THE MORPHOLOGY OF THE COMMON CRAYFISH: THE STRUCTURE AND THE
DEVELOPMENT OF THE INDIVIDUAL.
In the two preceding chapters the crayfish has been studied from
the point of view of the physiologist, who, regarding an animal as
a mechanism, endeavours to discover how it does that which it does.
And, practically, this way of looking at the matter is the same as
that of the teleologist. For, if all that we know concerning the
purpose of a mechanism is derived from observation of the manner in
which it acts, it is all one, whether we say that the properties and
the connexions of its parts account for its actions, or that its
structure is adapted to the performance of those actions.
Hence it necessarily follows that physiological phenomena can be
expressed in the language of teleology. On the assumption that the
preservation of the individual, and the continuance of the species,
are the final causes of the organization of an animal, the existence
of that organization is, in a certain sense, explained, when it is
shown that it is fitted for the attainment of those ends; although,
perhaps, the importance of {138} demonstrating the proposition that
a thing is fitted to do that which it does, is not very great.
But whatever may be the value of teleological explanations, there
is a large series of facts, which have as yet been passed over, or
touched only incidentally, of which they take no account. These
constitute the subject matter of _Morphology_, which is related to
physiology much as, in the not-living world, crystallography is
related to the study of the chemical and physical properties of
minerals.
Carbonate of lime, for example, is a definite compound of calcium,
carbon, and oxygen, and it has a great variety of physical and
chemical properties. But it may be studied under another aspect, as
a substance capable of assuming crystalline forms, which, though
extraordinarily various, may all be reduced to certain geometrical
types. It is the business of the crystallographer to work out the
relations of these forms; and, in so doing, he takes no note of the
other properties of carbonate of lime.
In like manner, the morphologist directs his attention to the
relations of form between different parts of the same animal, and
between different animals; and these relations would be unchanged
if animals were mere dead matter, devoid of all physiological
properties—a kind of mineral capable of a peculiar mode of growth.
A familiar exemplification of the difference between teleology and
morphology may be found in such works of human art as houses. {139}
A house is certainly, to a great extent, an illustration of
adaptation to purpose, and its structure is, to that extent,
explicable by teleological reasonings. The roof and the walls are
intended to keep out the weather; the foundation is meant to afford
support and to exclude damp; one room is contrived for the purpose
of a kitchen; another for that of a coal-cellar; a third for that
of a dining-room; others are constructed to serve as sleeping
rooms, and so on; doors, chimneys, windows, drains, are all more or
less elaborate contrivances directed towards one end, the comfort
and health of the dwellers in the house. What is sometimes called
sanitary architecture, now-a-days, is based upon considerations
of house teleology. But though all houses are, to begin with and
essentially, means adapted to the ends of shelter and comfort, they
may be, and too often are, dealt with from a point of view, in which
adaptation to purpose is largely disregarded, and the chief attention
of the architect is given to the form of the house. A house may
be built in the Gothic, the Italian, or the Queen Anne style; and
a house in any one of these styles of architecture may be just as
convenient or inconvenient, just as well or as ill adapted to the
wants of the resident therein, as any of the others. Yet the three
are exceedingly different.
To apply all this to the crayfish. It is, in a sense a house with
a great variety of rooms and offices, in which the work of the
indwelling life in feeding, breathing, moving, and reproducing
itself, is done. But the {140} same may be said of the crayfish’s
neighbours, the perch and the water-snail; and they do all these
things neither better nor worse, in relation to the conditions
of their existence, than the crayfish does. Yet the most cursory
inspection is sufficient to show that the “styles of architecture”
of the three are even more widely different than are those of the
Gothic, Italian, and Queen Anne houses.
That which Architecture, as an art conversant with pure form, is to
buildings, Morphology, as a science conversant with pure form, is
to animals and plants. And we may now proceed to occupy ourselves
exclusively with the morphological aspect of the crayfish.
* * * * *
As I have already mentioned, when dealing with the physiology of the
crayfish, the entire body of the animal, when reduced to its simplest
morphological expression, may be represented as a cylinder, closed
at each end, except so far as it is perforated by the alimentary
apertures (fig. 6); or we may say that it is a tube, inclosing
another tube, the edges of the two being continuous at their
extremities. The outer tube has a chitinous outer coat or cuticle,
which is continued on to the inner face of the inner tube. Neglecting
this for the present, the outermost part of the wall of the outer
tube, which answers to the _epidermis_ of the higher animals, and
the innermost part of the wall of the inner tube, which is an
_epithelium_, are formed by a layer of nucleated cells. A continuous
layer of cells, therefore, is everywhere to {141} be found on both
the external and the internal free surfaces of the body. So far as
these cells belong to the proper external wall of the body, they
constitute the _ectoderm_, and so far as they belong to its proper
internal wall, they compose the _endoderm_. Between these two layers
of nucleated cells lie all the other parts of the body, composed of
connective tissue, muscles, vessels, and nerves; and all these (with
the exception of the ganglionic chain, which we shall see properly
belongs to the ectoderm) may be regarded as a single thick stratum,
which, as it lies between the ectoderm and the endoderm, is called
the _mesoderm_.
If the intestine were closed posteriorly instead of opening by the
vent, the crayfish would virtually be an elongated sac, with one
opening, the mouth, affording an entrance into the alimentary cavity:
and, round this cavity, the three layers just referred to—endoderm,
mesoderm, and ectoderm—would be disposed concentrically.
* * * * *
We have seen that the body of the crayfish thus composed is obviously
separable into three regions—the _cephalon_ or head, the _thorax_,
and the _abdomen_. The latter is at once distinguished by the size
and the mobility of its segments: while the thoracic region is marked
off from that of the head, outwardly, only by the cervical groove.
But, when the carapace is removed, the lateral depression already
mentioned, in which the {142} scaphognathite lies, clearly indicates
the natural boundary between the head and the thorax. It has further
been observed that there are, in all, twenty pairs of appendages, the
six hindermost of which are attached to the abdomen. If the other
fourteen pairs are carefully removed, it will be found that the six
anterior belong to the head, and the eight posterior to the thorax.
The abdominal region may now be studied in further detail. Each of
its seven movable segments, except the telson, represents a sort of
morphological unit, the repetition of which makes up the whole fabric
of the body.
[Illustration: FIG. 36.—_Astacus fluviatilis._—A transverse section
through the nineteenth (fifth abdominal) somite (× 2). _e.m._,
extensor muscles; _f.m._, flexor muscles; _gn._ 12, the fifth
abdominal ganglion; _h.g._, hind-gut; _i.a.a._, inferior abdominal
artery; _s.a.a._, superior abdominal artery; _pl. XIX_, pleura of the
somite; _st. XIX_, its sternum; _t. XIX_, its tergum; _ep. XIX_, its
epimera; _19_, its appendages.]
If the abdomen is divided transversely between the {143} fourth and
fifth, and the fifth and sixth segments, the fifth will be isolated,
and can be studied apart. It constitutes what is called a _metamere_;
in which are distinguishable a central part termed the _somite_, and
two _appendages_ (fig. 36).
In the exoskeleton of the somites of the abdomen several regions
have already been distinguished; and although they constitute one
continuous whole, it will be convenient to speak of the _sternum_
(fig. 36, _st. XIX_), the _tergum_ (_t. XIX_), and, the _pleura_
(_pl. XIX_), as if they were separate parts, and to distinguish that
portion of the sternal region, which lies between the articulation of
the appendage and the pleuron, on each side, as the _epimeron_ (_ep.
XIX_). Adopting this nomenclature, it may be said of the fifth somite
of the abdomen, that it consists of a segment of the exoskeleton,
divisible into tergum, pleura, epimera, and sternum, with which two
appendages are articulated; that it contains a double ganglion (_gn._
12), a section of the flexor (_fm_) and extensor (_em_) muscles, and
of the alimentary (_hg_) and vascular (_s.a.a_, _i.a.a_) systems.
[Illustration: FIG. 37.—_Astacus fluviatilis._—Appendages of the
left side of the abdomen (× 3). A, the posterior face of the first
appendage of the male; B, the same of the female; C, posterior,
and C′, anterior faces of the second appendage of the male; D, the
third appendage of the male; E, the same of the female; F, the sixth
appendage. _a_, the rolled plate of the endopodite; _b_, the jointed
extremity of the same; _bp._, basipodite; _cx.p._, coxopodite;
_en.p._, endopodite; _ex.p._, exopodite.]
The appendage (fig. 36, _19_), which is attached to an articular
cavity situated between the sternum and the epimeron, is seen to
consist of a stalk or stem, which is made up of a very short basal
joint, the _coxopodite_ (fig. 37, D and E, _cx.p_), followed by a
long cylindrical second joint, the _basipodite_ (_b.p_), and receives
the name of _protopodite_. At its free end, it bears two flattened
narrow {145} plates, of which one is attached to the inner side of
the extremity of the protopodite, and is called the _endopodite_
(_en.p_), while the other is fixed a little higher up to the outer
side of that extremity, and is the _exopodite_ (_ex.p_). The
exopodite is shorter than the endopodite. The endopodite is broad
and is undivided for about half its length, from the attached end;
the other half is narrower and is divided into a number of small
segments, which, however, are not united by definite articulations,
but are merely marked off from one another by slight constrictions
of the exoskeleton. The exopodite has a similar structure, but its
undivided portion is shorter and narrower. The edges of both the
exopodite and the endopodite are fringed with long setæ.
In the female crayfish, the appendages of this and of the fourth and
third somites are larger than in the male (compare _D_ and _E_, fig.
37).
The fourth and fifth somites, with their appendages, may be described
in the same terms as the third, and in the sixth there is no
difficulty in recognising the corresponding parts of the somite; but
the appendages (fig. 37, _F_), which constitute the lateral portions
of the caudal fin, at first sight appear very different. In their
size, no less than in their appearance, they depart widely from the
appendages of the preceding somites. Nevertheless, each will be found
to consist of a basal stalk, answering to the protopodite (_cx.p_),
which however is very broad and thick, and is not divided into two
{146} joints; and of two terminal oval plates, which represent the
endopodite (_en.p_) and the exopodite (_ex.p_). The latter is divided
by a transverse suture into two pieces; and the edge of the larger or
basal moiety is beset with short spines, of which two, at the outer
end of the series, are larger than the rest.
The second somite is longer than the first (fig. 1); it has very
broad pleura, while those of the first somite are small and hidden by
the overlapping front margins of the pleura of the second somite.
In the female, the appendages of the second somite of the abdomen
are similar to those of the third, fourth, and fifth somites; but in
those of the first somite (fig. 37, _B_), there is a considerable
variation. Sometimes, in fact, the appendages of this somite are
altogether wanting; sometimes one is present, and not the other; and
sometimes both are found. But, when they exist, these appendages are
always small; and the protopodite is followed by only one imperfectly
jointed filament, which appears to represent the endopodite of the
other appendages.
In the male, the appendages of the first and second somites of the
abdomen are not only of relatively large size, but they are widely
different from the rest, those of the first somite departing from
the general type further than those of the second. In the latter
(_C, C′_) there is a protopodite (_cx.p_, _bp_) with the ordinary
structure, and it is followed by an endopodite (_en.p_) and an
exopodite {147} (_ex.p_); but the former is singularly modified.
The undivided basal part is large, and is produced on the inner side
into a lamella (_a_), which extends slightly beyond the end of the
terminal jointed portion (_b_). The inner half of this lamella is
rolled upon itself, in such a manner as to give rise to a hollow
cone, something like an extinguisher (_C′_, _a_).
The appendage of the first somite (_A_) is an unjointed styliform
body, which appears to represent the protopodite, together with
the basal part and the inner prolongation of the endopodite of the
preceding appendage. The terminal half of the appendage is really a
broad plate, slightly bifid at the summit, but the sides of the plate
are rolled in, in such a manner that the anterior half bends round
and partially incloses the posterior half. They thus give rise to a
canal, which is open at each end, and only partially closed behind.
These two pairs of curiously modified appendages are ordinarily
turned forwards and applied against the sterna of the posterior
part of the thorax, in the interval between the bases of the hinder
thoracic limbs (see fig. 3, _A_). They serve as conduits by which the
spermatic matter of the male is conveyed from the openings of the
ducts of the testes to its destination.
If we confine our attention to the third, fourth, and fifth metameres
of the abdomen of the crayfish, it is obvious that the several
somites and their appendages, and the various regions or parts into
which they are {148} divisible, correspond with one another, not
only in form, but in their relations to the general plan of the
whole abdomen. Or, in other words, a diagrammatic plan of one somite
will serve for all the three somites, with insignificant variations
in detail. The assertion that these somites are constructed upon
the same plan, involves no more hypothesis than the statement of an
architect, that three houses are built upon the same plan, though the
façades and the internal decorations may differ more or less.
In the language of morphology, such conformity in the plan of
organisation is termed _homology_. Hence, the several metameres in
question and their appendages, are _homologous_ with one another;
while the regions of the somites, and the parts of their appendages,
are also _homologues_.
When the comparison is extended to the sixth metamere, the homology
of the different parts with those of the other metameres, is
undeniable, notwithstanding the great differences which they present.
To recur to a previous comparison, the ground plan of the building
is the same, though the proportions are varied. So with regard to
the first and second metameres. In the second pair of appendages
of the male, the difference from the ordinary type of appendage is
comparable to that produced by adding a portico or a turret to the
building; while, in the first pair of appendages of the female, it is
as if one wing of the edifice were left unbuilt; {149} and, in those
of the male, as if all the rooms were run into one.
It is further to be remarked, that, just as of a row of houses
built upon the same plan, one may be arranged so as to serve as a
dwelling-house, another as a warehouse, and another as a lecture
hall, so the homologous appendages of the crayfish are made to
subserve various functions. And as the fitness of the dwelling-house,
the warehouse, and the lecture-hall for their several purposes would
not in the least help us to understand why they should all be built
upon the same general plan; so, the adaptation of the appendages
of the abdomen of the crayfish to the discharge of their several
functions does not explain why those parts are homologous. On the
contrary, it would seem simpler that each part should have been
constructed in such a manner as to perform its allotted function
in the best possible manner, without reference to the rest. The
proceedings of an architect, who insisted on constructing every
building in a town on the plan of a Gothic cathedral, would not be
explicable by considerations of fitness or convenience.
* * * * *
In the cephalothorax, the division into somites is not at first
obvious, for, as we have seen, the dorsal or tergal surface is
covered over by a continuous shield, distinguished into thoracic and
cephalic regions only by the cervical groove. Even here, however,
when a transverse section of the thorax is compared with that of the
{150} abdomen (figs. 15 and 36), it will be obvious that the tergal
and the sternal regions of the two answer to one another; while the
branchiostegites correspond with greatly developed pleura; and the
inner wall of the branchial chamber, which extends from the bases of
the appendages to the attachment of the branchiostegite, represents
an immensely enlarged epimeral region.
On examination of the sternal aspect of the cephalothorax the signs
of division into somites become plain (figs. 3 and 39, _A_). Between
the last two ambulatory limbs there is an easily recognisable sternum
(_XIV._), though it is considerably narrower than any of the sterna
of the abdominal somites, and differs from them in shape.
The deep transverse fold which separates this hindermost thoracic
sternum from the rest of the sternal wall of the cephalothorax, is
continued upwards on the inner or epimeral wall of the branchial
cavity; and thus the sternal and the epimeral portions of the
posterior thoracic somite are naturally marked off from those of the
more anterior somites.
[Illustration: FIG. 38.—_Astacus fluviatilis._—The mode of connexion
between the last thoracic and the first abdominal somites (× 3). _a_,
L-shaped bar; _cpe_, carapace; _cxp. 14_, coxopodite of the last
ambulatory leg; _plb._, place of attachment of the pleurobranchia;
_st. XV_, sternum, and _t. XV_, tergum of the first abdominal somite.]
The epimeral region of this somite presents a very curious structure
(fig. 38). Immediately above the articular cavities for the
appendages there is a shield-shaped plate, the posterior, convex edge
of which is sharp, prominent, and setose. Close to its upper boundary
the plate exhibits a round perforation (_plb._), to the margins of
which the stem of the hindermost {151} pleurobranchia (fig. 4, _plb.
14_) is attached; and in front of this, it is connected, by a narrow
neck, with an elongated triangular piece, which takes a vertical
direction, and lies in the fold which separates the posterior
thoracic somite from the next in front. The base of this piece unites
with the epimeron of the penultimate somite. Its apex is connected
with the anterior end of the horizontal arm of an L-shaped calcified
bar (fig. 38, _a_), the upper end of the vertical arm of which is
firmly, but moveably, connected with the anterior and lateral edge of
the tergum of the first abdominal somite (_t. XV._). The tendon of
one {152} of the large extensor muscles of the abdomen is attached
close to it.
The sternum and the shield-shaped epimeral plates constitute a
solid, continuously calcified, ventral element of the skeleton, to
which the posterior pair of legs is attached; and as this structure
is united with the somites in front of and behind it only by soft
cuticle, except where the shield-shaped plate is connected, by the
intermediation of the triangular piece, with the epimeron which lies
in front of it, it is freely movable backwards and forwards on the
imperfect hinge thus constituted.
In the same way, the first somite of the abdomen, and, consequently,
the abdomen as a whole, moves upon the hinges formed by the union of
the L-shaped pieces with the triangular pieces.
In the rest of the thorax, the sternal and the epimeral regions of
the several somites are all firmly united together. Nevertheless,
shallow grooves answering to folds of the cuticle, which run from the
intervals between the articular cavities for the limbs towards the
tergal end of the inner wall of the branchial chamber, mark off the
epimeral portions of as many somites as there are sterna, from one
another.
[Illustration: FIG. 39.—_Astacus fluviatilis._—The cephalothoracic
sterna and the endophragmal system (× 2). _A_, from beneath; _B_,
from above. _a_, _a′_, arthrophragms or partitions between the
articular cavities for the limbs; _c.ap_, cephalic apodeme; _cf_,
cervical fold; _epn. 1_, epimeron of the antennulary somite; _h_,
anterior, and _h′_, posterior horizontal process of endopleurite;
_lb_, labrum; _m_, mesophragm; _mt_, metastoma; _p_, paraphragm;
_I–XIV_, cephalothoracic sterna; _1–14_, articular cavities of the
cephalothoracic appendages. (The anterior cephalic sterna are bent
downwards in A so as to bring them into the same plane with the
remaining cephalothoracic sterna; in B these sterna are not shown.)]
A short distance above the articular cavities a transverse groove
separates a nearly square area of the lower part of the epimeron from
the rest. Towards the anterior and upper angle of this area, in the
two somites {153} which lie immediately in front of the hindermost,
there is a small round aperture for the attachment of the {154}
rudimentary branchia. These areæ of the epimera, in fact, correspond
with the shield-shaped plate of the hindermost somite. In the next
most anterior somite (that which bears the first pair of ambulatory
legs) there is only a small elevation in the place of the rudimentary
branchia; and in the anterior four thoracic somites nothing of the
kind is visible.
On the sternal aspect of the thorax (figs. 3 and 39, A) a triangular
space is interposed between the basal joints or coxopodites of the
penultimate and the ante-penultimate pairs of ambulatory legs, while
the coxopodites of the more anterior limbs are closely approximated.
The triangular area in question is occupied by two sterna (fig. 39,
A, _XII_, _XIII_), the lateral margins of which are raised into
flange-like ridges. The next two sterna (_X_, _XI_) are longer,
especially that which lies between the forceps (_X_), but they
are very narrow; while the lateral processes are reduced to mere
tubercles at the posterior ends of the sterna. Between the three
pairs of maxillipedes, the sterna (_VII_, _VIII_, _IX_) are yet
narrower, and become gradually shorter; but traces of the tubercles
at their posterior ends are still discernible. The most anterior of
these sternal rods passes into a transversely elongated plate, shaped
like a broad arrow (_V_, _VI_), which is constituted by the conjoined
sterna of the two posterior somites of the head.
Anteriorly to this, and between it and the posterior end of the
elongated oral aperture, the sternal region is {155} occupied only
by soft or imperfectly calcified cuticle, which, on each side of
the hinder part of the mouth, passes into one of the lobes of the
metastoma (_mt_). At the base of each of these lobes there is a
calcified plate, united by an oblique suture with another, which
occupies the whole length of the lobe and gives it firmness. The
soft narrow lip which constitutes the lateral boundary of the oral
aperture, and lies between it and the mandible, passes, in front,
into the posterior face of the labrum (_lb_).
In front of the mouth, the sternal region which appertains, in part,
to the antennæ, and, in part, to the mandibles, is obvious as a
broad plate (_III_), termed the _epistoma_. The middle third of the
posterior edge of the epistoma gives rise to a thickened transverse
ridge, with rounded ends, slightly excavated behind, and is then
continued into the labrum (_lb_), which is strengthened by three
pairs of calcifications, arranged in a longitudinal series. The
sides of the front edge of the epistoma are excavated, and bound the
articular cavities for the basal joints of the antennæ (_3_); but, in
the middle line, the epistoma is continued forwards into a spear-head
shaped process (figs. 39 and 40, _II_), to which the posterior end
of the antennulary sternum contributes. The antennulary sternum is
very narrow, and its anterior or upper end runs into a small but
distinct conical median spine (fig. 40, _t._). Upon this follows
an uncalcified plate, bent into the form of a half cylinder (_I_),
which lies between the inner ends of {156} the eye-stalks and is
united with adjacent parts only by flexible cuticle, so that it is
freely movable. This represents the whole of the sternal region, and
probably more, of the ophthalmic somite.
[Illustration: FIG. 40.—_Astacus fluviatilis._—The ophthalmic and
antennulary somites (× 3). _I_, ophthalmic, and _II_, antennulary
sternum; _1_, articular surface for eyestalk; _2_, for antennule;
_epm_, epimeral plate; _pcp_, procephalic process; _r_, base of
rostrum; _t_, tubercle.]
The sterna of fourteen somites are thus identifiable in the
cephalothorax. The corresponding epimera are represented, in the
thorax, by the thin inner walls of the branchial chamber; the pleura,
by the branchiostegites; and the terga, by so much of the median
region of the carapace as lies behind the cervical groove. That part
of the carapace which is situated in front of this groove occupies
the place of the terga of the head; while the low ridge, skirting
the oral and præ-oral region, in which it terminates laterally,
represents the pleura of the cephalic somites.
The epimera of the head are, for the most part, very narrow; but
those of the antennulary somite are broad plates (fig. 40, _epm._),
which constitute the posterior {157} wall of the orbits. I am
inclined to think that a transverse ridge, which unites these under
the base of the rostrum, represents the tergum of the antennulary
somite, and that the rostrum itself belongs to the next or antennary
somite.[11]
[11] There are some singular marine crustacea, the _Squillidæ_,
in which both the ophthalmic and the antennary somites are free
and movable, while the rostrum is articulated with the tergum of
the antennary somite.
The sharp convex ventral edge of the rostrum (fig. 41) is produced
into a single, or sometimes two divergent spines, which descend,
in front of the ophthalmic somite, towards the conical tubercle
mentioned above: it thus gives rise to an imperfect partition between
the orbits.
[Illustration: FIG. 41.—_Astacus fluviatilis._—The rostrum, seen from
the left side.]
The internal face of the sternal wall of the whole of the thorax and
of the post-oral part of the head, presents a complicated arrangement
of hard parts, which is known as the _endophragmal system_ (figs.
39, _B_, 42, and 43), and which performs the office of an internal
skeleton by affording attachment to muscles, and serving to protect
important viscera, while at the same time it ties the somites
together, and unites them into a solid whole. In reality, however,
the curious pillars and bulkheads which enter into the composition
of the endophragmal system are all {158} mere infoldings of the
cuticle, or _apodemes_; and, as such, they are shed along with the
other cuticular structures during the process of ecdysis.
Without entering into unnecessary details, the general principle
of the construction of the endophragmal skeleton may be stated as
follows. Four apodemes are developed between every two somites,
and as every apodeme is a fold of the cuticle, it follows that
the anterior wall of each belongs to the somite in front, and
the posterior wall to the somite behind. All four apodemes lie
in the ventral half of the somite and form a single transverse
series; consequently there are two nearer the middle line, which
are termed the _endosternites_, and two further off, which are the
_endopleurites_. The former lie at the inner, and the latter at the
outer ends of the partitions or _arthrophragms_ (fig. 39, A, _a, a′_,
fig. 42, _aph_), between the articular cavities for the basal joints
of the limbs, and they spring partly from the latter and partly from
the sternum and the epimera respectively.
The endosternite (fig. 42, _ens._) ascends vertically, with a slight
inclination forwards, and its summit narrows and assumes the form
of a pillar, with a flat, transversely elongated capital. The inner
prolongation of the capital is called the _mesophragm_ (_mph._), the
outer _paraphragm_ (_pph._). The mesophragms of the two endosternites
of a somite usually unite by a median suture, and thus form a
complete arch over the sternal canal (_s.c._), which lies between the
endosternites. {159}
The endopleurites (_en.pl._) are also vertical plates, but they
are relatively shorter, and their inner angles give off two nearly
horizontal processes, one of which passes obliquely forwards (fig.
39, B, _h_, fig. 42, _h.p._) and unites with the paraphragm of
the endosternite of the somite in front, while the other, passing
obliquely backwards (fig. 39, _h′_), becomes similarly connected with
the endosternite of the somite behind.
[Illustration: FIG. 42.—_Astacus fluviatilis._—A segment of the
endophragmal system (× 3). _aph_, arthrophragm; _arth_, arthrodial or
articular cavity; _cxp_, coxopodite of the ambulatory leg; _enpl_,
endopleurite; _ens_, endosternite; _epm_, epimeron; _hp_, horizontal
process of endopleurite; _mph_, mesophragm; _pph_, paraphragm; _s_,
sternum of somite; _sc_, sternal canal.]
The endopleurites of the last thoracic somite are rudimentary, and
its endosternites are small. On the other hand, the mesophragmal
processes of the endosternites of the two posterior somites of
the head (fig. 39, B, _c.ap_), by which the endophragmal system
terminates in front, are particularly strong and closely united
together. They thus, with their endopleurites, form a solid partition
between the stomach, which lies upon them, and the mass of {160}
coalesced anterior thoracic and posterior cephalic ganglia situated
beneath them. Strong processes are given off from their anterior and
outer angles, which curve round the tendons of the adductor muscles
of the mandibles, and give attachment to the abductors.
In front of the mouth there is no such endophragmal system as that
which lies behind it. But the anterior gastric muscles are attached
to two flat calcified plates, which appear to lie in the interior
of the head (though they are really situated in its upper and front
wall) on each side of the base of the rostrum, and are called the
_procephalic processes_ (figs. 40, 43, _p.cp_). Each of these plates
constitutes the posterior wall of a narrow cavity which opens
externally into the roof of the orbit, and has been regarded (though,
as it appears to me, without sufficient reason) as an olfactory
organ. I am disposed to think, though I have not been able to obtain
complete evidence of the fact, that the procephalic processes are
the representatives of the “procephalic lobes” which terminate the
anterior end of the body in the embryo crayfish. At any rate, they
occupy the same position relatively to the eyes and to the carapace;
and the hidden position of these processes, in the adult, appears to
arise from the extension of the carapace at the base of the rostrum
over the fore part of the originally free sternal surface of the
head. It has thus covered over the procephalic processes, in which
the sternal wall of the body terminated; and the cavities which lie
in front of them are {161} simply the interspaces left between
the inferior or posterior wall of the prolongation of the carapace
and the originally exposed external faces of these regions of the
cephalic integument.
* * * * *
Fourteen somites having thus been distinguished in the cephalothorax,
and six being obvious in the abdomen, it is clear that there is a
somite for every pair of appendages. And, if we suppose the carapace
divided into segments answering to these sterna, the whole body will
be made up of twenty somites, each having a pair of appendages.
As the carapace, however, is not actually divided into terga in
correspondence with the sterna which it covers, all we can safely
conclude from the anatomical facts is that it represents the tergal
region of the somites, not that it is formed by the coalescence of
primarily distinct terga. In the head, and in the greater part of
the thorax, the somites are, as it were, run together, but the last
thoracic somite is partly free and to a slight extent moveable,
while the abdominal somites are all free, and moveably articulated
together. At the anterior end of the body, and, apparently, from the
antennary somite, the tergal region gives rise to the rostrum, which
projects between and beyond the eyes. At the opposite extremity,
the telson is a corresponding median outgrowth of the last somite,
which has become moveably articulated therewith. The narrowing of the
sternal moieties of the anterior thoracic somites, {162} together
with the sudden widening of the same parts in the posterior cephalic
somites, gives rise to the lateral depression (fig. 39, _cf_) in
which the scaphognathite lies. The limit thus indicated corresponds
with that marked by the cervical groove upon the surface of the
carapace, and separates the head from the thorax. The three pair
of maxillipedes (_7, 8, 9_), the forceps (_10_), the ambulatory
{163} limbs (_11–14_), and the eight somites of which they are the
appendages (_VII–XIV_), lie behind this boundary and belong to the
thorax. The two pairs of maxillæ (_5, 6_) the mandibles (_4_), the
antennæ (_3_), the antennules (_2_), the eyestalks (_1_), and the
six somites to which they are attached (_I–VI_), lie in front of the
boundary and compose the head.
[Illustration: FIG. 43.—_Astacus fluviatilis._—Longitudinal section
of the anterior part of the cephalothorax (× 3). _I–IX_, sterna of
first nine cephalothoracic somites; _1_, eyestalk; _2_, basal joint
of antennule; _3_, basal joint of antenna; _4_, mandible; _a_, inner
division of the masticatory surface of the mandible; _a′_, apophysis
of the mandible for muscular attachment; _cp_, free edge of carapace;
_e_, endosternite; _enpl_, endopleurite; _epm_, epimeral plate; _l_,
labrum; _m_, muscular fibres connecting epimera with interior of
carapace; _mt_, metastoma; _pcp_, procephalic process.]
Another important point to be noticed is that, in front of the mouth,
the sternum of the antennary somite (fig. 43, _III_) is inclined at
an angle of 60° or 70° to the direction of the sterna behind the
mouth. The sternum of the antennulary somite (_II_) is at right
angles to the latter; and that of the eyes (_I_) looks upwards as
well as forwards. Hence, the front of the head beneath the rostrum,
though it looks forwards, or even upwards, is homologous with the
sternal aspect of the other somites. It is for this reason that the
feelers and the eyestalks take a direction so different from that of
the other appendages. The change of aspect of the sternal surface in
front of the mouth, thus effected, is what is termed the _cephalic
flexure_.
* * * * *
Since the skeleton which invests the trunk of the crayfish is made
up of a twenty-fold repetition of somites, homologous with those of
the abdomen, we may expect to find that the appendages of the thorax
and of the head, however unlike they may seem to be to those of the
abdomen, are nevertheless reducible to the same fundamental plan.
{164}
The third maxillipede is one of the most complete of these
appendages, and may be advantageously made the starting point of the
study of the whole series.
[Illustration: FIG. 44.—_Astacus fluviatilis._—The third or external
maxillipede of the left side (× 3). _e_, lamina, and _br_, branchial
filaments of the podobranchia; _cxp_, coxopodite; _cxs_, coxopoditic
setæ; _bp_, basipodite; _ex_, exopodite; _ip_, ischiopodite; _mp_,
meropodite; _cp_, carpopodite; _pp_, propodite; _dp_, dactylopodite.]
Neglecting details for the moment, it may be said that the appendage
consists of a basal portion (fig. 44, _cxp, bp_), {165} with two
terminal divisions (_ip_ to _dp_, and _ex_), which are directed
forwards, below the mouth, and a third, lateral appendage (_e, br_),
which runs up, beneath the carapace, into the branchial chamber.
The latter is the gill, or podobranchia, attached to this limb,
and it is something not represented in the abdominal limbs. But,
with regard to the rest of the maxillipede, it is obvious that the
basal portion (_cxp, bp_) represents the protopodite, and the two
terminal divisions the endopodite and the exopodite respectively. It
has been observed that, in the abdominal appendages, the extent to
which segmentation occurs in homologous parts varies indefinitely;
an endopodite, for example, may be a continuous plate, or may be
subdivided into many joints. In the maxillipede, the basal portion
is divided into two joints; and, as in the abdominal limb, the
first, or that which articulates with the thorax, is termed the
_coxopodite_ (_cxp_), while the second is the _basipodite_ (_bp_).
The stout, leg-like endopodite appears to be the direct continuation
of the basipodite; while the much more narrow and slender exopodite
articulates with its outer side. The exopodite (_ex_) is by no means
unlike one of the exopodites of the abdominal limbs, consisting as it
does of an undivided base and a many-jointed terminal filament. The
endopodite, on the contrary, is strong and massive, and is divided
into five joints, named, from that nearest to the base onwards,
_ischiopodite_ (_ip_), _meropodite_ (_mp_), _carpopodite_ (_cp_),
_propodite_ (_pp_), and _dactylopodite_ (_dp_). {166}
[Illustration: FIG. 45.—_Astacus fluviatilis._—A, the first; B, the
second maxillipede of the left side (× 3). _cxp_, coxopodite; _bp_,
basipodite; _e, br_, podobranchia; _ep_, epipodite; _en_, endopodite;
_ex_, exopodite; _ip_, ischiopodite; _mp_, meropodite; _cp_,
carpopodite; _pp_, propodite; _dp_, dactylopodite.]
The second maxillipede (fig. 45, B) has essentially the same
composition as the first, but the exopodite (_ex_) is relatively
larger, the endopodite (_ip–dp_) smaller and softer; and, while the
ischiopodite (_ip_) is the longest joint in the third maxillipede, it
is the meropodite (_mp_) which is longest in the second. In the first
maxillipede (fig. 45, A) a great modification has taken place. The
coxopodite (_cxp_) and the basipodite (_bp_) are broad thin plates
with setose cutting edges, while the endopodite (_en_) is short and
only two-jointed, and the undivided portion of the exopodite (_ex_)
is very long. The place of {167} the podobranchia is taken by a
broad soft membranous plate entirely devoid of branchial filaments
(_ep_). Thus, in the series of the thoracic limbs, on passing
forwards from the third maxillipede, we find that though the plan
of the appendages remains the same; (1) the protopodite increases
in relative size; (2) the endopodite diminishes; (3) the exopodite
increases; (4) the podobranchia finally takes the form of a broad
membranous plate and loses its branchial filaments.
Writers on descriptive Zoology usually refer to the parts of the
maxillipedes under different names from those which are employed
here. The protopodite and the endopodite taken together are commonly
called the _stem_ of the maxillipede, while the exopodite is the
_palp_, and the metamorphosed podobranchia, the real nature of which
is not recognised, is termed the _flagellum_.
When the comparison of the maxillipedes with the abdominal members,
however, had shown the fundamental uniformity of composition of the
two, it became desirable to invent a nomenclature of the homologous
parts which should be capable of a general application. The names
of protopodite, endopodite, exopodite, which I have adopted as
the equivalents of the “stem” and the “palp,” were proposed by
Milne-Edwards, who at the same time suggested _epipodite_ for the
“flagellum.” And the lamellar process of the first maxillipede is now
very generally termed an epipodite; while the podobranchiæ, which
have exactly the same relations to the following {168} limbs, are
spoken of as if they were totally different structures, under the
name of branchiæ or gills.
The flagellum or epipodite of the first maxillipede, however, is
nothing but the slightly modified stem of a podobranchia, which
has lost its branchial filaments; but the term “epipodite” may be
conveniently used for podobranchiæ thus modified. Unfortunately, the
same term is applied to certain lamelliform portions of the branchiæ
of other crustacea, which answer to the laminæ of the crayfishes’
branchiæ; and this ambiguity must be borne in mind, though it is of
no great moment.
On examining an appendage from that part of the thorax which lies
behind the third maxillipede, say, for example, the sixth thoracic
limb (the second walking leg) (fig. 46), the two joints of the
protopodite and the five joints of the endopodite are at once
identifiable, and so is the podobranchia; but the exopodite has
vanished altogether. In the eighth, or last, thoracic limb, the
podobranchia has also disappeared. The fifth and sixth limbs also
differ from the seventh and eighth, in being chelate; that is to
say, one angle of the distal end of the propodite is prolonged and
forms the fixed leg of the pincer. The produced angle is that which
is turned downwards when the limb is fully extended (fig. 46). In
the forceps, the great chela is formed in just the same way; the
only important difference lies in the fact that, as in the external
maxillipede, the basipodite and the ischiopodite are immoveably
united. Thus, {170} the limbs of the thorax are all reducible to
the same type as those of the abdomen, if we suppose that, in the
posterior five pair, the exopodites are suppressed; and that, in all
but the last, podobranchiæ are superadded.
[Illustration: FIG. 46.—_Astacus fluviatilis._—The second ambulatory
leg of the left side (× 3). _cxp_, coxopodite; _bp_, basipodite;
_br_, gill; _cxs_, coxopoditic setæ; _e_, lamina of gill or
epipodite; _ip_, ischiopodite; _mp_, meropodite; _cp_, carpopodite;
_pp_, propodite; _dp_, dactylopodite.]
Turning to the appendages of the head, the second maxilla (fig.
47, C) presents a further modification of the disposition of the
parts seen in the first maxillipede. The coxopodite (_cxp_) and
the basipodite (_bp_) are still thinner and more lamellar, and are
subdivided by deep fissures which extend from their inner edges.
The endopodite (_en_) is very small and undivided. In the place of
the exopodite and the epipodite there is only one great plate, the
scaphognathite (_sg_) which either is such an epipodite as that of
the first maxillipede with its anterior basal process much enlarged,
or represents both the exopodite and the epipodite. In the first
maxilla (B), the exopodite and the epipodite have disappeared,
and the endopodite (_en_) is insignificant and unjointed. In the
mandibles (A), the representative of the protopodite is strong
and transversely elongated. Its broad inner or oral end presents
a semicircular masticatory surface divided by a deep longitudinal
groove into two toothed ridges. The one of these follows the convex
anterior or inferior contour of the masticatory surface, projects
far beyond the other, and is provided with a sharp serrated edge;
the other (fig. 43, _a_) gives rise to the straight posterior or
superior contour of the masticatory surface, and is more obtusely
tuberculated. In front, the inner {171} ridge is continued into a
process by which the mandible articulates with the epistoma (fig. 47,
A, _ar_). The endopodite is represented by the three-jointed _palp_
(_p_), the terminal joint of which is oval and beset with numerous
strong setæ, which are especially abundant along its anterior edge.
[Illustration: FIG. 47.—_Astacus fluviatilis._—A, mandible; B, first
maxilla; C, second maxilla of the left side (× 3). _ar_, internal,
and _ar′_, external articular process of the mandible; _bp_,
basipodite; _cxp_, coxopodite; _en_, endopodite; _p_, palp of the
mandible; _sg_, scaphognathite; _x_, internal process of the first
maxilla.]
{172}
In the antenna (fig. 48, C) the protopodite is two-jointed. The
basal segment is small, and its ventral face presents the conical
prominence on the posterior aspect of which is the aperture of the
duct of the renal gland (_gg_). The terminal segment is larger and
is subdivided by deep longitudinal folds, one upon the dorsal and
one upon the ventral face, into two moieties which are more or less
moveable upon one another. In front and externally it bears the broad
flat _squame_ (_exp_) of the antenna, as an exopodite. Internally,
the long annulated “feeler” which represents the endopodite, is
connected with it by two stout basal segments.
[Illustration: FIG. 48.—_Astacus fluviatilis._—A, eye-stalk; B,
antennule; C, antenna of the left side (× 3). _a_, spine of the basal
joint of the antennule; _c_, corneal surface of the eye; _exp_,
exopodite or squame of the antenna; _gg_, aperture of the duct of the
green gland.]
{173}
The antennule (fig. 48, B) has a three-jointed stem and two terminal
annulated filaments, the outer of which is thicker and longer than
the inner, and lies rather above as well as external to the latter.
The peculiar form of the basal segment of the stem of the antennule
has already been adverted to (p. 116). It is longer than the other
two segments put together, and near the anterior end its sternal
edge is produced into a single strong spine (_a_). The stem of the
antennule answers to the protopodite of the other limbs, though its
division into three joints is unusual; the two terminal annulated
filaments represent the endopodite and the exopodite.
Finally, the eyestalk (A) has just the same structure as the
protopodite of an abdominal limb, having a short basal and a long
cylindrical terminal joint.
* * * * *
From this brief statement of the characters of the appendages, it
is clear that, in whatever sense it is allowable to say that the
appendages of the abdomen are constructed upon one plan, which is
modified in execution by the excess of development of one part over
another, or by the suppression of parts, or by the coalescence of one
part with another, it is allowable to say that all the appendages are
constructed on the same plan, and are modified on similar principles.
Given a general type of appendage consisting of a protopodite,
bearing a podobranchia, an endopodite and an exopodite, all the
actual appendages are readily derivable from that type. {174}
In addition, therefore, to their adaptation to the purposes which
they subserve, the parts of the skeleton of the crayfish show a
unity in diversity, such as, if the animal were a piece of human
workmanship, would lead us to suppose that the artificer was under
an obligation not merely to make a machine capable of doing certain
kinds of work, but to subordinate the nature and arrangement of the
mechanism to certain fixed architectural conditions.
The lesson thus taught by the skeletal organs is reiterated and
enforced by the study of the nervous and the muscular systems.
As the skeleton of the whole body is capable of resolution into
the skeletons of twenty separate metameres, variously modified
and combined; so is the entire ganglionic chain resolvable into
twenty pairs of ganglia various in size, distant in this region
and approximated in that; and so is the muscular system of the
trunk conceivable as the sum of twenty _myotomes_ or segments of
the muscular system appropriate to a metamere, variously modified
according to the degree of mobility of the different regions of the
organism.
* * * * *
The building up of the body by the repetition and the modification
of a few similar parts, which is so obvious from the study of the
general form of the somites and of their appendages, is still more
remarkably illustrated, if we pursue our investigations further, and
trace {175} out the more intimate structure of these parts. The
tough, outer coat, which has been termed the _cuticula_, except so
far as it presents different degrees of hardness, from the presence
or absence of calcareous salts, is obviously everywhere of the same
nature; and, by macerating a crayfish in caustic alkali, which
destroys all its other components of the body, it will be readily
enough seen that a continuation of the cuticular layer passes in at
the mouth and the vent, and lines the alimentary canal; furthermore,
that processes of the cuticle covering various parts of the trunk
and limbs extend inwards, and afford surfaces of attachment to the
muscles, as the _apodemata_ and _tendons_. In technical language, the
cuticular substance which thus enters so largely into the composition
of the bodily fabric of the crayfish is called a _tissue_.
The flesh, or _muscle_, is another kind of tissue, which is readily
enough distinguished from cuticular tissue by the naked eye; but, for
a complete discrimination of all the different tissues, recourse must
be had to the microscope, the application of which to the study of
the ultimate optical characters of the morphological constituents of
the body has given rise to that branch of morphology which is known
as _Histology_.
[Illustration: FIG. 49.—_Astacus fluviatilis._—The corpuscles of
the blood, highly magnified. _1–8_, show the changes undergone by a
single corpuscle during a quarter of an hour; _n_, the nucleus; _9_
and _10_ are corpuscles killed by magenta, and having the nucleus
deeply stained by the colouring matter.]
If we count every formed element of the body, which is separable from
the rest by definite characters, as a tissue, there are no more than
eight kinds of such tissues in the crayfish; that is to say, every
solid constituent {176} of the body consists of one or more of the
following eight histological groups:—
1. Blood corpuscles; 2. Epithelium; 3. Connective tissue; 4. Muscle;
5. Nerve; 6. Ova; 7. Spermatozoa; 8. Cuticle.
1. A drop of freshly-drawn blood of the crayfish contains multitudes
of small particles, the _blood corpuscles_, which rarely exceed
1‐700th, and usually are about 1‐1000th, of an inch in diameter (fig.
49). They are sometimes pale and delicate, but generally more or
less dark, from containing a number of minute strongly refracting
granules, and they are ordinarily exceedingly irregular in form. If
one of them is watched {177} continuously for two or three minutes,
its shape will be seen to undergo the constant but slow changes to
which passing reference has already been made (p. 69). One or other
of the irregular prolongations will be drawn in, and another thrown
out elsewhere. The corpuscle, in fact, has an inherent contractility,
like one of those low organisms, known as an _Amœba_, whence its
motions are frequently called _amœbiform_. In its interior, an
ill-marked oval contour may be seen, indicating the presence of a
spheroidal body, about 1‐2000th of an inch in diameter, which is the
nucleus of the corpuscle (_n_). The addition of some re-agents, such
as dilute acetic acid, causes the corpuscles at once to assume a
spherical shape, and renders the nucleus very conspicuous (fig. 49,
_9_ and _10_). The blood corpuscle is, in fact, a simple nucleated
cell, composed of a contractile protoplasmic mass, investing a
nucleus; it is suspended freely in the blood; and, though as much
a part of the crayfish organism as any other of its histological
elements, leads a quasi-independent existence in that fluid.
2. Under the general name of _epithelium_, may be included a form
of tissue, which everywhere underlies the exoskeleton (where it
corresponds with the epidermis of the higher animals), and the
cuticular lining of the alimentary canal, extending thence into the
hepatic cæca. It is further met with in the generative organs, and
in the green gland. Where it forms the subcuticular layer of the
integument and of the alimentary canal, it is found to {178} consist
of a protoplasmic substance (fig. 50), in which close set nuclei
(_n_) are imbedded. If a number of blood corpuscles could be supposed
to be closely aggregated together into a continuous sheet, they would
give rise to such a structure as this; and there can be no doubt
that it really is an aggregate of nucleated cells, though the limits
between the individual cells are rarely visible in the fresh state.
In the liver, however, the cells grow, and become detached from one
another in the wider and lower parts of the cæca, and their essential
nature is thus obvious.
[Illustration: FIG. 50.—_Astacus fluviatilis._—Epithelium, from the
epidermic layer subjacent to the cuticle, highly magnified. _A_, in
vertical section; _B_, from the surface. _n_, nuclei.]
3. Immediately beneath the epithelial layer follows a tissue,
disposed in bands or sheets, which extend to the subjacent parts,
invest them, and connect one with another. Hence this is called
_connective tissue_.
[Illustration: FIG. 51.—_Astacus fluviatilis._—Connective tissue;
_A_, second form; _B_, third form. _a_, cavities; _n_, nuclei. Highly
magnified.]
The connective tissue presents itself under three forms. In the
first there is a transparent homogeneous-looking matrix, or ground
substance, through which are scattered many nuclei. In fact,
this form of connective tissue {179} very closely resembles the
epithelial tissue, except that the intervals between the nuclei are
wider, and that the substance in which they are imbedded cannot be
broken up into a separate cell-body for each nucleus. In the second
form (fig. 51, _A_) the matrix exhibits fine wavy parallel lines, as
if it were marked out into imperfect fibres. In this form, as in the
next to be described, more or less spherical cavities, which contain
a clear fluid, are excavated in the matrix; and the number of {180}
these is sometimes so great, that the matrix is proportionally
very much reduced, and the structure acquires a close superficial
similarity to that of the parenchyma of plants. This is still more
the case with a third form, in which the matrix itself is marked off
into elongated or rounded masses, each of which has a nucleus in its
interior (fig. 51, _B_). Under one form or another, the connective
tissue extends throughout the body, ensheathing the various organs,
and forming the walls of the blood sinuses.
The third form is particularly abundant in the outer investment
of the heart, the arteries, the alimentary canal, and the nervous
centres. About the cerebral and anterior thoracic ganglia, and on
the exterior of the heart, it usually contains more or less fatty
matter. In these regions, many of the nuclei, in fact, are hidden by
the accumulation round them of granules of various sizes, some of
which are composed of fat, while others consist of a proteinaceous
material. These aggregates of granules are usually spheroidal; and,
with the matrix in which they are imbedded and the nucleus which
they surround, they are often readily detached when a portion of the
connective tissue is teased out, and are then known as _fat cells_.
From what has been said respecting the distribution of the connective
tissue, it is obvious that if all the other tissues could be removed,
this tissue would form a continuous whole, and represent a sort of
model, or cast, of the whole body of the crayfish. {181}
[Illustration: FIG. 52.—_Astacus fluviatilis._—A, a single muscular
fibre, transverse diameter 1‐110th of an inch; B, a portion of
the same more highly magnified; C, a smaller portion treated with
alcohol and acetic acid still more highly magnified; D and E, the
splitting up of a part of a fibre, treated with picro-carmine, into
fibrillæ; F, the connection of a nervous with a muscular fibre which
has been treated with alcohol and acetic acid. _a_, darker, and _b_,
clearer portions of the fibrillæ; _n_, nuclei; _nv_, nerve fibre;
_s_, sarcolemma; _t_, tendon; _1–5_, successive dark granular striæ
answering to the granular portions, _a_, of each fibrilla.]
4. The _muscular tissue_ of the crayfish always has the form of
bands or fibres, of very various thickness, marked, when viewed by
transmitted light, by alternate darker and {182} lighter striæ,
transversely to the axis of the fibres (fig. 52 A). The distance of
the transverse striæ from one another varies with the condition of
the muscle, from 1‐4,000th of an inch in the quiescent state to as
little as 1‐30,000th of an inch in that of extreme contraction. The
more delicate muscular fibres, like those of the heart and those of
the intestine, are imbedded in the connective tissue of the organ,
but have no special sheaths. The fibres which make up the more
conspicuous muscles of the trunk and limbs, on the other hand, are
much larger, and are invested by a thin, transparent, structureless
sheath, which is termed the _sarcolemma_. Nuclei are scattered, at
intervals, through the striated substance of the muscle; and, in the
larger muscular fibres, a layer of nucleated protoplasm lies between
the sarcolemma and the striated muscle substance.
[Illustration: FIG. 53.—_Astacus fluviatilis._—A, living muscular
fibres very highly magnified; _B_, a fibrilla treated with solution
of sodium chloride; _C_, a fibrilla treated with strong nitric acid,
_s_, septal lines; _sz_, septal zones; _is_, interseptal zones; _a_,
transverse line in the interseptal zone.]
{183}
This much is readily seen in a specimen of muscular fibre taken from
any part of the body, and whether alive or dead. But the results
of the ultimate optical analysis of these appearances, and the
conclusions respecting the normal structure of striped muscle which
may be legitimately drawn from them, have been the subjects of much
controversy.
Quiescent muscular fibres from the chela of the forceps of a
crayfish, examined while still living, without the addition of any
extraneous fluid, and with magnifying powers of not less than seven
or eight hundred diameters, exhibit the following appearance. At
intervals of about 1‐4000th of an inch, very delicate but dark and
well-defined transverse lines are visible; and these, on careful
focussing, appear beaded, as if they were made of a series of
close-set minute granules not more than 1‐20,000th to 1‐30,000th of
an inch in diameter. These may be termed the _septal lines_ (fig. 52,
D and E, _a_; C, _1–5_; fig. 53, _s_). On each side of every septal
line there is a very narrow perfectly transparent band, which may be
distinguished as the _septal zone_ (fig. 53, _sz_). Upon this follows
a relatively broad band of a substance which has a semi-transparent
aspect, like very finely ground glass, and hence appears somewhat
dark relatively to the septal zone. Upon this _inter-septal zone_
(_i s_) follows another septal zone, then a septal line, another
septal zone, an inter-septal zone, and so on throughout the whole
length of the fibre. {184}
In the perfectly unaltered state of the muscle no other transverse
markings than these are discernible. But it is always possible to
observe certain longitudinal markings; and these are of three kinds.
In the first place, the nuclei which, in the perfectly fresh muscle,
are delicate transparent oval bodies, are lodged in spaces which
taper off at each end into narrow longitudinal clefts (fig. 52, A,
B). Prolongations of the protoplasmic sheath of the fibre extend
inwards and fill these clefts. Secondly, there are similar clefts
interposed between these, but narrow and merely linear throughout.
Sometimes these clefts contain fine granules. Thirdly, even in
the perfectly fresh muscle, extremely faint parallel longitudinal
striæ 1‐7,000th of an inch, or thereabouts, apart, traverse the
several zones, so that longer or shorter segments of the successive
septal lines are inclosed between them. A transverse section of
the muscle appears divided into rounded or polygonal areæ of the
same diameter, separated from one another here and there by minute
interstices. Moreover, on examination of perfectly fresh muscle with
high magnifying powers, the septal lines are hardly ever straight
for any distance, but are broken up into short lengths, which answer
to one or more of the longitudinal divisions, and stand at slightly
different heights.
The only conclusion to be drawn from these appearances seems to
me to be that the substance of the muscle is composed of distinct
_fibrils_; and that the longitudinal {185} striæ and the rounded
areæ of the transverse section are simply the optical expressions of
the boundaries of these fibrils. In the perfectly unaltered state of
the tissue, however, the fibrils are so closely packed that their
boundaries are scarcely discernible.
Thus each muscular _fibre_ may be regarded as composed of larger and
smaller bundles of _fibrils_ imbedded in a nucleated protoplasmic
framework which ensheaths the whole and is itself invested by the
sarcolemma.
As the fibre dies, the nuclei acquire hard, dark contours and their
contents become granular, while at the same time the fibrils acquire
sharp and well-defined boundaries. In fact, the fibre may now be
readily teased out with needles, and the fibrils isolated.
In muscle which has been treated with various reagents, such as
alcohol, nitric acid, or solution of common salt, the fibrils
themselves may be split up into filaments of extreme tenuity, each of
which appears to answer to one of the granules of the septal lines.
Such an isolated _muscle filament_ looks like a very fine thread
carrying minute beads at regular intervals.
The septal lines resist most reagents, and remain visible in muscular
fibres which have been subjected to various modes of treatment;
but they may have the appearance of continuous bars, or be more
or less completely resolved into separate granules, according to
circumstances. On the other hand, what is to be seen in {186} the
interspace between every two septal lines depends upon the reagent
employed. With dilute acids and strong solutions of salt, the
inter-septal substance swells up and becomes transparent, so that it
ceases to be distinguishable from the septal zone. At the same time
a distinct but faint transverse line may appear in the middle of its
length. Strong nitric acid, on the contrary, renders the inter-septal
substance more opaque, and the septal zones consequently appear very
well defined.
In living and recently dead muscle, as well as in muscles which
have been preserved in spirit or hardened with nitric acid, the
inter-septal zones polarize light; and hence, in the dark field of
the polarizing microscope, the fibre appears crossed by bright bands,
which correspond with the inter-septal zones, or at any rate, with
the middle parts of them. The substance which forms the septal zones,
on the contrary, produces no such effect, and consequently remains
dark; while the septal lines again have the same property as the
inter-septal substance, though in a less degree.
In fibres which have been acted upon by solution of salt, or dilute
acids, the inter-septal zones have lost their polarizing property.
As we know that the reagents in question dissolve the peculiar
constituent of muscle, _myosin_, it is to be concluded that the
inter-septal substance is chiefly composed of myosin.
Thus a fibril may be considered to be made up of {187}
segments of different material arranged in regular order;
S–sz–IS–sz–S–sz–IS–sz–S: S representing the septal line; sz, the
septal zone; IS, the inter-septal zone. Of these, IS is the chief if
not the only seat of the myosin; what the composition of sz and of S
may be is uncertain, but the supposition, that, in the living muscle,
sz is a mere fluid, appears to me to be wholly inadmissible.
When living muscle contracts, the inter-septal zones become shorter
and wider and their margins darker, while the septal zones and the
septal lines tend to become effaced—as it appears to me simply
in consequence of the approximation of the lateral margins of
the inter-septal zones. It is probable that the substance of the
intermediate zone is the chief, if not the only, seat of the activity
of the muscle during contraction.
5. The elements of the _nervous tissue_ are of two kinds,
_nerve-cells_, and _nerve fibres_; the former are found in the
ganglia, and they vary very much in size (fig. 54, B). Each
ganglionic corpuscle consists of a cell body produced into one or
more processes which sometimes, if not always, end in nerve fibres.
A large, clear spherical nucleus is seen in the interior of the
nerve-cell; and in the centre of this is a well defined, small round
particle, the _nucleolus_. The corpuscle, when isolated, is often
surrounded by a sort of sheath of small nucleated cells. {188}
[Illustration: FIG. 54.—_Astacus fluviatilis._—A, one of the (double)
abdominal ganglia, with the nerves connected with it (× 25); B,
a nerve cell or ganglionic corpuscle (× 250). _a_, sheath of the
nerves; _c_, sheath of the ganglion; _co, co′_, commissural cords
connecting the ganglia with those in front, and those behind them.
_gl.c_, points to the ganglionic corpuscles of the ganglia; _n_,
nerve fibres.]
The nerve fibres (fig. 55) of the crayfish are remarkable for
the large size which some of them attain. In the central nervous
system a few reach as much as 1‐200th of an inch in diameter; and
fibres of 1‐300th or 1‐400th of an inch in diameter are not rare
in the main branches. Each fibre is a tube, formed of a strong and
elastic, sometimes fibrillated, sheath, in which nuclei are imbedded
at irregular intervals; and, when the nerve trunk gives {189}
off a branch, more or fewer of these tubes divide, sending off a
prolongation into each branch.
[Illustration: FIG. 55.—_Astacus fluviatilis._—Three nerve fibres,
with the connective tissue in which they are imbedded (magnified
about 250 diameters); _n_, nuclei.]
When quite fresh, the contents of the tubes are perfectly pellucid,
and without the least indication of structure; and, from the manner
in which the contents exude from the cut ends of the tubes, it is
evident that they consist of a fluid of gelatinous consistency. As
the fibre dies, and under the influence of water and of many chemical
re-agents, the contents break up into globules or become turbid and
finely granular.
Where motor nerve fibres terminate in the muscles to which they are
distributed, the sheath of each fibre becomes continuous with the
sarcolemma of the muscle, and the subjacent protoplasm is commonly
raised into a small prominence which contains several nuclei (fig.
52, F). These are called the _terminal_ or _motor plates_. {190}
6, 7. The _ova_ and the _spermatozoa_ have already been described
(pp. 132–135).
* * * * *
It will be observed that the blood corpuscles, the epithelial
tissues, the ganglionic corpuscles, the ova and the spermatozoa, are
all demonstrably nucleated cells, more or less modified. The first
form of connective tissue is so similar to epithelial tissue, that
it may obviously be regarded as an aggregate of as many cells as it
presents nuclei, the matrix representing the more or less modified
and confluent bodies of the cells, or products of these. But if this
be so, then the second and third forms have a similar composition,
except so far as the matrix of the cells has become fibrillated,
or vacuolated, or marked off into masses corresponding with the
several nuclei. By a parity of reasoning, muscular tissue may also be
considered a cell aggregate, in which the inter-nuclear substance has
become converted into striated muscle; while, in the nerve fibres,
a like process of metamorphosis may have given rise to the pellucid
gelatinous nerve substance. But, if we accept the conclusions thus
suggested by the comparison of the various tissues with one another,
it follows that every histological element, which has now been
mentioned, is either a simple nucleated cell, a modified nucleated
cell, or a more or less modified cell aggregate. In other words,
every tissue is resolvable into nucleated cells. {191}
[Illustration: FIG. 56.—_Astacus fluviatilis._—The structure of the
cuticle. _A_, transverse section of a joint of the forceps (× 4);
_s_, setæ; _B_, a portion of the same (× 30); _C_, a portion of B
more highly magnified. _a_, epiostracum; _b_, ectostracum; _c_,
endostracum; _d_, canal of seta; _e_, canals filled with air; _s_,
seta. _D_, section of an intersternal membrane of the abdomen, the
portion to the right in the natural condition, the remainder pulled
apart with needles (× 20); _E_, small portion of the same, highly
magnified; _a_, intermediate substance; _b_, laminæ. _F_, a seta,
highly magnified; _a_ and _b_, joints.]
{192}
A notable exception to this generalisation, however, obtains in the
case of the _cuticular structures_, in which no cellular components
are discoverable. In its simplest form, such as that presented by
the lining of the intestine, the cuticle is a delicate, transparent
membrane, thrown off from the surface of the subjacent cells, either
by a process of exudation, or by the chemical transformation of
their superficial layer. No pores are discernible in this membrane,
but scattered over its surface there are oval patches of extremely
minute, sharp conical processes, which are rarely more than 1‐5,000th
of an inch long. Where the cuticle is thicker, as in the stomach and
in the exoskeleton, it presents a stratified appearance, as if it
were composed of a number of laminæ, of varying thickness, which had
been successively thrown off from the subjacent cells.
Where the cuticular layer of the integument is uncalcified, for
example, between the sterna of the abdominal somites, it presents an
external, thin, dense, wrinkled lamina, the _epiostracum_, followed
by a soft substance, which, on vertical section, presents numerous
alternately more transparent and more opaque bands, which run
parallel with one another and with the free surfaces of the slice
(fig. 56, D). These bands are very close-set, often not more than
1‐5000th of an inch apart near the outer and the inner surfaces, but
in the middle of the section they are more distant.
If a thin vertical slice of the soft cuticle is gently {193} pulled
with needles in the direction of its depth, it stretches to eight or
ten times its previous diameter, the clear intervals between the dark
bands becoming proportionally enlarged, especially in the middle of
the slice, while the dark bands themselves become apparently thinner,
and more sharply defined. The dark bands may then be readily drawn
to a distance of as much as 1‐300th of an inch from one another; but
if the slice is stretched further, it splits along, or close to, one
of the dark lines. The whole of the cuticular layer is stained by
such colouring matters as hæmatoxylin; and, as the dark bands become
more deeply coloured than the intermediate transparent substance, the
transverse stratification is made very manifest by this treatment.
Examined with a high magnifying power, the transparent substance is
seen to be traversed by close-set, faint, vertical lines, while the
dark bands are shown to be produced by the cut edges of delicate
laminæ, having a finely striated appearance, as if they were composed
of delicate parallel wavy fibrillæ.
In the calcified parts of the exoskeleton a thin, tough, wrinkled
epiostracum (fig. 56, B, _a_), and, subjacent to this, a number of
alternately lighter and darker strata are similarly discernible:
though all but the innermost laminæ are hardened by a deposit of
calcareous salts, which are generally evenly diffused, but sometimes
take the shape of rounded masses with irregular contours.
Immediately beneath the epiostracum, there is a zone {194} which
may occupy a sixth or a seventh of the thickness of the whole, which
is more transparent than the rest, and often presents hardly any
trace of horizontal or vertical striation. When it appears laminated,
the strata are very thin. This zone may be distinguished as the
_ectostracum_ (_b_), from the _endostracum_ (_c_), which makes up the
rest of the exoskeleton. In the outer part of the endostracum, the
strata are distinct, and may be as much as 1‐500th of an inch thick,
but in the inner part they become very thin, and the lines which
separate them may be not more than 1‐8000th of an inch apart. Fine,
parallel, close-set, vertical striæ (_e_) traverse all the strata of
the endostracum, and may usually be traced through the ectostracum,
though they are always faint, and sometimes hardly discernible, in
this region. When a high magnifying power is employed, it is seen
that these striæ, which are about 1‐7000th of an inch apart, are
not straight, but that they present regular short undulations, the
alternate convexities and concavities of which correspond with the
light and the dark bands respectively.
If the hard exoskeleton has been allowed to become partially or
wholly dry before the section is made, the latter will look white
by reflected and black by transmitted light, in consequence of the
places of the striæ being taken by threads of air of such extreme
tenuity, that they may measure not more than 1‐30,000th of an inch in
diameter. It is to be concluded, therefore, that {195} the striæ are
the optical indications of parallel undulating canals which traverse
the successive strata of the cuticle, and are ordinarily occupied by
a fluid. When this dries up, the surrounding air enters, and more or
less completely fills the tubes. And that this is really the case
may be proved by making very thin sections parallel with the face of
the exoskeleton, for these exhibit innumerable minute perforations,
set at regular distances from one another, which correspond with the
intervals between the striæ in the vertical section; and sometimes
the contours of the areæ which separate the apertures are so well
defined as to suggest a pavement of minute angular blocks, the
corners of which do not quite meet.
When a portion of the hard exoskeleton is decalcified, a chitinous
substance remains, which presents the same structure as that just
described, except that the epiostracum is more distinct; while the
ectostracum appears made up of very thin laminæ, and the tubes are
represented by delicate striæ, which appear coarser in the region of
the dark zones. As in the naturally soft parts of the exoskeleton,
the decalcified cuticle may be split into flakes, and the pores are
then seen to be disposed in distinct areæ circumscribed by clear
polygonal borders. These perforated areæ appear to correspond with
individual cells of the ectoderm, and the canals thus answer to the
so-called “pore-canals,” which are common in cuticular structures and
in the walls of many cells which bound free surfaces. {196}
The whole exoskeleton of the crayfish is, in fact, produced by the
cells which underlie it, either by the exudation of a chitinous
substance, which subsequently hardens, from them; or, as is more
probable, by the chemical metamorphosis of a superficial zone of the
bodies of the cells into chitin. However this may be, the cuticular
products of adjacent cells at first form a simple, continuous, thin
pellicle. A continuation of the process by which it was originated
increases the thickness of the cuticle; but the material thus added
to the inner surface of the latter is not always of the same nature,
but is alternately denser and softer. The denser material gives rise
to the tough laminæ, the softer to the intermediate transparent
substance. But the quantity of the latter is at first very small,
whence the more external laminæ are in close apposition. Subsequently
the quantity of the intermediate substance increases, and gives rise
to the thick stratification of the middle region, while it remains
insignificant in the inner region of the exoskeleton.
The cuticular structures of the crayfish differ from the nails,
hairs, hoofs, and similar hard parts of the higher animals, insomuch
as the latter consist of aggregations of cells, the bodies of which
have been metamorphosed into horny matter. The cuticle, with all its
dependencies, on the contrary, though no less dependent on cells
for its existence, is a derivative product, the formation of which
does not involve the complete {197} metamorphosis and consequent
destruction of the cells to which it owes its origin.
The calcareous salts by which the calcified exoskeleton is hardened
can only be supplied by the infiltration of a fluid in which they are
dissolved from the blood; while the distinctive structural characters
of the epiostracum, the ectostracum, and the endostracum, are the
results of a process of metamorphosis which goes on _pari passu_ with
this infiltration. To what extent this metamorphosis is a properly
vital process; and to what extent it is explicable by the ordinary
physical and chemical properties of the animal membrane on the one
hand, and the mineral salts on the other, is a curious, and at
present, unsolved problem.
The outer surface of the cuticle is rarely smooth. Generally it is
more or less obviously ridged or tuberculated; and, in addition,
presents coarser or finer hair-like processes which exhibit every
gradation from a fine microscopic down to stout spines. As these
processes, though so similar to hairs in general appearance, are
essentially different from the structures known as hairs in the
higher animals, it is better to speak of them as _setæ_.
These setæ (fig. 56, F) are sometimes short, slender, conical
filaments, the surface of which is quite smooth; sometimes
the surface is produced into minute serrations, or scale-like
prominences, disposed in two or more series; in other setæ, the
axis gives off slender lateral {198} branches; and in the most
complicated form the branches are ornamented with lateral branchlets.
For a certain distance from the base of the seta, its surface is
usually smooth, even when the rest of its extent is ornamented with
scales or branches. Moreover, the basal part of the seta is marked
off from its apical moiety by a sort of joint which is indicated by
a slight constriction, or by a peculiarity in the structure of the
cuticula at this point. A seta almost always takes its origin from
the bottom of a depression or pit of the layer of cuticle, from which
it is developed, and at its junction with the latter it is generally
thin and flexible, so that the seta moves easily in its socket. Each
seta contains a cavity, the boundaries of which generally follow the
outer contours of the seta. In a good many of the setæ, however, the
parietes, near the base of the seta, are thickened in such a manner
as almost, or completely, to obliterate the central cavity. However
thick the cuticle may be at the point from which the setæ take their
origin, it is always traversed by a funnel-shaped canal (fig. 56, B,
_d_), which usually expands beneath the base of the seta. Through
this canal the subjacent ectoderm extends up to the base of the seta,
and can even be traced for some distance into its interior.
It has already been mentioned that the apodemata and the tendons of
the muscles are infoldings of the cuticle, embraced and secreted by
corresponding involutions of the ectoderm. {199}
Thus the body of the crayfish is resolvable, in the first place, into
a repetition of similar segments, the _metameres_, each of which
consists of a somite and two appendages; the metameres are built
up out of a few simple _tissues_; and, finally, the tissues are
either aggregates of more or less modified nucleated _cells_, or are
products of such cells. Hence, in ultimate morphological analysis,
the crayfish is a multiple of the histological unit, the nucleated
cell.
What is true of the crayfish, is certainly true of all animals, above
the very lowest. And it cannot yet be considered certain that the
generalization fails to hold good even of the simplest manifestations
of animal life; since recent investigations have demonstrated the
presence of a nucleus in organisms in which it had hitherto appeared
to be absent.
However this may be, there is no doubt that in the case of man and
of all vertebrated animals, in that of all arthropods, mollusks,
echinoderms, worms, and inferior organisms down to the very lowest
sponges, the process of morphological analysis yields the same result
as in the case of the crayfish. The body is built up of tissues, and
the tissues are either obviously composed of nucleated cells; or,
from the presence of nuclei, they may be assumed to be the results of
the metamorphosis of such cells; or they are cuticular structures.
The essential character of the nucleated cell is that it consists
of a protoplasmic substance, one part of which differs somewhat
in its physical and chemical characters {200} from the rest, and
constitutes the nucleus. What part the nucleus plays in relation to
the functions, or vital activities, of the cell is as yet unknown;
but that it is the seat of operations of a different character from
those which go on in the body of the cell is clear enough. For, as we
have seen, however different the several tissues may be, the nuclei
which they contain are very much alike; whence it follows, that if
all these tissues were primitively composed of simple nucleated
cells, it must be the bodies of the cells which have undergone
metamorphosis, while the nuclei have remained relatively unchanged.
On the other hand, when cells multiply, as they do in all growing
parts, by the division of one cell into two, the signs of the process
of internal change which ends in fission are apparent in the nucleus
before they are manifest in the body of the cell; and, commonly, the
division of the former precedes that of the latter. Thus a single
cell body may possess two nuclei, and may become divided into two
cells by the subsequent aggregation of the two moieties of its
protoplasmic substance round each of them, as a centre.
In some cases, very singular structural changes take place in the
nuclei in the course of the process of cell-division. The granular
or fibrillar contents of the nucleus, the wall of which becomes less
distinct, arrange themselves in the form of a spindle or double
cone, formed of extremely delicate filaments; and in the plane
{201} of the base of the double cone the filaments present knots
or thickenings, just as if they were so many threads with a bead in
the middle of each. When the nuclear spindle is viewed sideways,
these beads or thickenings give rise to the appearance of a disk
traversing the centre of the spindle. Soon each bead separates into
two, and these move away from one another, but remain connected by
a fine filament. Thus the structure which had the form of a double
cone, with a disk in the middle, assumes that of a short cylinder,
with a disk and a cone at each end. But as the distance between the
two disks increases, the uniting filaments lose their parallelism,
converge in the middle, and finally separate, so that two separate
double cones are developed in place of the single one. Along with
these changes in the nucleus, others occur in the protoplasm of the
cell body, and its parts commonly display a tendency to arrange
themselves in radii from the extremities of the cones as a centre;
while, as the separation of the two secondary nuclear spindles
becomes complete, the cell body gradually splits from the periphery
inwards, in a direction at right angles to the common axis of the
spindles and between their apices. Thus two cells are formed, where,
previously, only one existed; and the nuclear spindles of each soon
revert to the globular form and confused arrangement of the contents,
characteristic of nuclei in their ordinary state. The formation of
these nuclear spindles is very beautifully seen in the epithelial
cells of the testis of the {202} crayfish (fig. 33, p. 132); but I
have not been able to find distinct evidence of it elsewhere in this
animal; and although the process has now been proved to take place in
all the divisions of the animal kingdom, it would seem that nuclei
may, and largely do, undergo division, without becoming converted
into spindles.
The most cursory examination of any of the higher plants shows that
the vegetable, like the animal body, is made up of various kinds of
tissues, such as pith, woody fibre, spiral vessels, ducts, and so on.
But even the most modified forms of vegetable tissue depart so little
from the type of the simple cell, that the reduction of them all to
that common type is suggested still more strongly than in the case
of the animal fabric. And thus the nucleated cell appears to be the
morphological unit of the plant no less than of the animal. Moreover,
recent inquiry has shown that in the course of the multiplication of
vegetable cells by division, the nuclear spindles may appear and run
through all their remarkable changes by stages precisely similar to
those which occur in animals.
The question of the universal presence of nuclei in cells may be left
open in the case of Plants, as in that of Animals; but, speaking
generally, it may justly be affirmed that the nucleated cell is the
morphological foundation of both divisions of the living world; and
the great generalisation of Schleiden and Schwann, that there is a
fundamental agreement in structure and {203} development between
plants and animals, has, in substance, been merely confirmed and
illustrated by the labours of the half century which has elapsed
since its promulgation.
Not only is it true that the minute structure of the crayfish is, in
principle, the same as that of any other animal, or of any plant,
however different it may be in detail; but, in all animals (save some
exceptional forms) above the lowest, the body is similarly composed
of three layers, ectoderm, mesoderm, and endoderm, disposed around
a central alimentary cavity. The ectoderm and the endoderm always
retain their epithelial character; while the mesoderm, which is
insignificant in the lower organisms, becomes, in the higher, far
more complicated even than it is in the crayfish.
Moreover, in the whole of the _Arthropoda_, and the whole of the
_Vertebrata_, to say nothing of other groups of animals, the body,
as in the crayfish, is susceptible of distinction into a series of
more or less numerous segments, composed of homologous parts. In
each segment these parts are modified according to physiological
requirements; and by the coalescence, segregation, and change of
relative size and position of the segments, well characterized
regions of the body are marked out. And it is remarkable that
precisely the same principles are illustrated by the morphology of
plants. A flower with its whorls of sepals, petals, stamens and
carpels has the same relation to a stem {204} with its whorls of
leaves, as a crayfish’s head has to its abdomen, or a dog’s skull to
its thorax.
* * * * *
It may be objected, however, that the morphological generalisations
which have now been reached, are to a considerable extent of a
speculative character; and that, in the case of our crayfish, the
facts warrant no more than the assertion that the structure of that
animal may be consistently interpreted, on the supposition that the
body is made up of homologous somites and appendages, and that the
tissues are the result of the modification of homologous histological
elements or cells; and the objection is perfectly valid.
There can be no doubt that blood corpuscles, liver cells, and ova
are all nucleated cells; nor any that the third, fourth, and fifth
somites of the abdomen are constructed upon the same plan; for
these propositions are mere statements of the anatomical facts. But
when, from the presence of nuclei in connective tissue and muscles,
we conclude that these tissues are composed of modified cells; or
when we say that the ambulatory limbs of the thorax are of the
same type as the abdominal limbs, the exopodite being suppressed,
the statement, as the evidence stands at present, is no more than
a convenient way of interpreting the facts. The question remains,
has the muscle actually been formed out of nucleated cells? Has the
ambulatory limb ever possessed an exopodite, and lost it? {205}
The answer to these questions is to be sought in the facts of
individual and ancestral development.
An animal not only is, but becomes; the crayfish is the product of
an egg, in which not a single structure visible in the adult animal
exists: in that egg the different tissues and organs make their
appearance by a gradual process of evolution; and the study of this
process can alone tell us whether the unity of composition suggested
by the comparison of adult structures, is borne out by the facts of
their development in the individual or not. The hypothesis that the
body of the crayfish is made up of a series of homologous somites and
appendages, and that all the tissues are composed of nucleated cells,
might be only a permissible, because a useful, mode of colligating
the facts of anatomy. The investigation of the actual manner in which
the evolution of the body of the crayfish has been effected, is the
only means of ascertaining whether it is anything more. And, in this
sense, development is the criterion of all morphological speculations.
* * * * *
The first obvious change which takes place in an impregnated ovum is
the breaking up of the yelk into smaller portions, each of which is
provided with a nucleus, and is termed a _blastomere_. In a general
morphological sense, a blastomere is a nucleated cell, and differs
from an ordinary cell only in size, and in the usual, though by no
means invariable, abundance of granular contents; and blastomeres
insensibly pass into ordinary cells, as {206} the process of
division of the yelk into smaller and smaller portions goes on.
In a great many animals, the splitting-up into blastomeres is
effected in such a manner that the yelk is, at first, divided into
equal, or nearly equal, masses; that each of these again divides into
two; and that the number of blastomeres thus increases in geometrical
progression until the entire yelk is converted into a mulberry-like
body, termed a _morula_, made up of a great number of small
blastomeres or nucleated cells. The whole organism is subsequently
built up by the multiplication, the change of position, and the
metamorphosis of these products of yelk division.
In such a case as this, yelk division is said to be _complete_. An
unessential modification of complete yelk division is seen when, at
an early period, the blastomeres produced by division are of unequal
sizes; or when they become unequal in consequence of division taking
place much more rapidly in one set than in another.
In many animals, especially those which have large ova, the
inequality of division is pushed so far that only a portion of
the yelk is affected by the process of fission, while the rest
serves merely as _food-yelk_, for nutriment to the blastomeres
thus produced. Over a greater or less extent of the surface of the
egg, the protoplasmic substance of the yelk segregates itself from
the rest, and, constituting a _germinal layer_, breaks up into the
blastomeres, which multiply at the expense of the {207} food-yelk,
and fabricate the body of the embryo. This process is termed
_partial_ or _incomplete_ yelk division.
The crayfish is one of those animals in the egg of which the yelk
undergoes partial division. The first steps of the process have not
yet been thoroughly worked out, but their result is seen in ova which
have been but a short time laid (fig. 57, A). In such eggs, the great
mass of the substance of the vitellus is destined to play the part
of food-yelk; and it is disposed in conical masses, which radiate
from a central spheroidal portion to the periphery of the yelk
(_v_). Corresponding with the base of each cone, there is a clear
protoplasmic plate, which contains a nucleus; and as these bodies are
all in contact by their edges, they form a complete, though thin,
investment to the food-yelk. This is termed the _blastoderm_ (_bl_).
Each nucleated protoplasmic plate adheres firmly to the corresponding
cone of granular food-yelk, and, in all probability, the two together
represent a blastomere; but, as the cones only indirectly subserve
the growth of the embryo, while the nucleated peripheral plates form
an independent spherical sac, out of which the body of the young
crayfish is gradually fashioned, it will be convenient to deal with
the latter separately.
[Illustration: FIG. 57.—_Astacus fluviatilis._—Diagrammatic sections
of embryos; partly after Reichenbach, partly original (× 20). A. An
ovum in which the blastoderm is just formed. B. An ovum in which
the invagination of the blastoderm to constitute the hypoblast or
rudiment of the mid-gut has taken place. (This nearly answers to the
stage represented in fig. 58, _A._) C. A longitudinal section of an
ovum, in which the rudiments of the abdomen, of the hind-gut, and
of the fore-gut have appeared. (This nearly answers to the stage
represented in fig. 58, E.) D. A similar section of an embryo in
nearly the same stage of development as that represented in C, fig.
59. E. An embryo just hatched, in longitudinal section; _a_, anus;
_bl_, blastoderm; _bp_, blastopore; _e_, eye; _ep. b._, epiblast;
_fg_, fore-gut; _fg_^1, its œsophageal, and _fg_^2, its gastric
portion; _h_, heart; _hg_, hind-gut; _m_, mouth; _mg_, hypoblast,
archenteron, or mid-gut; _v_, yelk. The dotted portions in D and E
represent the nervous system.]
Thus, at this period, the body of the developing crayfish is nothing
but a spherical bag, the thin walls of which are composed of a
single layer of nucleated cells, while its cavity is filled with
food-yelk. The first modification {209} which is effected in the
vesicular blastoderm manifests itself on that face of it which is
turned towards the pedicle of the egg. Here the layer of cells
becomes thickened throughout an oval area about 1‐25th of an inch in
diameter. Hence, when the egg is viewed by reflected light, a whitish
patch of corresponding form and size appears in this region. This may
be termed the _germinal disk_. Its long axis corresponds with that of
the future crayfish.
Next, a depression (fig. 58, A, _bp_) appears in the hinder third
of the germinal disk, in consequence of this part of the blastoderm
growing inwards, and thus giving rise to a small wide-mouthed
pouch, which projects into the food-yelk with which the cavity of
the blastoderm is filled (fig. 57, B, _mg_). As this infolding, or
invagination of the blastoderm, goes on, the pouch thus produced
increases, while its external opening, termed the _blastopore_ (fig.
57, B, and 58, A–E, _bp_), diminishes in size. Thus the body of
the embryo crayfish, from being a simple bag becomes a double bag,
such as might be produced by pushing in the wall of an incompletely
distended india-rubber ball with the finger. And, in this case, if
the interior of the bag contained porridge, the latter would very
fairly represent the food-yelk.
[Illustration: FIG. 58.—_Astacus fluviatilis._—Surface views of the
earlier stages in the development of the embryo, from the appearance
of the blastopore (A) to the assumption of the nauplius form (F)
(after Reichenbach, × about 23). _bp_, blastopore; _c_, carapace;
_fg_, fore-gut involution; _h_, heart; _hg_, hind-gut involution;
_lb_, labrum; _mg_, medullary groove; _o_, optic pit; _p_, endodermal
plug partly filling up the blastopore; _pc_, procephalic processes;
_ta_, abdominal elevation; _2_, antennules; _3_, antennæ; _4_,
mandibles.]
By this invagination a most important step has been taken in the
development of the crayfish. For, though the pouch is nothing but an
ingrowth of part of the blastoderm, the cells of which its wall is
composed {211} henceforward exhibit different tendencies from those
which are possessed by the rest of the blastoderm. In fact, it is
the primitive alimentary apparatus or _archenteron_, and its wall is
termed the _hypoblast_. The rest of the blastoderm, on the contrary,
is the primitive epidermis, and receives the name of _epiblast_.
If the food-yelk were away, and the archenteron enlarged until the
hypoblast came in contact with the epiblast, the entire body would be
a double-walled sac, containing an alimentary cavity, with a single
external aperture. This is the _gastrula_ condition of the embryo;
and some animals, such as the common fresh-water polype, are little
more than permanent _gastrulæ_.
Although the gastrula has not the slightest resemblance to a
crayfish, yet, as soon as the hypoblast and the epiblast are thus
differentiated, the foundations of some of the most important systems
of organs of the future crustacean are laid. The hypoblast will
give rise to the epithelial lining of the mid-gut; the epiblast
(which answers to the ectoderm in the adult) to the epithelia of the
fore-gut and hind-gut, to the epidermis, and to the central nervous
system.
The mesodermal structures, that is to say the connective tissue,
the muscles, the heart and vessels, and the reproductive organs,
which lie between the ectoderm and the endoderm, are not derived
directly from either the epiblast or the hypoblast, but have a
_quasi_-independent origin, from a mass of cells which first makes
its {212} appearance in the neighbourhood of the blastopore, between
the hypoblast and the epiblast, though they are probably derived
from the former. From this region they gradually spread, first over
the sternal, and then on to the tergal aspect of the embryo, and
constitute the _mesoblast_.
Epiblast, hypoblast, and mesoblast are at first alike constituted
of nothing but nucleated cells, and they increase in dimensions by
the continual fission and growth of these cells. The several layers
become gradually modelled into the organs which they constitute,
before the cells undergo any notable modification into tissues.
A limb, for example, is, at first, a mere cellular outgrowth, or
bud, composed of an outer coat of epiblast with an inner core of
mesoblast; and it is only subsequently that its component cells are
metamorphosed into well-defined epidermic and connective tissues,
vessels and muscles.
The embryo crayfish remains only a short while in the gastrula stage,
as the blastopore soon closes up, and the archenteron takes the form
of a sac, flattened out between the epiblast and the food-yelk, with
which its cells are in close contact (fig. 57, C and D).[12] Indeed,
as development proceeds, the cells of the hypoblast actually feed
upon the substance of the food-yelk, and turn it to account for the
general nutrition of the body. {213}
[12] Whether, as some observers state, the hypoblastic cells
grow over and inclose the food-yelk or not, is a question that
may be left open. I have not been able to satisfy myself of this
fact.
The sternal area of the embryo gradually enlarges until it occupies
one hemisphere of the yelk; in other words, the thickening of the
epiblast gradually extends outwards. Just in front of the blastopore,
as it closes, the middle of the epiblast grows out into a rounded
elevation (fig. 58, _t a_; fig. 59, _ab_), which rapidly increases
in length, and at the same time turns forwards. This is the rudiment
of the whole abdomen of the crayfish. Further forwards, two broad
and elongated, but flatter thickenings appear; one on each side of
the middle line (fig. 58, _p c_). As the free end of the abdominal
papilla now marks the hinder extremity of the embryo, so do these
two elevations, which are termed the _procephalic lobes_, define
its anterior termination. The whole sternal region of the body will
be produced by the elongation of that part of the embryo which lies
between these two limits.
A narrow longitudinal groove-like depression appears on the surface
of the epiblast, in the middle line, between the procephalic lobes
and the base of the abdominal papilla (fig. 58, C–F, _m g_). About
its centre, this groove becomes further depressed by the ingrowth
of the epiblast, which constitutes its floor, and gives rise to a
short tubular sac, which is the rudiment of the whole fore-gut (fig.
57, C, and fig. 58, E, _f g_). At first, this epiblastic ingrowth
does not communicate with the archenteron, but, after a while, its
blind end combines with the front and lower part of the hypoblast,
and an opening is formed by {214} which the cavity of the fore-gut
communicates with that of the mid-gut (fig. 57, E). Thus a gullet
and stomach, or rather the parts which will eventually give rise to
all these, are constituted. And it is important to remark that, in
comparison with the mid-gut, they are, at first, very small.
In the same way, the epiblast covering the sternal face of the
abdominal papilla undergoes invagination and is converted into a
narrow tube which is the origin of the whole hind-gut (fig. 57, C,
and fig. 58, E, _hg_). This, like the fore-gut, is at first blind;
but the shut front end soon applying itself to the hinder wall of the
archenteric sac, the two coalesce and open into one another (fig.
57, E). Thus the complete alimentary canal, consisting of a very
narrow, tubular, fore- and hind-gut, derived from the epiblast, and
a wider and more sac-like mid-gut, formed of the whole hypoblast, is
constituted.
The procephalic lobes become more convex; while, behind them, the
surface of the epiblast rises into six elevations disposed in pairs,
one on each side of the median groove. The hindermost of these, which
lie at the sides of the mouth, are the rudiments of the mandibles
(fig. 58, E and F, _4_); the other two become the antennæ (_3_) and
the antennules (_2_), while, at a later period, processes of the
procephalic lobes give rise to the eyestalks.
A short distance behind the abdomen, the epiblast rises into a
transverse ridge, which is concave forwards, {215} while its ends
are prolonged on each side nearly as far as the mouth. This is the
commencement of the free edge of the carapace (fig. 58, E and F,
and fig. 59, A, _c_)—the lateral parts of which, greatly enlarging,
become the branchiostegites (fig. 59, D, _c_).
In many animals allied to crayfish, the young, when it has reached
a stage in its development, which answers to this, undergoes rapid
changes of outward form and of internal structure, without making any
essential addition to the number of the appendages. The appendages
which represent the antennules, the antennæ, and the mandibles
elongate and become oar-like locomotive organs; a single median
eye is developed, and the young leaves the egg as an active larva,
which is known as a _Nauplius_. The crayfish, on the other hand,
is wholly incapable of an independent existence at this stage,
and continues its embryonic life within the egg case; but it is a
remarkable circumstance that the cells of the epiblast secrete a
delicate cuticula, which is subsequently shed. It is as if the animal
symbolized a nauplius condition by the development of this cuticle,
as the fœtal whalebone whale symbolizes a toothed condition by
developing teeth which are subsequently lost and never perform any
function.
[Illustration: FIG. 59.—_Astacus fluviatilis._—Ventral (A, B, C,
F) and lateral (D, E) views of the embryo in successive stages of
development (after Rathke, × 15). A is a little more advanced than
the embryo represented in fig. 58, F: D, E, and F are views of the
young crayfish when nearly ready to be hatched: in E, the carapace
is removed, and the limbs and abdomen are spread out. 1–14, the
cephalic and thoracic appendages; _ab_, abdomen; _br_, branchiæ;
_c_, carapace; _ep_, epipodite of the first maxillipede; _gg_,
green gland; _h_, heart; _lb_, labrum; _lr_, liver; _m_, mandibular
muscles.]
In fact, in the crayfish, the nauplius condition is soon left behind.
The sternal disk spreads more and more over the yelk; as the region
between the mouth and the root of the abdomen elongates, slight
transverse {217} depressions indicate the boundaries of the posterior
cephalic and the thoracic somites; and pairs of elevations, similar
to the rudiments of the antennules and antennæ, appear upon them in
regular order from before backwards (fig. 59, C).
In the meanwhile, the extremity of the abdomen flattens out and takes
on the form of an oval plate, the middle of the posterior margin of
which is slightly truncated or notched; while, finally, transverse
constrictions mark off six segments, the somites of the abdomen, in
front of this. Along with these changes, four pairs of tubercles grow
out from the sternal faces of the four middle abdominal somites,
and constitute the rudiments of the four middle pairs of abdominal
appendages. The first abdominal somite exhibits only two hardly
perceptible elevations in place of the appendages of the others,
while the sixth seems, at first, to have none. The appendages of the
sixth somite, however, are already formed, though, singularly enough,
they lie beneath the cuticle of the telson and are set free only
after the first ecdysis.
The rostrum grows out between the procephalic lobes; it remains
relatively very short up to the time that the young crayfish quits
the egg, and is directed more downwards than forwards. The lateral
portions of the carapacial ridge, becoming deeper, are converted into
the branchiostegites, and the cavities which they overarch are the
branchial chambers. The transverse portion of {218} the ridge, on
the other hand, remains relatively short, and constitutes the free
posterior margin of the carapace.
As these changes take place, the abdomen and the sternal region of
the thorax are constantly enlarging in proportion to the rest of
the ovum; and the food-yelk which lies in the cephalothorax is,
_pari passu_, being diminished. Hence the cephalothorax constantly
becomes relatively smaller and the tergal aspect of the carapace less
spherical; although, even when the young crayfish is ready to be
hatched, the difference between it and the adult in the form of the
cephalothoracic region, and in the size of the latter relatively to
the abdomen, is very marked.
The simple bud-like outgrowths of the somites, in which all the
appendages take their origin, are rapidly metamorphosed. The
eyestalks (fig. 59, _1_) soon attain a considerable relative size.
The extremities of the antennules (_2_) and of the antennæ (_3_)
become bifurcated; and the two divisions of the antennule remain
broad, thick, and of nearly the same size up to birth. On the other
hand, the inner or endopoditic division of the antenna becomes
immensely lengthened, and at the same time annulated, while the outer
or exopoditic division remains relatively short, and acquires its
characteristic scale-like form.
The labrum (_lb_) arises as a prolongation of the middle sternal
region in front of the mouth, while the bilobed metastoma is an
outgrowth of the sternal region behind it. {219}
The posterior cephalic and the thoracic appendages (_5–14_) elongate
and gradually approach the form which they possess in the adult. I
have not been able to discover, at any period of development, an
outer division or exopodite in any of the five posterior thoracic
limbs. And this is a very remarkable circumstance, inasmuch as such
an exopodite exists in the closely allied lobster in the larval
state; and, in many of the shrimp and prawn-like allies of the
crayfish, a complete or rudimentary exopodite is found in these
limbs, even in the adult condition.
When the crayfish is hatched (fig. 60) it differs from the adult in
many ways—not only is the cephalothorax more convex and larger in
proportion to the abdomen; but the rostrum is short and bent down
between the eyes. The sterna of the thorax are wider relatively,
and hence there is a greater interval between the bases of the legs
than in the adult. The proportion of the limbs to one another and
to the body are nearly the same as in the adult, but the chelæ of
the forceps are more slender. The tips of the chelæ are all strongly
incurved (fig. 8, B, p. 41), and the dactylopodites of the two
posterior thoracic limbs are hook-like. The appendages of the first
abdominal somite are undeveloped, and those of the last are inclosed
within the telson, which is, as has already been said, of a broad
oval form, usually notched in the middle of its hinder margin, and
devoid of any indication of transverse division. Its margins are
produced into a single series of short conical {220} processes, and
the disposition of the vascular canals in its interior gives it the
appearance of being radially striated.
The setæ, so abundant in the adult, are very scanty in the newly
hatched young; and the great majority of those which exist are simple
conical prolongations of the uncalcified cuticle, the bases of which
are not sunk in pits and which are devoid of lateral scales or
processes.
[Illustration: FIG. 60.—_Astacus fluviatilis._—Newly-hatched young
(× 6).]
The young animals are firmly attached to the abdominal appendages of
the parent in the manner already described. They are very sluggish,
though they move when touched; and at this period they do not feed,
but {221} are nourished by the food-yelk, of which a considerable
store still remains in the cephalothorax.
I imagine that they are set free during the first ecdysis, and
that the appendages of the sixth abdominal somite are at that time
expanded, but nothing is definitely known at present of these changes.
* * * * *
The foregoing sketch of the general nature of the changes which
take place in the egg of the crayfish suffice to show that its
development is, in the strictest sense of the word, a process of
evolution. The egg is a relatively homogeneous mass of living
protoplasmic matter, containing much nutritive material; and the
development of the crayfish means the gradual conversion of this
comparatively simple body into an organism of great complexity. The
yelk becomes differentiated into formative and nutritive portions.
The formative portion is subdivided into histological units: these
arrange themselves into a blastodermic vesicle; the blastoderm
becomes differentiated into epiblast, hypoblast, and mesoblast;
and the simple vesicle assumes the gastrula condition. The layers
of the gastrula shape themselves into the body of the crayfish and
its appendages, while along with this, the cells of which all the
parts are built, become metamorphosed into tissues, each with its
characteristic properties. And all these wonderful changes are the
necessary consequences of the interaction of the molecular forces
resident in the substance of the {222} impregnated ovum, with the
conditions to which it is exposed; just as the forms evolved from a
crystallising fluid are dependent upon the chemical composition of
the dissolved matter and the influence of surrounding conditions.
Without entering into details which lie beyond the scope of the
present work, something must be said respecting the manner in which
the complicated internal organisation of the crayfish is evolved from
the cellular double sac of the gastrula stage.
It has been seen that the fore-gut is at first an insignificant
tubular involution of the epiblast in the region of the mouth. It
is, in fact, a part of the epiblast turned inwards, and the cells
of which it is composed secrete a thin cuticular layer, as do those
of the rest of the epiblast, which gives rise to the ectodermal or
epidermic part of the integument. As the embryo grows, the fore-gut
enlarges much faster than the mid-gut, increasing in height and from
before backwards, while its side-walls remain parallel, and are
separated by only a narrow cavity. At length, it takes on the shape
of a triangular bag (fig. 57, D, _fg_), attached by its narrow end
around the mouth and immersed in the food-yelk, which it gradually
divides into two lobes, one on the right and one on the left side.
At the same time a vertical plate of mesoblastic tissue, from which
the great anterior and posterior muscles are eventually developed,
connects it with the roof and with the front wall of the carapace.
{223} Becoming constricted in the middle, the fore-gut next appears
to consist of two dilatations of about equal size, connected by a
narrower passage (fig. 57, E, _fg_^1, _fg_^2). The front dilatation
becomes the œsophagus and the cardiac division of the stomach; the
hinder one, the pyloric division. At the sides of the front end of
the cardiac division two small pouches are formed shortly after
birth; in each of these a thick laminated deposit of chitin takes
place, and constitutes a minute crab’s-eye or gastrolith, which
has the same structure as in the adult, and is largely calcified.
This fact is the more remarkable as, at this time, the exoskeleton
contains very little calcareous deposit. In the position of the
gastric teeth, folds of the cellular wall of corresponding shape are
formed, and the chitinous cuticle of which the teeth are composed is,
as it were, modelled upon them.
The hind-gut occupies the whole length of the abdomen, and its cells
early arrange themselves into six ridges, and secrete a cuticular
layer.
The mid-gut, or hypoblastic sac, very soon gives off numerous small
prolongations on each side of its hinder extremity, and these are
converted into the cæca of the liver (fig. 57, E, _mg_). The cells
of its tergal wall are in close contact with the adjacent masses of
food-yelk; and it is probable that the gradual absorption of the
food-yelk is chiefly effected by these cells. At birth, however, the
lateral lobes of the food-yelk are still large, and occupy the space
left between the stomach and liver {224} on the one hand, and the
cephalic integument on the other.
The mesoblastic cells give rise to the layer of connective tissue
which forms the deeper portion of the integument, and to that which
invests the alimentary canal; to all the muscles; and to the heart,
the vessels, and the corpuscles of the blood. The heart appears very
early as a solid mass of mesoblastic cells in the tergal region of
the thorax, just in front of the origin of the abdomen (figs. 57, 58,
59, _h_). It soon becomes hollow, and its walls exhibit rhythmical
contractions.
The branchiæ are, at first, simple papillæ of the integument of the
region from which they take their rise. These papillæ elongate into
stems, which give off lateral filaments. The podobranchiæ are at
first similar to the arthrobranchiæ, but an outgrowth soon takes
place near the free end of the stem, and becomes the lamina, while
the attached end enlarges into the base.
The renal organ is stated to arise by a tubular involution of the
epiblast, which soon becomes convoluted, and gives rise to the green
gland.
The central nervous system is wholly a product of the epiblast. The
cells which lie at the sides of the longitudinal groove already
mentioned (fig. 58, _mg_), grow inwards, and give rise to two cords
which are at first separate from one another and continuous with
the rest of the epiblast. At the front end of the groove a {225}
depression arises, and its cells form a mass which connects these
two cords in front of the mouth, and gives rise to the cerebral
ganglia. The epiblastic linings of two small pits (fig. 58, _o_)
which appear very early on the surface of the procephalic lobes,
are also carried inwards in the same way, and, uniting with the
foregoing, produce the optic ganglia.
The cells of the longitudinal cords become differentiated into nerve
fibres and nerve cells, and the latter, gathering towards certain
points, give rise to the ganglia which eventually unite in the middle
line. By degrees, the ingrowth of epiblastic cells, from which all
these structures are developed, becomes completely separated from
the rest of the epiblast, and is invested by mesoblastic cells. The
central nervous system, therefore, in a crayfish, as in a vertebrated
animal, is at first, as a part of the ectoderm, morphologically one
with the epidermis; and the deep and protected position which it
occupies in the adult is only a consequence of the mode in which the
nervous portion of the ectoderm grows inwards and becomes detached
from the epidermic portion.
The visual rods of the eye are merely modified cells of the ectoderm.
The auditory sac is formed by an involution of the ectoderm of the
basal joint of the antennule. At birth it is a shallow wide-mouthed
depression, and contains no otoliths.
Lastly, the reproductive organs result from the segregation and
special modification of cells of the mesoblast {226} behind the
liver. Rathke states that the sexual apertures are not visible until
the young crayfish has attained the length of an inch; and that the
first pair of abdominal appendages of the male appear still later in
the form of two papillæ, which gradually elongate and take on their
characteristic forms.
{227}