oniscus (fig. 241 A) but is relatively much shorter in
Cymothoa. Anteriorly it dilates into the two procephalic lobes. In
Cymothoa it next becomes segmented; and the anterior segments are
formed nearly simultaneously, and those of the abdomen somewhat later.
At the same time a median depression appears dividing the blastoderm
longitudinally into two halves. The appendages are formed later than
their segments, and the whole of them are formed nearly
simultaneously, with the exception of the last thoracic, which does
not appear till comparatively late after the hatching of the embryo.
The late development of the seventh thoracic segment and appendage is
a feature common to the majority of the Isopoda (Fritz Müller). In
Oniscus the limbs are formed in nearly the same way as in Cymothoa,
but in Asellus they do not arise quite simultaneously. First of all,
the two antennæ and mandibles (the future palp) appear, inaugurating a
stage often spoken of as the Nauplius stage, which is supposed to
correspond with the free Nauplius stage of Penæus and Euphausia. At
this stage a cuticle is shed (Van Beneden) which remains as an
envelope surrounding the larva till the time of hatching. Similar
cuticular envelopes are formed in many Isopoda. Subsequently the
appendages of the thorax appear, and finally those of the abdomen.
Later than the appendages there arise behind the mouth two prominences
which resemble appendages, but give rise to a bilobed lower lip
(Dohrn).
In Asellus and Oniscus the ventral plate moulds itself to the shape of
the egg, and covers the greater part of the dorsal as well as of the
ventral side (fig. 241 A). As a result of this the ventral surface of
the embryo is throughout convex; and in Asellus a deep fold appears on
the back of the embryo, so that the embryo appears coiled up within
the egg with its ventral side outwards and its head and tail in
contact. In Oniscus the ventral surface is convex, but the dorsal
surface is never bent in as in Asellus. In Cymothoa the egg is very
big and the ventral plate does not extend nearly so far round to the
dorsal side as in Asellus, in consequence of which the ventral surface
is not nearly so convex as in other Isopoda. At the same time the
telson is early formed, and is bent forwards so as to lie on the under
side of the part of the blastoderm in front. In having this ventral
curvature of the telson Cymothoa forms an exception amongst Isopods;
and in this respect is intermediate between the embryos of Asellus and
those of the Amphipoda.
Amphipoda. Amongst the Amphipoda the segmentation is usually
centrolecithal. In the case of Gammarus locusta (Ed. van Beneden and
Bessels, No. 503) it commences with an unequal but total segmentation
like that of the Frog (_vide_ p. 97), and the separation of a central
yolk mass is a late occurrence; and it is noticeable that the part of
the egg with the small segments eventually becomes the ventral
surface. In the fresh-water species of Gammarus (G. pulex and
fluviatilis) the segmentation is more like that of Insects; the
blastoderm cells being formed nearly simultaneously over a large part
of the surface of the egg.
Both forms of segmentation give rise to a blastoderm covering the
whole egg, which soon becomes thickened on the ventral surface. There
is formed, as in the Isopoda, a larval membrane at about the time when
the blastoderm is completed. Very soon after this the egg loses its
spherical shape, and becomes produced into a pointed extremity--the
future abdomen--which is immediately bent over the ventral surface of
the part in front. The ventral curvature of the hinder part of the
embryo at so early an age stands in marked contrast to the usual
condition of Isopod embryos, and is only approached in this group, so
far as is known, in the case of Cymothoa.
At the formation of the first larval membrane the blastoderm cells
separate themselves from it, except at one part on the dorsal surface.
The patch of cells adherent at this part gives rise to a dorsal organ,
comparable with that in Oniscus, connecting the embryo and its first
larval skin. A perforation appears in it at a later period.
The segments and limbs of the Amphipoda are all formed before the
larva leaves the egg.
Cladocera. The segmentation (Grobben, No. 455) takes place on the
normal centrolecithal type, but is somewhat unequal. Before the close
of the segmentation there may be seen at the apex of the vegetative
pole one cell marked off from the remainder by its granular aspect. It
gives rise to the generative organs. One of the cells adjoining it
gives rise to the hypoblast, and the other cells which surround it
form the commencement of the mesoblast. The remaining cells of the
ovum form the epiblast. By a later stage the hypoblast cell is divided
into thirty-two cells and the genital cell into four, while the
mesoblast forms a circle of twelve cells round the genital mass.
The hypoblast soon becomes involuted; the blastopore probably closes,
and the hypoblast forms a solid cord of cells which eventually becomes
the mesenteron. The stomodæum is said to be formed at the point of
closure of the blastopore. The mesoblast passes inwards and forms a
mass adjoining the hypoblast, and somewhat later the genital mass also
becomes covered by the epiblast. The proctodæum appears to be formed
later than the stomodæum.
The embryo as first shewn by Dohrn passes through a Nauplius stage in
the brood-pouch, but is hatched, except in the case of the winter eggs
of Leptodora, in a form closely resembling the adult.
Copepoda. Amongst the free Copepoda the segmentation and formation of
the layers have recently been investigated by Hoek (No. 512). He finds
that there is, in both the fresh-water and marine forms studied by
him, a centrolecithal segmentation similar to that of Palæmon and
Pagurus (_vide_ p. 112), which might from the surface be supposed to
be complete and nearly regular. After the formation of the blastoderm
an invagination of some of its cells takes place and is completed in
about a quarter of an hour. The opening becomes closed. This
invagination is compared by Hoek to the invagination in Astacus, and
is believed by him to give rise to the mesenteron. Its point of
closing corresponds with the hind end of the embryo. On the ventral
surface there appear two transverse furrows dividing the embryo into
three segments, and a median longitudinal furrow which does not extend
to the front end of the foremost segment. The three pairs of Nauplius
appendages and upper lip become subsequently formed as outgrowths from
the sides of the ventral blastodermic thickening.
Amongst the parasitic Copepoda there are found two distinct types of
segmentation, analogous to those in the Isopoda. In the case of
Condracanthus the segmentation is somewhat irregular, but on the type
of Eupagurus, etc. (_vide_ p. 112). In the other group (Anchorella,
Clavella, Congericola, Caligus, Lerneopoda) the segmentation nearly
resembles the ordinary meroblastic type (_vide_ p. 120), and is to be
explained in the same manner as in the cases of Oniscus and Cymothoa.
The first blastodermic cells sometimes appear in a position
corresponding with the head end of the embryo (Anchorella), at other
times at the hind end (Clavella), and sometimes in the middle of the
ventral surface. The dorsal surface of the yolk is always the latest
to be inclosed by the blastoderm cells. A larval cuticle similar to
that of the Isopoda is formed at the same time as the blastoderm. At
the sides of the ventral thickening of the blastoderm there grow out
the Nauplius appendages, of which only the first two appear in
Anchorella. In Anchorella and Lerneopoda the embryos are not hatched
at the Nauplius stage, but after the Nauplius appendages have been
formed a fresh cuticle--the Nauplius cuticle--is shed, and within it
the embryo develops till it reaches the so-called Cyclops stage
(_vide_ p. 490). The embryo within the egg has its abdomen curved
dorsalwards as amongst the Isopoda.
Cirripedia. The segmentation of Balanus and Lepas commences by the
segregation of the constituents of the egg into a more protoplasmic
portion, and a portion formed mainly of food material. The former
separates from the latter as a distinct segment, and then divides into
two not quite equal portions. The division of the protoplasmic part of
the embryo continues, and the resulting segments grow round the single
yolk segment. The point where they finally enclose it is situated on
the ventral surface (Lang) at about the position of the mouth (?).
After being enclosed by the protoplasmic cells the yolk divides, and
gives rise to a number of cells, which probably supply the material
for the walls of the mesenteron. The external layer of protoplasm
forms the so-called blastoderm, and soon (Arnold, Lang) becomes
thickened on the dorsal surface.
The embryo is next divided by two constrictions into three segments;
and there are formed the three appendages corresponding to these,
which are at first simple. The two posterior soon become biramous. The
larva leaves the egg before any further appendages become formed.
_Comparative development of the organs._
Central nervous system. The ventral nerve cord of the Crustacea
develops as a thickening of the epiblast along the median ventral
line; the differentiation of which commences in front, and thence
extends backwards. The ventral cord is at first unsegmented. The
supra-oesophageal ganglia originate as thickenings of the epiblast of
the procephalic lobes.
The details of the above processes are still in most cases very
imperfectly known. The fullest account we have is that of Reichenbach
(No. 488) for Astacus. He finds that the supra-oesophageal ganglia and
ventral cord arise as a continuous formation, and not independently as
would seem to be the case in Chætopoda. The supra-oesophageal ganglia
are formed from the procephalic lobes. The first trace of them is
visible in the form of a pair of pits, one on each side of the middle
line. These pits become in the Nauplius stage very deep, and their
walls are then continued into two ridges where the epiblast is several
cells deep, which pass backwards one on each side of the mouth. The
walls of the pits are believed by Reichenbach to give rise to the
optic portions of the supra-oesophageal ganglia, and the epiblastic
ridges to the remainder of the ganglia and to the circum-oesophageal
commissures. At a much later stage, when the ambulatory feet have
become formed, a median involution of epiblast in front of the mouth
and between the two epiblast ridges gives rise to a central part of
the supra-oesophageal ganglia. Five elements are thus believed by
Reichenbach to be concerned in the formation of these ganglia, viz.
two epiblast pits, two epiblast ridges, and an involution of epiblast
between the latter. It should be noted however that the fate neither
of the pair of pits, nor of the median involution, appears to have
been satisfactorily worked out. The two epiblast ridges, which pass
back from the supra-oesophageal ganglia on each side of the mouth, are
continued as a pair of thickenings of the epiblast along the sides of
a median ventral groove. This groove is deep in front and shallows out
posteriorly. The thickenings on the sides of this groove no doubt give
rise to the lateral halves of the ventral cord, and the cells of the
groove itself are believed by Reichenbach, but it appears to me
without sufficient evidence, to become invaginated also and to assist
in forming the ventral cord. When the ventral cord becomes separated
from the epiblast the two halves of it are united in the middle line,
but it is markedly bilobed in section.
In the Isopoda it would appear both from Bobretzky's and Bullar's
observations that the ventral nerve cord arises as an unpaired
thickening of the epiblast _in which there is no trace of anything
like a median involution_. After this thickening has become separated
from the epiblast a slight median furrow indicates its constitution
out of two lateral cords. The supra-oesophageal ganglia are stated to
be developed quite simply as a pair of thickenings of the procephalic
lobes, but whether they are from the first continuous with the ventral
cord does not appear to have been determined.
The later stages in the differentiation of the ventral cord are, so
far as is known, very similar throughout the Crustacea. The ventral
cord is, as has been stated, at first unsegmented (fig. 241 A, _vg_),
but soon becomes divided by a series of constrictions into as many
ganglia as there are pairs of appendages or segments (fig. 241 B,
_vg_).
There appears either on the ventral side (Oniscus) or in the centre
(Astacus, Palæmon) of the two halves of each segment or ganglion a
space filled with finely punctuated material, which is the
commencement of the commissural portion of the cords. The commissural
tissue soon becomes continuous through the length of the ventral cord,
and is also prolonged into the supra-oesophageal ganglia.
After the formation of the commissural tissue the remaining cells of
the cord form the true ganglion cells. A gradual separation of the
ganglia next takes place, and the cells become confined to the
ganglia, which are finally only connected by a double band of
commissural tissue. The commissural tissue not only gives rise to the
longitudinal cords connecting the successive ganglia, but also to the
transverse commissures which unite the two halves of the individual
ganglia.
The ganglia usually, if not always, appear at first to correspond in
number with the segments, and the smaller number so often present in
the adult is due to the coalescence of originally distinct ganglia.
Organs of special sense. Comparatively little is known on this head.
The compound eyes are developed from the coalescence of two
structures, both however epiblastic, viz. (1) part of the superficial
epiblast of the procephalic lobes; (2) part of the supra-oesophageal
ganglia. The former gives rise to the corneal lenses, the crystalline
cones, and the pigment surrounding them; the latter to the rhabdoms
and the cells which encircle them. Between these two parts a
mesoblastic pigment is interposed.
Of the development of the auditory and olfactory organs almost nothing
is known.
Dorsal organ. In a considerable number of the Malacostraca and
Branchiopoda a peculiar organ is developed from the epiblast in the
anterior dorsal region. This organ has been called the dorsal organ.
It appears to be of a glandular nature, and is usually very large in
the embryo or larva and disappears in the adult; but in some
Branchiopoda it persists through life. In most cases it is unpaired,
but in some instances a paired organ appears to take its place.
Various views as to its nature have been put forward. There is but
little doubt of its being glandular, and it is possible that it is a
provisional renal organ, though so far as I know concretions have not
yet been found in it.
Its development has been most fully studied in the Isopoda.
[FIG. 242. DIAGRAMMATIC SECTION OF CYMOTHOA SHEWING THE DORSAL
ORGAN. (From Bullar.)]
In Cymothoa (Bullar, No. 499) there appears on the dorsal surface, in
the region which afterwards becomes the first thoracic segment, an
unpaired linear thickening of the blastoderm. This soon becomes a
circular patch, the central part of which is invaginated so as to
communicate with the exterior by a narrow opening only (fig. 242). It
becomes at the same time attached to the inner egg membrane. It
retains this condition till the close of larval life.
In Oniscus (Dohrn, No. 500; Bobretzky, No. 498) there appears very
early a dorsal patch of thickened cells. These cells become attached
at their edge to the inner egg membrane and gradually separated from
the embryo, with which they finally only remain in connection by a
hollow column of cells (fig. 241 A, _do_). The original patch now
gradually spreads over the inner egg membrane, and forms a transverse
saddle-shaped band of flattened cells which engirths the embryo on all
but the ventral side.
In the Amphipods the epiblast cells remain attached for a small area
on the dorsal surface to the first larval skin, when this is formed.
This patch of cells, often spoken of as a micropyle apparatus, forms a
dorsal organ equivalent to that in Oniscus. A perforation is formed in
it at a later period. A perhaps homologous structure is found in the
embryos of Euphausia, Cuma, etc.
[FIG. 243. DIAGRAMMATIC SECTION OF AN EMBRYO OF ASELLUS AQUATICUS TO
SHEW THE PAIRED DORSAL ORGAN. (From Bullar; after E. van Beneden.)]
In many Branchiopoda a dorsal organ is found. Its development has been
studied by Grobben in Moina. It persists in the adult in Branchipus,
Limnadia, Estherea, etc.
In the Copepoda a dorsal organ is sometimes found in the embryo;
Grobben at any rate believes that he has detected an organ of this
nature in the embryo of Cyclops serrulatus.
A paired organ which appears to be of the same nature has been found
in Asellus and Mysis.
In Asellus (Rathke (No. 501), Dohrn (No. 500), Van Beneden (No. 497))
this organ originates as two cellular masses at the sides of the body
just behind the region of the procephalic lobes. Each of them becomes
trifoliate and bends towards the ventral surface. In each of their
lobes a cavity arises and finally the three cavities unite, forming a
trilobed cavity open to the yolk. This organ eventually becomes so
large that it breaks through the egg membranes and projects at the
sides of the embryo (fig. 243). Though formed before the appendages it
does not attain its full development till considerably after the
latter have become well established.
In Mysis it appears during the Nauplius stage as a pair of cavities
lined by columnar cells, which atrophy very early.
Various attempts have been made to identify organs in other Arthropod
embryos with the dorsal organ of the Crustacea, but the only organ at
all similar which has so far been described is one found in the embryo
of Linguatula (_vide_ Chapter XIX.), but there is no reason to think
that this organ is really homologous with the dorsal organ of the
Crustacea.
The mesoblast. The mesoblast in the types so far investigated arises
from the same cells as the hypoblast, and appears as a somewhat
irregular layer between the epiblast and the hypoblast. It gives rise
to the same parts as in other forms, but it is remarkable that it does
not, in most Decapods and Isopods (and so far we do not know about
other forms), become divided into somites, at any rate with the same
distinctness that is usual in Annelids and Arthropods. Not only so,
but there is at first no marked division into a somatic and splanchnic
layer with an intervening body cavity. Some of the cells become
differentiated into the muscles of the body wall and limbs; and other
cells, usually in the form of a very thin layer, into the muscles of
the alimentary tract. In the tail of _Palæmon_ Bobretzky noticed that
the cells about to form the muscles of the body were imperfectly
divided into cubical masses corresponding with the segments; which
however, in the absence of a central cavity, differed from typical
mesoblastic somites. In _Mysis_ Metschnikoff states that the mesoblast
becomes broken up into distinct somites. Further investigations on
this subject are required. The body cavity has the form of irregular
blood sinuses amongst the internal organs.
Heart. The origin and development of the heart and vascular system are
but very imperfectly known.
In Phyllopods (Branchipus) Claus (No. 454) has shewn that the heart is
formed by the coalescence of the lateral parts of the mesoblast of the
ventral plates. The chambers are formed successively as the segments
to which they belong are established, and the anterior chambers are in
full activity while the posterior are not yet formed.
In Astacus and Palæmon, Bobretzky finds that at the stage before the
heart definitely appears there may be seen a solid mass of mesoblast
cells in the position which it eventually occupies[209]; and considers
it probable that the heart originates from this mass. At the time when
the heart can first be made out and before it has begun to beat, it
has the form of an oval sack with delicate walls separated from the
mesenteron by a layer of splanchnic mesoblast. Its cavity is filled
with a peculiar plasma which also fills up the various cavities in the
mesoblast. Around it a pericardial sack is soon formed, and the walls
of the heart become greatly thickened. Four bands pass off from the
heart, two dorsalwards which become fixed to the integument, and two
ventralwards. There is also a median band of cells connecting the
heart with the dorsal integument. The main arteries arise as direct
prolongations of the heart. Dohrn's observations on Asellus greatly
strengthen the view that the heart originates from a solid mesoblastic
mass, in that he was able to observe the hollowing out of the mass in
the living embryo (cf. the development of the heart in Spiders). Some
of the central cells (nuclei, Dohrn) become blood corpuscles. The
formation of these is not, according to Dohrn, confined to the heart,
but takes place _in situ_ in all the parts of the body (antennæ,
appendages, etc.). The corpuscles are formed as free nuclei and are
primarily derived from the yolk, which at first freely communicates
with the cavities of the appendages.
[209] Reichenbach describes these cells, and states that there is
a thickening of the epiblast adjoining them. In one place he
states that the heart arises from this thickening of epiblast,
and in another that it arises from the mesoblast. An epiblastic
origin of the heart is extremely improbable.
Alimentary tract. In Astacus the formation of the mesenteron by
invagination, and the absorption of the yolk by the hypoblast cells,
have already been described. On the absorption of the yolk the
mesenteron has the form of a sack, the walls of which are formed of
immensely long cells--the yolk pyramids--at the base of which the
nucleus is placed (fig. 238 B). This sack gives rise both to the
portion of the alimentary canal between the abdomen and the stomach
and to the liver. The epithelial wall of both of these parts is formed
by the outermost portions of the pyramids with the nuclei and
protoplasm becoming separated off from the yolk as a layer of flat
epithelial cells. The yolk then breaks up and forms a mass of
nutritive material filling up the cavity of the mesenteron.
The differentiation both of the liver and alimentary tract proper
first takes place on the ventral side, and commences close to the
point where the proctodæum ends, and extends forward from this point.
A layer of epithelial cells is thus formed on the ventral side of the
mesenteron which very soon becomes raised into a series of
longitudinal folds, one of which in the middle line is very
conspicuous. The median fold eventually, by uniting with a
corresponding fold on the dorsal side, gives rise to the true
mesenteron; while the lateral folds form parallel hepatic cylinders,
which in front are not constricted off from the alimentary tract. The
lateral parts of the dorsal side of the mesenteron similarly give rise
to hepatic cylinders. The yolk pyramids of the anterior part of the
mesenteron, which projects forwards as a pair of diverticula on each
side to the level of the stomach, are not converted into hepatic
cylinders till after the larva is hatched.
The proctodæum very early opens into the mesenteron, but the stomodæum
remains closed till the differentiation of the mid-gut is nearly
completed. The proctodæum gives rise to the abdominal part of the
intestine, and the stomodæum to the oesophagus and stomach. The
commencement of the masticatory apparatus in the latter appears very
early as a dorsal thickening of the epithelium.
The primitive mesenteron in Palæmon differentiates itself into the
permanent mid-gut and liver in a manner generally similar to that in
Astacus, though the process is considerably less complicated. A
distinct layer of cells separates itself from the outer part of the
yolk pyramids, and gives rise to the glandular lining both of the
mid-gut and of the liver. The differentiation of this layer commences
behind, and the mid-gut very soon communicates freely with the
proctodæum. The lateral parts of the primitive mesenteron become
constricted into four wings, two directed forwards and two backwards;
these, after the yolk in them has become absorbed, constitute the
liver. The median part simply becomes the mesenteron. The stomachic
end of the stomodæum lies in contact with the mesenteron close to the
point where it is continued into the hepatic diverticula, and, though
the partition wall between the two becomes early very thin, a free
communication is not established till the yolk has been completely
absorbed.
The alimentary tract in the Isopoda is mainly if not entirely formed
from the proctodæum and stomodæum, both of which arise before any
other part of the alimentary system as epiblastic invaginations, and
gradually grow inwards (fig. 244). In Oniscus the liver is formed as
two discs at the surface of the yolk on each side of the anterior part
of the body. Their walls are composed of cubical cells derived from
the yolk cells, the origin of which was spoken of on p. 516. These two
discs gradually take the form of sacks (fig. 244 B, _li._) freely open
on their inner side to the yolk. As these sacks continue to grow the
stomodæum and proctodæum do not remain passive. The stomodæum, which
gives rise to the oesophagus and stomach of the adult, soon exhibits a
posterior dilatation destined to become the stomach, on the dorsal
wall of which a well-marked prominence--the earliest trace of the
future armature--is soon formed (fig. 244 B, _zp_). The proctodæum
(_pr_) grows with much greater rapidity than the stomodæum, and its
end adjoining the yolk becomes extremely thin or even broken through.
In the earliest stages it was surrounded by the yolk cells, but in its
later growth the yolk cells become gradually reduced in number and
appear to recede before it--so much so that one is led to conclude
that the later growth of the proctodæum takes place at the expense of
the yolk cells.
[FIG. 244. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS
MURARIUS. (After Bobretzky.)
_st._ stomodæum; _pr._ proctodæum; _hy._ hypoblast formed of large
nucleated cells imbedded in yolk; _m._ mesoblast; _vg._ ventral
nerve cord; _sg._ supra-oesophageal ganglion; _li._ liver; _do._
dorsal organ; _zp._ rudiment of masticatory apparatus.]
The liver sacks become filled with a granular material without a trace
of cells; their posterior wall is continuous with the yolk cells, and
their anterior lies close behind the stomach. The proctodæum
continually grows forwards till it approaches close to the stomodæum,
and the two liver sacks, now united into one at their base, become
directly continuous with the proctodæum. By the stage when this
junction is effected the yolk cells have completely disappeared. It
seems then that in Oniscus the yolk cells (hypoblast) are mainly
employed in giving rise to the walls of the liver; but that they
probably also supply the material for the later growth of the apparent
proctodæum. It becomes therefore necessary to conclude that the
latter, which might seem, together with the stomodæum, to form the
whole alimentary tract, does in reality correspond to the proctodæum
and mesenteron together, though the digestive fluids are no doubt
mainly secreted not in the mesenteron but in the hepatic diverticula.
The proctodæum and stomodæum at first meet each other without
communicating, but before long the partition between the two is broken
through.
In Cymothoa (Bullar, No. 499) the proctodæum and stomodæum develop in
the same manner as in Oniscus, but the hypoblast has quite a different
form. The main mass of the yolk, which is much greater than in
Oniscus, is not contained in definite yolk cells, but the hypoblast is
represented by (1) two solid masses of cells, derived apparently from
the inner layer of blastoderm cells, which give rise to the liver; and
(2) by a membrane enclosing the yolk in which nuclei are present.
The two hepatic masses lie on the surface of the yolk, and each of
them becomes divided into three short cæcal tubes freely open to the
yolk. The stomodæum soon reaches its full length, but the proctodæum
grows forwards above the yolk till it meets the stomodæum. By the time
this takes place the liver cæca have grown into three large tubes
filled with fluid, and provided with a muscular wall. They now lie
above the yolk, and no longer communicate directly with the cavity of
the yolk-sack, but open together with the yolk-sack into the point of
junction of the proctodæum and stomodæum. The yolk-sack of Cymothoa no
doubt represents part of the mesenteron, but there is no evidence in
favour of any part of the apparent proctodæum representing it also,
though it is quite possible that it may do so. The relations of the
yolk-sack and hepatic diverticula in Cymothoa appear to hold good for
Asellus and probably for most Isopoda.
The differences between the Decapods and Isopods in the development of
the mesenteron are not inconsiderable, but they are probably to be
explained by the relatively larger amount of food-yolk in the latter
forms. The solid yolk in the Isopods on this view represents the
primitive mesenteron of Decapods after the yolk has been absorbed by
the hypoblast cells. Starting from this standpoint we find that in
both groups the lateral parts of the mesenteron become the liver. In
Decapods the middle part becomes directly converted into the mid-gut,
the differentiation of it commencing behind and proceeding forwards.
In the Isopods, owing to the mesenteron not having a distinct cavity,
the differentiation of it, which proceeds forwards as in Decapods,
appears simply like a prolongation forwards of the proctodæum, the
cells for the prolongation being probably supplied from the yolk. In
Cymothoa the food-yolk is so bulky that a special yolk-sack is
developed for its retention, which is not completely absorbed till
some time after the alimentary canal has the form of a continuous
tube. The walls of this yolk-sack are morphologically a specially
developed part of the mesenteron.
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CHAPTER XIX.
POECILOPODA, PYCNOGONIDA, TARDIGRADA, AND LINGUATULIDA; AND
COMPARATIVE SUMMARY OF ARTHROPODAN DEVELOPMENT.
The groups dealt with in the present Chapter undoubtedly belong to the
Arthropoda. They are not closely related, and in the case of each
group it is still uncertain with which of the main phyla they should
be united. It is possible that they may all be offshoots from the
Arachnidan phylum.
POECILOPODA.
The development of Limulus has been studied by Dohrn (No. 533) and
Packard (No. 534). The ova are laid in the sand near the spring-tide
marks. They are enveloped in a thick chorion formed of several layers;
and (during the later stages of development at any rate) there is a
membrane within the chorion which exhibits clear indications of cell
outlines[210].
[210] The nature of the inner membrane is obscure. It is believed
by Packard to be moulted after the formation of the limbs, and
_to be equivalent to the amnion of Insects_, while by Dohrn it is
regarded as a product of the follicle cells.
There is a centrolecithal segmentation, which ends in the formation of
a blastoderm enclosing a central yolk mass. A ventral plate is then
formed, which is thicker in the region where the abdomen is eventually
developed. Six segments soon become faintly indicated in the
cephalothoracic region, the ends of which grow out into prominent
appendages (fig. 245 A); of these there are six pairs, which increase
in size from before backwards. A stomodæum (m) is by this time
established _and is placed well in front of the foremost pair of
appendages_[211].
[211] Dohrn finds at first only five appendages, but thinks that
the sixth (the anterior one) may have been present but invisible.
In the course of the next few days the two first appendages of the
abdominal region become formed (_vide_ fig. 245 C shewing those
abdominal appendages at a later stage), and have a very different
shape and direction to those of the cephalothorax. The appendages of
the latter become flexed in the middle in such a way that their ends
become directed towards the median line (fig. 245 B). The body of the
embryo (fig. 245 B) is now distinctly divided into two regions--the
cephalothoracic in front, and the abdominal behind, both divided into
segments.
[FIG. 245. THREE STAGES IN THE DEVELOPMENT OF LIMULUS POLYPHEMUS.
(Somewhat modified from Packard.)
A. Embryo in which the thoracic limbs and mouth have become
developed on the ventral plate. The outer line represents what
Packard believes to be the amnion.
B. Later embryo from the ventral surface.
C. Later embryo, just before the splitting of the chorion from the
side. The full number of segments of the abdomen, and three
abdominal appendages, have become established; _m._ mouth; I-IX.
appendages.]
Round the edge of the ventral plate there is a distinct ridge--the
rudiment of the cephalothoracic shield.
With the further growth of the embryo the chorion becomes split and
cast off, the embryo being left enclosed within the inner membrane.
The embryo has a decided ventral flexure, and the abdominal region
grows greatly and forms a kind of cap at the hinder end, while its
vaulted dorsal side becomes divided into segments (fig. 245 C). Of
these there are according to Dohrn seven, but according to Packard
nine, of which the last forms the rudiment of the caudal spine.
In the thoracic region the nervous system is by this stage formed as a
ganglionated cord (Dohrn), with no resemblance to the peculiar
oesophageal ring of the adult. The mouth is stated by Dohrn to lie
between the second pair of limbs, so that, if the descriptions we have
are correct, it must have by this stage changed its position with
reference to the appendages. Between the thorax and abdomen two
papillæ have arisen which form the so-called lower lip of the adult,
but from their position and late development they can hardly be
regarded as segmental appendages. In the course of further changes all
the parts become more distinct, while the membrane in which the larva
is placed becomes enormously distended (fig. 246 A). The rudiments of
the compound eyes are formed on the third (Packard) or fourth (Dohrn)
segment of the cephalothorax, and the simple eyes near the median line
in front. The rudiments of the inner process of the chelæ of the
cephalothoracic appendages arise as buds. The abdominal appendages
become more plate-like, and the rudiments of a third pair appear
behind the two already present. The heart appears on the dorsal
surface.
An ecdysis now takes place, and in the stage following the limbs have
approached far more closely to their adult state (fig. 246 A). The
cephalothoracic appendages become fully jointed; the two anterior
abdominal appendages (VII.) have approached, and begin to resemble the
operculum of the adult, and on the second pair is formed a small inner
ramus. The segmentation of the now vaulted cephalothorax becomes less
obvious, though still indicated by the arrangement of the yolk masses
which form the future hepatic diverticula.
[FIG. 246. TWO STAGES IN THE DEVELOPMENT OF LIMULUS POLYPHEMUS.
(After Dohrn.)
A. An advanced embryo enveloped in the distended inner membrane
shortly before hatching; from the ventral side. B. A later embryo
at the Trilobite stage, from the dorsal side.
I., VII., VIII. First, seventh, and eighth appendages. _cs._ caudal
spine; _se._ simple eye; _ce._ compound eye.]
Shortly after this stage the embryo is hatched, and at about the time
of hatching acquires a form (fig. 246 B) in which it bears, as pointed
out by Dohrn and Packard, the most striking resemblance to a
Trilobite.
Viewed from the dorsal surface (fig. 246 B) it is divided into two
distinct regions, the cephalothoracic in front and the abdominal
behind. The cephalothoracic has become much flatter and wider, has
lost all trace of its previous segmentation, and has become distinctly
trilobed. The central lobe forms a well-marked keel, and at the line
of insertion of the rim-like edge of the lateral lobes are placed the
two pairs of eyes (_se_ and _ce_). The abdominal region is also
distinctly trilobed and divided into nine segments; the last, which is
merely formed of a median process, being the rudiment of the caudal
spine. The edges of the second to the seventh are armed with a spine.
The changes in the appendages are not very considerable. The anterior
pair nearly meet in the middle line in front or the mouth; and the
latter structure is completely covered by an upper lip. Each abdominal
appendage of the second pair is provided with four gill lamellæ,
attached close to its base.
Three weeks after hatching an ecdysis takes place, and the larva
passes from a trilobite into a limuloid form. The segmentation of the
abdomen has become much less obvious, and this part of the embryo
closely resembles its permanent form. The caudal spine is longer, but
is still relatively short. A fourth pair of abdominal appendages is
established, and the first pair have partially coalesced, while the
second and third pairs have become jointed, their outer ramus
containing four and their inner three joints. Additional gill lamellæ
attached to the two basal joints of the second and third abdominal
appendages have appeared.
The further changes are not of great importance. They are effected in
a series of successive moults. The young larvæ swim actively at the
surface.
Our, in many respects, imperfect knowledge of the development of
Limulus is not sufficient to shew whether it is more closely related
to the Crustacea or to the Arachnida, or is an independent phylum.
The somewhat Crustacean character of biramous abdominal feet, etc. is
not to be denied, but at the same time the characters of the embryo
appear to me to be decidedly more arachnidan than crustacean. The
embryo, when the appendages are first formed, has a decidedly
arachnidan facies. It will be remembered that when the limbs are first
formed they are all _post-oral_. They resemble in this respect the
limbs of the Arachnida, and it seems to be probable that the anterior
pair is equivalent to the cheliceræ of Arachnida, which, as shewn in a
previous section, are really post-oral appendages in no way homologous
with antennæ[212].
[212] Dohrn believes that he has succeeded in shewing that the
first pair of appendages of Limulus is innervated in the embryo
from the supra-oesophageal ganglia. His observations do not
appear to me conclusive, and arguing from what we know of the
development of the Arachnida, the innervation of these appendages
in the adult can be of no morphological importance.
The six thoracic appendages may thus be compared with the six
Arachnidan appendages; which they resemble in their relation to the
mouth, their basal cutting blades, etc.
The existence of abdominal appendages behind the six cephalothoracic
does not militate against the Arachnidan affinities of Limulus,
because in the Arachnida rudimentary abdominal appendages are always
present in the embryo. The character of the abdominal appendages is
probably secondarily adapted to an aquatic respiration, since it is
likely (for the reasons already mentioned in connection with the
Tracheata) that if Limulus has any affinities with the stock of the
Tracheata it is descended from air-breathing forms, and has acquired
its aquatic mode of respiration. The anastomosis of the two halves of
the generative glands is an Arachnidan character, and the position of
the generative openings in Limulus is more like that in the Scorpion
than in Crustacea.
A fuller study of the development would be very likely to throw
further light on the affinities of Limulus, and if Packard's view
about the nature of the inner egg membrane were to be confirmed,
strong evidence would thereby be produced in favour of the Arachnidan
affinities.
(533) A. Dohrn. "Untersuch. üb. Bau u. Entwick. d. Arthropoden
(Limulus polyphemus)." _Jenaische Zeitschrift_, Vol. VI., 1871.
(534) A. S. Packard. "The development of Limulus polyphemus." _Mem.
Boston Soc. Nat. History_, Vol. II., 1872.
PYCNOGONIDA.
The embryos, during the first phases of their development, are always
carried by the _male_ in sacks which are attached to a pair of
appendages (the third) specially formed for this purpose. The
segmentation of the ovum is complete, and there is in most forms
developed within the eggshell a larva with three pairs of two-jointed
appendages, and a rostrum placed between the front pair.
It will be convenient to take Achelia lævis, studied by Dohrn (No.
536), as type.
The larva of Achelia when hatched is provided with the typical three
pairs of appendages. The foremost of them is chelate, and the two
following pairs are each provided with a claw. Of the three pairs of
larval appendages Dohrn states that he has satisfied himself that the
anterior is innervated by the supra-oesophageal ganglion, and the two
posterior by separate nerves coming from two imperfectly united
ventral ganglia. The larva is provided with a median eye formed of two
coalesced pigment spots, and with a simple stomach.
The gradual conversion of the larva into the adult takes place by the
elongation of the posterior end of the body into a papilla, and the
formation there, at a later period, of the anus; while at the two
sides of the anal papilla rudiments of a fresh pair of appendages--the
first pair of ambulatory limbs of the adult--make their appearance.
The three remaining pairs of limbs become formed successively as
lateral outgrowths, and their development is accomplished in a number
of successive ecdyses. As they are formed cæca from the stomach become
prolonged into them. For each of them there appears a special
ganglion. While the above changes are taking place the three pairs of
larval appendages undergo considerable reduction. The anterior pair
singly becomes smaller, the second loses its claw, and the third
becomes reduced to a mere stump. In the adult the second pair of
appendages becomes enlarged again and forms the so-called palpi, while
the third pair develops in the male into the egg-carrying appendages,
but is aborted in the female. The first pair form appendages lying
parallel to the rostrum, which are sometimes called pedipalpi and
sometimes antennæ.
The anal papilla is a rudimentary abdomen, and, as Dohrn has shewn,
contains rudiments of two pairs of ganglia.
The larvæ of Phoxichilidium are parasitic in various Hydrozoa
(Hydractinia, etc.). After hatching they crawl into the Hydractinia
stock. They are at first provided with the three normal pairs of
larval appendages. The two hinder of these are soon thrown off, and
the posterior part of the trunk, with the four ambulatory appendages
belonging to it, becomes gradually developed in a series of moults.
The legs, with the exception of the hindermost pair, are fully formed
at the first ecdysis after the larva has become free. In the genus
Pallene the metamorphosis is abbreviated, and the young are hatched
with the full complement of appendages.
The position of the Pycnogonida is not as yet satisfactorily settled.
The six-legged larva has none of the characteristic features of the
Nauplius, except the possession of the same number of appendages.
The number of appendages (7) of the Pycnogonida does not coincide with
that of the Arachnida. On the other hand, the presence of chelate
appendages innervated in the adult by the supra-oesophageal ganglia
rather points to a common phylum for the Pycnogonida and Arachnida;
though as shewn above (p. 455) all the appendages in the embryo of
true Arachnida are innervated by post-oral ganglia. The innervation of
these appendages in the larvæ of Pycnogonida requires further
investigation. Against such a relationship the extra pair of
appendages in the Pycnogonida is no argument, since the embryos of
most Arachnida are provided with four such extra pairs. The two groups
must no doubt have diverged very early.
BIBLIOGRAPHY.
(535) G. Cavanna. "Studie e ricerche sui Picnogonidi." _Pubblicazioni
del R. Instituto di Studi superiori in Firenze_, 1877.
(536) An. Dohrn. "Ueber Entwicklung u. Bau d. Pycnogoniden."
_Jenaische Zeitschrift_, Vol. V. 1870, and "Neue Untersuchungen üb.
Pycnogoniden." _Mittheil. a. d. zoologischen Station zu Neapel_, Bd.
I. 1878.
(537) G. Hodge. "Observations on a species of Pycnogon, etc." _Annal.
and Mag. of Nat. Hist._ Vol. IX. 1862.
(538) C. Semper. "Ueber Pycnogoniden u. ihre in Hydroiden
schmarotzenden Larvenformen." _Arbeiten a. d. zool.-zoot. Instit.
Würzburg_, Vol. I. 1874.
PENTASTOMIDA.
The development and metamorphosis of Pentastomum tænoides have been
thoroughly worked out by Leuckart (No. 540) and will serve as type for
the group.
In the sexual state it inhabits the nasal cavities of the dog. The
early embryonic development takes place as the ovum gradually passes
down the uterus. The segmentation appears to be complete; and gives
rise to an oval mass in which the separate cells can hardly be
distinguished. This gradually differentiates itself into a
characteristic embryo, divided into a tail and trunk. The tail is
applied to the ventral surface of the trunk, and on the latter two
pairs of stump-like unsegmented appendages arise, each provided with a
pair of claws. At the anterior extremity of the body is formed the
mouth, with a ventral spine and lateral hook, which are perhaps
degenerated jaws. The spine functions as a boring apparatus, and an
apparatus with a similar function is formed at the end of the tail. A
larval cuticle now appears, which soon becomes detached from the
embryo, except on the dorsal surface, where it remains firmly united
to a peculiar papilla. This papilla becomes eventually divided into
two parts, one of which remains attached to the cuticle, while the
part connected with the embryo forms a raised cross placed in a
cup-shaped groove. The whole structure has been compared, on
insufficient grounds, to the dorsal organ of the Crustacea.
The eggs, containing the embryo in the condition above described, are
eventually carried out with the nasal slime, and, if transported
thence into the alimentary cavity of a rabbit or hare, the embryos
become hatched by the action of the gastric juice. From the alimentary
tract of their new host they make their way into the lungs or liver.
They here become enveloped in a cyst, in the interior of which they
undergo a very remarkable metamorphosis. They are, however, so minute
and delicate that Leuckart was unable to elucidate their structure
till eight weeks after they had been swallowed. At this period they
are irregularly-shaped organisms, with a most distant resemblance to
the earlier embryos. They are without their previous appendages, but
the alimentary tract is now distinctly differentiated. The remains of
two cuticles in the cyst seem to shew that the above changes are
effected in two ecdyses.
In the course of a series of ecdyses the various organs of the larval
form known as Pentastomum denticulatum continue to become
differentiated. After the first (=third) ecdysis the oesophageal
nerve-ring and sexually undifferentiated generative organs are
developed. At the fourth (=sixth) ecdysis the two pairs of hooks of
the adult are formed in pockets which appeared at a somewhat earlier
stage; and the body acquires an annulated character. At a somewhat
earlier period rudiments of the external generative organs indicate
the sex of the larva.
After a number of further ecdyses, which are completed in about six
months after the introduction of the embryos into the intermediate
host, the larva attains its full development, and acquires a form in
which it has long been known as Pentastomum denticulatum. It now
leaves its cyst and begins to move about. It is in a state fit to be
introduced into its final host; but if it be not so introduced it may
become encysted afresh.
If the part of a rabbit or hare infected by a Pentastomum denticulatum
be eaten by a dog or wolf, the parasite passes into the nasal cavity
of the latter, and after further changes of cuticle becomes a fully
developed sexual Pentastomum tænioides, which does not differ to any
very marked extent from P. denticulatum.
In their general characters the larval migrations of Pentastomum are
similar to those of the Cestodes.
The internal anatomy of the adult Pentastomum, as well as the
characters of the larva with two pairs of clawed appendages, are
perhaps sufficient to warrant us in placing it with the Arthropoda,
though it would be difficult to shew that it ought not to be placed
with such a form as Myzostomum (_vide_ p. 369). There do not appear to
be any sufficient grounds to justify its being placed with the Mites
amongst the Arachnida. If indeed the rings of the body of the
Pentastomida are to be taken as implying a true segmentation, it is
clear that the Pentastomida cannot be associated with the Mites.
BIBLIOGRAPHY.
(539) P. J. van Beneden. "Recherches s. l'organisation et le
développement d. Linguatules." _Ann. d. Scien. Nat._, 3 Ser., Vol. XI.
(540) R. Leuckart. "Bau u. Entwicklungsgeschichte d. Pentastomen."
Leipzig and Heidelberg. 1860.
TARDIGRADA.
Very little is known with reference to the development of the
Tardigrada. A complete and regular segmentation (von Siebold,
Kaufmann, No. 541) is followed by the appearance of a groove on the
ventral side indicating a ventral flexure. At about the time of the
appearance of the groove the cells become divided into an epiblastic
investing layer and a central hypoblastic mass.
The armature of the pharynx is formed very early at the anterior
extremity, and the limbs arise in succession from before backwards.
The above imperfect details throw no light on the systematic position
of this group.
_Tardigrada._
(541) J. Kaufmann. "Ueber die Entwicklung u. systematische Stellung d.
Tardigraden." _Zeit. f. wiss. Zool._, Bd. III. 1851.
_Summary of Arthropodan Development._
The numerous characters common to the whole of the Arthropoda led
naturalists to unite them in a common phylum, but the later researches
on the genealogy of the Tracheata and Crustacea tend to throw doubts
on this conclusion, while there is not as yet sufficient evidence to
assign with certainty a definite position in either of these classes
to the smaller groups described in the present chapter. There seems to
be but little doubt that the Tracheata are descended from a
terrestrial Annelidan type related to Peripatus. The affinities of
Peripatus to the Tracheata are, as pointed out in a previous chapter
(p. 386), very clear, while at the same time it is not possible to
regard Peripatus simply as a degraded Tracheate, owing to the fact
that it is provided with such distinctly Annelidan organs as
nephridia, and that its geographical distribution shews it to be a
very ancient form.
The Crustacea on the other hand are clearly descended from a
Phyllopod-like ancestor, which can be in no way related to Peripatus.
The somewhat unexpected conclusion that the Arthropoda have a double
phylum is on the whole borne out by the anatomy of the two groups.
Without attempting to prove this in detail, it may be pointed out that
the Crustacean appendages are typically biramous, while those of the
Tracheata are never at any stage of development biramous[213]; and the
similarity between the appendages of some of the higher Crustacea and
those of many Tracheata is an adaptive one, and could in no case be
used as an argument for the affinity of the two groups.
[213] The biflagellate antennæ of Pauropus amongst the Myriapods
can hardly be considered as constituting an exception to this
rule.
The similarity of many organs is to be explained by both groups being
descendants of Annelidan ancestors. The similarity of the compound eye
in the two groups cannot however be explained in this way, and is one
of the greatest difficulties of the above view. It is moreover
remarkable that the eye of Peripatus[214] is formed on a different
type to either the single or compound eyes of most Arthropoda.
[214] I hope to shew this in a paper I am preparing on the
anatomy of Peripatus.
The conclusion that the Crustacea and Tracheata belong to two distinct
phyla is confirmed by a consideration of their development. They have
no doubt in common a centrolecithal segmentation, but, as already
insisted on, the segmentation is no safe guide to the affinities.
In the Tracheata the archenteron is never, so far as we know, formed
by an invagination[215], while in Crustacea the evidence is in favour
of such an invagination being the usual, and, without doubt, the
primitive, mode of origin.
[215] Stecker's description of an invagination in the Chilognatha
cannot be accepted without further confirmation; _vide_ p. 388.
The mesoblast in the Tracheata is formed in connection with a median
thickening of the ventral plate. The unpaired plate of mesoblast so
formed becomes divided into two bands, one on each side of the middle
line.
In both Spiders and Myriopods, and probably Insects, the two plates of
mesoblast are subsequently divided into somites, the lumen of which is
continued into the limbs.
In Crustacea the mesoblast usually originates from the walls of the
invagination, which gives rise to the mesenteron.
It does not become divided into two distinct bands, but forms a layer
of scattered cells between the epiblast and hypoblast, and does not
usually break up into somites; and though somites are stated in some
cases to be found they do not resemble those in the Tracheata.
The proctodæum is usually formed in Crustacea before and rarely
later[216] than the stomodæum. The reverse is true for the Tracheata.
In Crustacea the proctodæum and stomodæum, especially the former, are
very long, and usually give rise to the greater part of the alimentary
tract, while the mesenteron is usually short.
[216] This is stated to be the case in Moina (Grobben).
In the Tracheata the mesenteron is always considerable, and the
proctodæum is always short. The derivation of the Malpighian bodies
from the proctodæum is common to most Tracheata. Such diverticula of
the proctodæum are not found in Crustacea.
CHAPTER XX.
ECHINODERMATA[217].
[217] The following classification of the Echinodermata is
employed in this chapter:
I. Holothuroidea.
II. Asteroidea.
III. Ophiuroidea.
IV. Echinoidea.
V. Crinoidea.
The development of the Echinodermata naturally falls into two
sections:--
(1) The development of the germinal layers and of the systems of
organs; (2) the development of the larval appendages and the
metamorphosis.
_The Development of the Germinal Layers and of the Systems of
Organs._
The development of the systems of organs presents no very important
variations within the limits of the group.
Holothuroidea. The Holothurians have been most fully studied (Selenka,
No. 563), and may be conveniently taken as type.
The segmentation is nearly regular, though towards its close, and in
some instances still earlier, a difference becomes apparent between
the upper and the lower poles.
At the close of segmentation (fig. 247 A) the egg has a nearly
spherical form, and is constituted of a single layer of columnar cells
enclosing a small segmentation cavity. The lower pole is slightly
thickened, and the egg rotates by means of fine cilia.
An invagination now makes its appearance at the lower pole (fig. 247
B), and simultaneously there become budded off _from the cells
undergoing the invagination_ amoeboid cells, which eventually form the
muscular system and the connective tissue. These cells very probably
have a bilaterally symmetrical origin. This stage represents the
gastrula stage which is common to all Echinoderms. The invaginated
sack is the archenteron. As it grows larger one side of the embryo
becomes flattened, and the other more convex. On the flattened side a
fresh invagination arises, the opening of which forms the permanent
mouth, the opening of the first invagination remaining as the
permanent anus (fig. 248 A).
[FIG. 247. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA
VIEWED IN OPTICAL SECTION. (After Selenka.)
A. Blastosphere stage at the close of segmentation. B. Gastrula
stage.
_mr._ micropyle; _fl._ chorion; _s.c._ segmentation cavity; _bl_.
blastoderm; _ep._ epiblast; _hy._ hypoblast; _ms._ amoeboid cells
derived from hypoblast; _a.e._ archenteron.]
These changes give us the means of attaching definite names to the
various parts of the embryo. It deserves to be noted in the first
place that the embryo has assumed a distinctly bilateral form. There
is present a more or less concave surface extending from the mouth to
near the anus, which will be spoken of as the ventral surface. The
anus is situated at the posterior extremity. The convex surface
opposite the ventral surface forms the dorsal surface, which
terminates anteriorly in a rounded præ-oral prominence.
It will be noticed in fig. 248 A that in addition to the primitive
anal invagination there is present a vesicle (_v.p._). This vesicle is
directly formed by a constriction of the primitive archenteron (fig.
249 _Vpv._), and is called by Selenka the vaso-peritoneal vesicle. It
gives origin to the epithelioid lining of the body cavity and
water-vascular system of the adult[218]. In the parts now developed we
have the rudiments of all the adult organs.
[218] The origin of the vaso-peritoneal vesicle is not quite the
same in all the species. In Holothuria tubulosa it is separated
from the cæcal end of the archenteron; the remainder of which
then grows towards the oral invagination. In Cucumaria the
archenteron forks (fig. 249); and one fork forms the
vaso-peritoneal vesicle, and the other the major part of the
mesenteron.
The mouth and anal involutions (after the separation of the
vaso-peritoneal vesicle) meet and unite, a constriction indicating
their point of junction (fig. 248 B). Eventually the former gives rise
to the mouth and oesophagus, and the latter to the remainder of the
alimentary canal[219].
[219] There appears to be some uncertainty as to how much of the
larval oesophagus is derived from the stomodæal invagination.
[FIG. 248. THREE STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA
VIEWED FROM THE SIDE IN OPTICAL SECTION. (After Selenka.)
_m._ mouth; _oe._ oesophagus; _st._ stomach; _i._ intestine; _a_
anus; _l.c._ longitudinal ciliated band; _v.p._ vaso-peritoneal
vesicle; _p.v._ peritoneal vesicle; _p.r._ right peritoneal vesicle;
_pl._ left peritoneal vesicle; _w.v._ water-vascular vesicle; _p._
dorsal pore of water-vascular system; _ms._ muscle cells.]
The vaso-peritoneal vesicle undergoes a series of remarkable changes.
After its separation from the archenteron it takes up a position on
the left side of this, elongates in an antero-posterior direction, and
from about its middle sends a narrow diverticulum towards the _dorsal_
surface of the body, where an opening to the exterior becomes formed
(fig. 248 B, _p._). The diverticulum becomes the madreporic canal, and
the opening the dorsal pore.
The vaso-peritoneal vesicle next divides into two, an anterior vesicle
(fig. 248 B, _w.v._), from which is derived the epithelium of the
water-vascular system, and a posterior (fig. 248 B, _p.v._), which
gives rise to the epithelioid lining of the body cavity. The anterior
vesicle (fig. 248 C, _w.v._) becomes five-lobed, takes a
horseshoe-shaped form, and grows round the oesophagus (fig. 256,
_w.v.r_). The five lobes form the rudiments of the water-vascular
prolongations into the tentacles. The remaining parts of the
water-vascular system are also developed as outgrowths of the original
vesicle. Five of these, alternating with the original diverticula,
form the five ambulacral canals, from which diverticula are produced
into the ambulacral feet; a sixth gives rise to the Polian vesicle.
The remaining parts of the original vesicle form the water-vascular
ring.
We must suppose that eventually the madreporic canal loses its
connection with the exterior so as to hang loosely in the interior,
though the steps of this process do not appear to have been made out.
[FIG. 249. LONGITUDINAL SECTION THROUGH AN EMBRYO OF CUCUMARIA
DOLIOLUM AT THE END OF THE FOURTH DAY.
_Vpv._ vaso-peritoneal vesicle; _ME._ mesenteron; _Blp._, _Ptd._
blastopore, proctodæum.]
The original hinder peritoneal vesicle grows rapidly, and divides into
two (fig. 248 C, _pl._ and _pr._), which encircle the two sides of the
alimentary canal, and meet above and below it. The outer wall of each
of them attaches itself to the skin, and the inner one to the
alimentary canal and water-vascular system; in both cases the walls
remain separated from the adjacent parts by a layer of the amoeboid
cells already spoken of. The cavity of the peritoneal vesicles becomes
the permanent body cavity. Where the walls of the two vesicles meet on
the dorsal side, a mesentery, suspending the alimentary canal and
dividing the body cavity longitudinally, is often formed. In other
parts the partition walls between the two sacks appear to be absorbed.
The amoeboid cells, which were derived from the invaginated cells,
arrange themselves as a layer round all the organs (fig. 249). Some of
them remain amoeboid, attach themselves to the skin, and form part of
the cutis; and in these cells the calcareous spicula of the larva and
adult are formed. Others form the musculature of the larval alimentary
tract, while the remainder give rise to the musculature and connective
tissue of the adult.
The development of the vascular system is not known, but the discovery
of Kowalevsky, confirmed by Selenka, that from the walls of the
water-vascular system corpuscles are developed, identical with those
in the blood-vessels, indicates that it probably develops in
connection with the water-vascular system. The observations of
Hoffmann and Perrier on the communication of the two systems in the
Echinoidea point to the same conclusion. Though nothing very definite
is known with reference to the development of the nervous system,
Metschnikoff suggests that it develops in connection with the
thickened bands of epiblast which are formed by a metamorphosis of the
ciliated bands of the embryo, and accompany the five radial tubes
(_vide_ p. 555). In any case its condition in the adult leaves no
doubt of its being a derivative of the epiblast.
From the above description the following general conclusions may be
drawn:--
(1) The blastosphere stage is followed by a gastrula stage.
(2) The gastrula opening forms the permanent anus, and the mouth is
formed by a fresh invagination.
(3) The mesoblast arises entirely from the invaginated cells, but in
two ways:--
(_a_) As scattered amoeboid cells, which give origin to the muscles
and connective tissue (including the cutis) of the body wall and
alimentary tract.
(_b_) As a portion separated off from the archenteron, which gives
rise both to the epithelioid lining of the body cavity, and of the
water-vascular system.
(4) The oesophagus is derived from an invagination of the epiblast,
and the remainder of the alimentary canal from the archenteron.
(5) The embryonic systems of organs pass directly into those of the
adult.
The development of Synapta diverges, as might be expected, to a very
small extent from that of Holothuria.
Asteroidea. In Asterias the early stages of development conform to our
type. There arise, however, two bilaterally symmetrical
vaso-peritoneal diverticula from the archenteron. These diverticula
give rise both to the lining of the body cavity and water-vascular
system. With reference to the exact changes they undergo there is,
however, some difference of opinion. Agassiz (543) maintains that both
vesicles are concerned in the formation of the water-vascular system,
while Metschnikoff (560) holds that the water-vascular system is
entirely derived from the anterior part of the larger left vesicle,
while the right and remainder of the left vesicle form the body
cavity. Metschnikoff's statements appear to be the most probable. The
anterior part of the left vesicle, after separating from the
posterior, grows into a five-lobed rosette (fig. 260, _i_), and a
madreporic canal (_h_) with a dorsal pore opening to the exterior. The
rosette appears not to grow round the oesophagus, as in the cases
hitherto described. But the latter is stated to disappear, and a new
oesophagus to be formed, which pierces the rosette, and places the old
mouth in communication with the stomach. Except where the anus is
absent in the adult, the larval anus probably persists.
Ophiuroidea. The early development of the Ophiuroidea is not so fully
known as that of other types. Most species have a free-swimming larva,
but some (Amphiura) are viviparous.
The early stages of the free-swimming larvæ have not been described,
but I have myself observed in the case of Ophiothrix fragilis that the
segmentation is uniform, and is followed by the normal invagination.
The opening of this no doubt remains as the larval anus, and there are
probably two outgrowths from this to form the vaso-peritoneal
vesicles. Each of these divides into two parts, an anterior lying
close to the oesophagus, and a posterior close to the stomach. The
anterior on the right side aborts; that on the left side becomes the
water-vascular vesicle, early opens to the exterior, and eventually
grows round the oesophagus, which, as in Holothurians, becomes the
oesophagus of the adult. The posterior vesicles give rise to the
lining of the body cavity, but are stated by Metschnikoff to be at
first solid, and only subsequently to acquire a cavity--the permanent
body cavity. The anus naturally disappears, since it is absent in the
adult. In the viviparous type the first stages are imperfectly known,
but it appears that the blastopore vanishes before the appearance of
the mouth. The development of the vaso-peritoneal bodies takes place
as in the free-swimming larvæ.
Echinoidea. In the Echinoidea (Agassiz, No. 542, Selenka, No. 564)
there is a regular segmentation and the normal invagination (fig. 250
A). The amoeboid mesoblast cells arise as two laterally placed masses,
and give rise to the usual parts. The archenteron grows forward and
bends towards the ventral side (fig. 250 B). It becomes (fig. 250 C)
divided into three chambers, of which the two hindermost (_d_ and _c_)
form the stomach and intestine; while the anterior forms the
oesophagus, and gives rise to the vaso-peritoneal vesicles. These
latter appear as a pair of outgrowths (fig. 251), but become
constricted off as _a single two-horned vesicle_, which subsequently
divides into two. The left of these is eventually divided, as in
Asteroids, into a peritoneal and water-vascular sack, while the right
forms the right peritoneal sack. An oral invagination on the flattened
ventral side meets the mesenteron after its separation from the
vaso-peritoneal vesicle. The larval anus persists, as also does the
larval mouth, but owing to the manner in which the water-vascular
rosette is established the larval oesophagus appears to be absorbed,
and to be replaced by a fresh oesophagus.
[FIG. 250. THREE SIDE VIEWS OF EARLY STAGES IN THE DEVELOPMENT OF
STRONGYLOCENTRUS. (From Agassiz.)
_a._ anus (blastopore); _d._ stomach; _o._ oesophagus; _c._ rectum;
_w._ vaso-peritoneal vesicle; _v._ ciliated ridge; _r._ calcareous
rod.]
Crinoidea. Antedon, the only Crinoid so far studied (Götte, No. 549),
presents some not inconsiderable variations from the usual Echinoderm
type. The blastopore is placed on the somewhat flattened side of the
oval blastosphere, and not, as is usual, at the hinder end.
[FIG. 251. DORSO-VENTRAL VIEW OF AN EARLY LARVA OF STRONGYLOCENTRUS.
(From Agassiz.)
_a._ anus; _d._ stomach; _o._ oesophagus; _w._ vaso-peritoneal
vesicle; _r._ calcareous rod.]
The blastopore completely closes, and is not converted into the
permanent anus. The archenteron gives rise to the epithelioid lining
of both body cavity and water-vascular system. These parts do not,
however, appear as a single or paired outgrowth from the archenteron,
but as three distinct outgrowths which are not formed contemporaneously.
Two of them are first formed, and become the future body cavity; but
their lumens remain distinct. Originally appearing as lateral
outgrowths, the right one assumes a dorsal position and sends a
prolongation into the stalk (fig. 252, _rp´_), and the left one
assumes first a ventral, and then an oral position (fig. 252, _lp_).
[FIG. 252. LONGITUDINAL SECTION THROUGH AN ANTEDON LARVA. (From
Carpenter; after Götte.)