THE MORPHOLOGICAL COMPOSITION OF ANIMALS,
_CONTINUED_.
§ 208. Insects, Arachnids, Crustaceans, and Myriapods, are all members
of that higher division of the _Annulosa_[26] called _Articulata_
or now more generally _Arthropoda_. Though in these creatures the
formation of segments may be interpreted as a disguised gemmation;
and though, in some of them, the number of segments increases by this
modified budding after leaving the egg, as it does among the Annelids;
yet the process is not nearly so dominant: the segments are usually
much less numerous than we find them in the types last considered. In
most cases, too, the segments are in a greater degree differentiated
one from another, at the same time that they are severally more
differentiated within themselves. Nor is there any instance of
spontaneous fission taking place in the series of segments composing
an articulate animal. On the contrary, the integration, always great
enough permanently to unite the segments, is frequently carried so
far as to hide very completely the individualities of some or many of
them; and occasionally, as among the Acari, the consolidation, or the
arrest of segmentation, is so decided as to leave scarcely a trace
of the articulate structure: the type being in these cases indicated
chiefly by the presence of those characteristically-formed limbs, which
give the alternative name _Arthropoda_ to all the higher _Annulosa_.
Omitting the parasitic orders, which, as in other cases, are aberrant
members of their sub-kingdom, comparisons between the different orders
prove that the higher are strongly distinguished from the lower, by
the much greater degree in which the individuality of the tertiary
aggregate dominates over the individualities of those secondary
aggregates called segments or “somites,” of which it is composed. The
successive Figs. 170–176, representing (without their limbs) a Julus,
a Scolopendra, an isopodous Crustacean, and four kinds of decapodous
Crustaceans, ending with a Crab, will convey at a glance an idea of
the way in which that greater size and heterogeneity reached by the
higher types, is accompanied by an integration which, in the extreme
cases, nearly obliterates all traces of composite structure. In the
Crab the posterior segments, usually folded underneath the shell, alone
preserve their primitive distinctness. So completely confluent are the
rest, that it seems absurd to say that a Crab’s carapace is composed of
as many segments as there are pairs of limbs, foot-jaws, and antennæ
attached to it; and were it not that during early stages of the Crab’s
development the segmentation is faintly marked, the assertion might be
considered illegitimate.
[Illustration: Figs. 170–176.]
That all articulate animals are thus composed from end to end
of homologous segments, is, however, an accepted doctrine among
naturalists. It is a doctrine that rests on careful observation of
three classes of facts--the correspondences of parts in the successive
“somites” of an adult articulate animal; the still more marked
correspondences of such parts as they exist in the embryonic or larval
articulate animal; and the maintenance of such correspondences in some
types, which are absent in types otherwise near akin to them. The
nature of the conclusion which these evidences unite in supporting,
will best be shown by the annexed copies from the lecture-diagrams
of Prof. Huxley; exhibiting the typical structures of a Myriapod, an
Insect, a Spider, and a Crustacean, with their relations to a common
plan, as interpreted by him.
[Illustration: Figs. 177–186.]
Treating of these homologies, Prof. Huxley says “that a striking
uniformity of composition is to be found in the heads of, at any rate,
the more highly organized members of these four classes; and that,
typically, the head of a Crustacean, an Arachnid, a Myriapod, or an
Insect, is composed of six somites (or segments corresponding with
those of the body) and their appendages, the latter being modified so
as to serve the purpose of sensory and manducatory organs.”[27]
Thus even in the higher _Arthropoda_, the much greater consolidation
and much greater heterogeneity do not obliterate all evidence of the
fact, that the organism is an aggregate of the third order. Comparisons
show that it is divisible into a number of proximate units, each of
which is akin in certain fundamental traits to its neighbours, and each
of which is an aggregate of the second order, in so far as it is an
organized combination of those aggregates of the first order which we
call morphological units or cells. And that these segments or somites,
which make up an annulose animal, were originally aggregates of the
second order having independent individualities, is an hypothesis which
gathers further support from the contrast between the higher and the
lower Arthropods, as well as from the contrast between the Arthropods
in general and the Annelids. For if that masking of the individualities
of the segments which we find distinguishes the higher forms from the
lower, has been going on from the beginning, as we may fairly assume;
it is to be inferred that the individualities of the segments in the
lower forms, were originally more marked than they now are. Reversing
those processes of change by which the most developed _Annulosa_ have
arisen from the least developed; and applying in thought this reversed
process to the least developed, as they were described in the last
Chapter; we are brought to the conception of attached segments that are
all completely alike, and have their individualities in no appreciable
degree subordinated to that of the chain they compose. From which there
is but one step to the conception of gemmiparously-produced individuals
which severally part one from another as soon as they are formed.
§ 209. We must now return to a junction whence we diverged some time
ago. As before explained under the head of Classification, organisms
do not admit of uniserial arrangement, either in general or in detail;
but everywhere form groups within groups. Hence, having traced the
phases of morphological composition up to the highest forms in any
sub-kingdom, we find ourselves at the extremity of a great branch, from
which there is no access to another great branch, except by going back
to some place of bifurcation low down in the tree.
There exist such similarities of shape and structure between the larval
forms of low Molluscs and those of Annelids and Rotifers, as to show
that there was an early type common to them all; and its probable
characters, suggested by comparison, seem to imply that it had arisen
from some cœlenterate type, intermediate between the _Cnidaria_ and
the _Ctenophora_. But there is this noteworthy difference between the
molluscan larva and the allied larvæ, that it gives origin to only one
animal and not to a group of animals, united or disunited. No true
Mollusc multiplies by gemmation, either continuous or discontinuous;
but the product of every fertilized germ is a single individual.
It is a significant fact that here, where for the first time we have
homogenesis holding throughout an entire sub-kingdom, we have also
throughout an entire sub-kingdom no case in which the organism is
divisible into two, three, or more, like parts. There is neither any
such clustering or branching as a cœlenterate or molluscoid animal
usually displays; nor is there any trace of that segmentation which
characterizes the _Annulosa_. Among these animals in which no single
egg produces several individuals, no individual is separable into
several homologous divisions. This connexion will be seen to have
a probable meaning, on remembering that it is the converse of the
connexion which obtains among the _Annulosa_, considered as a group.
A Mollusc, then, is an aggregate of the second order. Not only in the
adult animal is there no sign of a multiplicity of like parts that
have become obscured by integration; but there is no sign of such
multiplicity in the embryo. And this unity is just as conspicuous in
the lowest Lamellibranch as in the highest Cephalopod.
[Illustration: Figs. 188–190.]
It may be well to note, however, more especially because it illustrates
a danger of misinterpretation presently to be guarded against, that
there are certain Molluscs which simulate the segmented structure.
Externally a _Chiton_, Fig. 188, appears to be made up of divisions
substantially like those of the creature Fig. 189; and one who judged
only by externals, would say that the creature Fig. 190 differs as
much from the creature Fig. 189, as this does from the preceding one.
But the truth is, that while 190 and 189 are closely-allied types, 189
differs from 188 much more widely than a man does from a fish. And the
radical distinction between them is this:--Whereas in the Crustacean
the segmentation is carried transversely through the whole mass of the
body, so as to render the body more or less clearly divisible into
a series of parts which are similarly composed; in the Mollusc the
segmentation is limited to the shell carried on its upper surface,
and leaves its body as completely undivided as is that of a common
slug.[28] Were the body cut through at each of the divisions, the
section of it attached to each portion of the shell would be unlike
all the other sections. Here the segmentation has a purely functional
derivation--is adaptive instead of genetic. The similarly-formed and
similarly-placed parts, are not homologous in the same sense as are the
appendages of a phænogamic axis or the limbs of an insect.
§ 210. In studying the remaining and highest sub-kingdom of animals, it
is important to recognize this radical difference in meaning between
that likeness of parts which is produced by likeness of modifying
forces, and that likeness of parts which is due to primordial identity
of origin. On our recognition of this difference depends the view we
take of certain doctrines that have long been dominant, and have still
a wide currency.
Among the _Vertebrata_, as among the _Mollusca_, homogenesis is
universal. The two sub-kingdoms are like one another and unlike the
remaining sub-kingdoms in this, that in all the types they severally
include, a single fertilized ovum produces only a single individual. It
is true that as the eggs of certain gasteropods occasionally exhibit
spontaneous fission of the vitelline mass, which may or may not result
in the formation of two individuals; so among vertebrate animals we
now and then meet with double monsters, which appear to imply such a
spontaneous fission imperfectly carried out. But these anomalies serve
to render conspicuous the fact, that in both these sub-kingdoms the
normal process is the integration of the whole germ-mass into a single
organism, which at no phase of its development displays any tendency to
separate into two or more parts.
Equally as throughout the _Mollusca_, there holds throughout the
_Vertebrata_ the correlative fact, that not even in its lowest any
more than in its highest types, is the body divisible into homologous
segments. The vertebrate animal, under its simplest as under its most
complex form, is like the molluscous animal in this, that you cannot
cut it into transverse slices, each of which contains a digestive
organ, a respiratory organ, a reproductive organ, &c. The organs of the
least-developed fish as well as those of the most developed mammal,
form but a single physiological whole; and they show not the remotest
trace of having ever been divisible into two or more physiological
wholes. That segmentation which the vertebrate animal usually exhibits
throughout part of its organization, is the same in origin and meaning
as the segmentation of a _Chiton’s_ shell; and no more implies in the
vertebrate animal a composite structure, than do the successive pairs
of branchiæ of the _Doto_, or the transverse rows of branchiæ in the
_Eolis_, imply composite structure in the molluscous animal. To some
this will seem a very questionable proposition; and had we no evidence
beyond that which adult vertebrate animals of developed types supply,
it would be a proposition not easy to substantiate. But abundant
support for it is to be found in the structure of the vertebrate
embryo, and in the comparative morphology of the _Vertebrata_ in
general.
Embryologists teach us that the primordial relations of parts are most
clearly displayed in the early stages of evolution; and that they
generally become partially or completely disguised in its later stages.
Hence, were the vertebrate animal on the same level as the annulose
animal in degree of composition--did it similarly consist of segments
which are homologous in the sense that they are the proximate units
of composition; we ought to find this fundamental fact most strongly
marked at the outset. As in the annelid-embryo the first conspicuous
change is the elongation and division into segments, by constrictions
that encircle the whole body; and as in the arthropod embryo the
blastoderm becomes marked out transversely into pieces which extend
themselves round the yelk before the internal organization has made
any appreciable progress; so in the embryo of every vertebrate animal,
had it an analogous composition, the first decided change should be
a segmentation implicating the entire mass. But it is not so. Sundry
important differentiations occur before any divisions begin to show
themselves. There is the defining of that elongated, elevated area with
its longitudinal groove, which becomes the seat of subsequent changes;
there is the formation of the notochord lying beneath this groove;
there is the growth upwards of the boundaries of the groove into the
dorsal laminæ, which rapidly develop and fold over in the region of the
head. Rathke, as quoted and indorsed by Prof. Huxley, describes the
subsequent changes as follows:--“The gelatinous investing mass, which,
at first, seems only to constitute a band to the right and to the left
of the notochord forms around it, in the further course of development,
a sheath, which ends in a point posteriorly. Anteriorly, it sends
out two processes which underlie the lateral parts of the skull, but
very soon coalesce for a longer or shorter distance. Posteriorly, the
sheath projects but little beyond the notochord; but, anteriorly, for a
considerable distance, as far as the infundibulum. It sends upwards two
plates, which embrace the future central parts of the nervous system
laterally, probably throughout their entire length.” That is to say, in
the _Vertebrata_ the first step is the marking out on the blastoderm
of an integrated structure, within which segments subsequently appear.
When these do appear, they are for some time limited to the middle
region of the spinal axis; and no more then than ever after, do they
implicate the general mass of the body in their transverse divisions.
On the contrary, before vertebral segmentation has made much progress,
the rudiments of the vascular system are laid down in a manner
showing no trace of any primordial correspondence of its parts with
the divisions of the axis. Equally at variance with the belief that
the vertebrate animal is essentially a series of homologous parts,
is the heterogeneity which exists among these parts on their first
appearance. Though in the head of an adult articulate animal there
is little sign of divisibility into segments like those of the body;
yet such segments, with their appropriate ganglia and appendages, are
easily identifiable in the articulate embryo. But in the _Vertebrata_
this antithesis is reversed. At the time when segmentation has become
decided in the dorsal region of the spine, there is no trace of
segments in the parts which are to form the skull--nothing whatever
to suggest that the skull is being formed out of divisions homologous
with vertebræ.[29] And minute observation no more discloses any such
homology than does general appearance. “Remak,” says Prof. Huxley,
“has more fully proved than any other observer, the segmentation into
‘urwirbel,’ or proto-vertebræ, which is characteristic of the vertebral
column, stops at the occipital margin of the skull--the base of which,
before ossification, presents no trace of that segmentation which
occurs throughout the vertebral column.”
[Illustration: Fig. 191.]
Consider next the evidence supplied by comparative morphology. In
preceding sections (§§ 206, 208) it has been shown that among annulose
animals, the divisibility into homologous parts is most clearly
demonstrable in the lowest types. Though in decapodous Crustaceans,
in Insects, in Arachnids, there is difficulty in identifying some
or many of the component somites; and though, when identified, they
display only partial correspondences; yet on descending to Annelids,
the composition of the entire body out of such somites becomes
conspicuous, and the homology between each somite and its neighbours
is shown by the repetition of one another’s structural details, as
well as by their common gemmiparous origin: indeed, in some cases we
have the homology directly demonstrated by seeing a somite of the body
transformed into a head. If, then, a vertebrate animal had a segmental
composition of kindred nature, we ought to find it most clearly
marked in the lowest _Vertebrata_ and most disguised in the highest
_Vertebrata_. But here, as before, the fact is just the reverse. Among
the _Vertebrata_ of developed type, such segmentation as really exists
remains conspicuous--is but little obscured even in parts of the spinal
column formed out of integrated vertebræ. Whereas in the undeveloped
vertebrate type, segmentation is scarcely at all traceable.[30] The
_Amphioxus_, Fig. 191, is not only without ossified vertebræ; not
only is it without cartilaginous representatives of them; but it is
even without anything like distinct membranous divisions. The spinal
column exists as a continuous notochord: the only signs of incipient
segmentation being given by its membranous sheath, in the upper part
of which “quadrate masses of somewhat denser tissue seem faintly to
represent neural spines.” Moreover, throughout sundry groups of fishes
and amphibians, the segmentation remains very imperfect: only certain
peripheral appendages of the vertebræ becoming defined and solidified,
while in place of the bodies of the vertebræ there still continues the
undivided notochord. Thus, instead of being morphologically composed
of vertebral segments, the vertebrate animal in its primitive form is
entirely without vertebral segments; and vertebral segments begin to
appear only as we advance towards developed forms. Once more, evidence
equally adverse to the current hypothesis meets us on observing that
the differences between the parts supposed to be homologous, are as
great at first as at last. Did the vertebrate animal primordially
consist of homologous segments from snout to tail; then the segments
said to compose the skull ought, in the lowest _Vertebrata_, to show
themselves much more like the remaining segments than they do in the
highest _Vertebrata_. But they do not. Fishes have crania made up of
bones that are no more clearly arrangeable into segments like vertebræ,
than are the cranial bones of the highest mammal. Nay, indeed, the
case is much stronger. The simplest fish possessing a skeleton, has a
cranium composed of cartilage that is not segmented at all!
Besides being inconsistent with the leading truths of Embryology
and Comparative Morphology, the hypothesis of Goethe and Oken is
inconsistent with itself. The facts brought forward to show that there
exists an archetypal vertebra, and that the vertebrate animal is
composed of archetypal vertebræ arranged in a series, and severally
modified to fit their positions--these facts, I say, so far from
proving as much, suffice, when impartially considered, to disprove it.
No assigned, nor any conceivable, attribute of the supposed archetypal
vertebra is uniformly maintained. The parts composing it are constant
neither in their number, nor in their relative positions, nor in their
modes of ossification, nor in the separateness of their several
individualities when present. There is no fixity of any one element,
or connexion, or mode of development, which justifies even a suspicion
that vertebræ are modelled after an ideal pattern. To substantiate
these assertions here would require too much space, and an amount of
technical detail wearisome to the general reader. The warrant for them
will be found in a criticism on the osteological works of Prof. Owen,
originally published in the _British and Foreign Medico-Chirurgical
Review_ for Oct. 1858. This criticism I add in the Appendices, for the
convenience of those who may wish to study the question more fully.
(See Appendix B.)
Everything, then, goes to show that the segmental composition which
characterises the apparatus of external relation in most _Vertebrata_,
is not primordial or genetic, but functionally determined or adaptive.
Our inference must be that the vertebrate animal is an aggregate of
the second order, in which a relatively superficial segmentation has
been produced by mechanical intercourse with the environment. We
shall hereafter see that this conception leads us to a consistent
interpretation of the facts--shows us why there has arisen such unity
in variety as exists in every vertebral column, and why this unity
in variety is displayed under countless modifications in different
skeletons.[31]
§ 211. On glancing back at the facts brought together in these two
chapters, we see it to be probable that there has gone on among animals
a process like that which we saw reason to think has gone on among
plants. Minute aggregates of those physiological units which compose
living protoplasm, exist as _Protozoa_: some of them incoherent,
indefinite, and almost homogeneous, and others of them more coherent,
definite, and heterogeneous. By union of these nucleated particles
of sarcode, are produced various indefinite aggregates of the second
order--Sponges, _Polycytharia_, Foraminifers, &c.; in which the
compound individuality is scarcely enough marked to subordinate the
primitive individualities. But in other types, as in _Hydra_, the lives
of the morphological units are in a considerable degree, though not
wholly, merged in the life of the integrated body they form. As the
primary aggregate, when it passes a certain size, undergoes fission or
gemmation; so does the secondary aggregate. And as on the lower stage
so on the higher, we see cases in which the gemmiparously-produced
individuals part as soon as formed, and other cases in which they
continue united, though in great measure independent. This massing
of secondary aggregates into tertiary aggregates, is variously
carried on among the _Hydrozoa_, the _Actinozoa_, the _Polyzoa_,
and the _Tunicata_. In most of the types so produced, the component
individualities are very little subordinated to the individuality of
the composite mass--there is only physical unity and not physiological
unity; but in certain of the oceanic _Hydrozoa_, the individuals are
so far differentiated and combined as very much to mask them. Forms
showing us clearly the transition to well-developed individuals of
the third order, are not to be found. Nevertheless, in the great
sub-kingdom _Annulosa_, there are traits of structure, development,
and mode of multiplication, which go far to show that its members are
such individuals of the third order; and in the relations to external
conditions involved by the mode of union, we find an adequate cause for
that obscuration of the secondary individualities which we must suppose
has taken place. The two other great subdivisions, _Mollusca_ and
_Vertebrata_, between the lower members of which there are suggestive
points of community, present us only with aggregates of the second
order, that have in many cases become very large and very complex.
We find in them no trace of the union of gemmiparously-produced
individuals. Neither the molluscous nor the vertebrate animal shows
the faintest trace of a segmentation affecting the totality of
its structure; and we see good grounds for concluding that such
segmentation as exceptionally occurs in the one and usually occurs in
the other, is superinduced.
* * * * *
[NOTE:--A critic calls in question the statement on p. 121 respecting
the _Amphioxus_. At the outset, however, he admits that in the
_Amphioxus_ “the central nervous system and the notochord are not
segmented.” In the Annelid, however, the central nervous system
is segmented, and there is segmentation of the part which, as a
supporting structure, is analogous to the notochord in respect
of function--the outer part which represents the exo-skeleton in
contrast to the endo-skeleton. He goes on to say that “the gut is
not involved [in the segmentation] and exhibits in _Amphioxus_ just
as it does in worms differentiations entirely independent of the
segmentation of the mesoblast.” Part of this statement is, I think,
not congruous with all the facts. In _Protodrilus_, one of the lowest
of the _Archiannelida_, “the intestine is moniliform, there being a
constriction between each segment” and the next. (Shipley.) Complete
segmentation of the intestine is obviously impossible, since, were
the canal divided into portions by septa, no food could pass. But the
fact that the gut has these successive expansions and constrictions,
corresponding to the successive segments, and giving to each segment
a partially-separate stomach, shows that segmentation has gone as
far as consists with the carrying on of the lives of the segments.
No such partial segmentation exists in the _Amphioxus_. Thus, then,
three fundamental structures--the directive structure, the supporting
structure, and the alimentary structure--are respectively simple in
the lowest vertebrate and segmented, or partially segmented, in the
lowest Annelid. Again, while it is said that the gill-clefts exhibit
segmentation, it is admitted that this has no relevance to any
constitutional segmentation: “they are segmented on a plan of their
own” irrespective of other organs. Another allegation is that the
ovaries of _Amphioxus_ are segmented. Their segmentation, however, like
that of the gills, is isolated, and may be considered as illustrating
those repetitions of like parts seen in supernumerary vertebræ in
various creatures--a repetition which becomes habitual if the resulting
structure is advantageous to the species. On the statement that while
the _Amphioxus_ has no rudiments of a renal system the Elasmobranch
embryo has such rudiments, which are as distinctly segmented as the
nephridia of a worm, two comments may be made. The first is that if
in these Vertebrates the nephridia bear a relation to the general
structure like that which they do in Annelids, then one would expect
to find the segmental arrangement shown in the lowest type, as in
Annelids, rather than in a type considerably advanced in development.
Should it be replied that in the _Amphioxus_ an excretory system had
not yet arisen, though one is required for the higher organization
of an Elasmobranch, then the answer may be that since the segmental
arrangement in the Elasmobranch corresponds with that of the myotomes,
it has no reference to any primordial segmentation, since the myotomes
have been functionally generated. The second comment is that whereas
the nephridia of the Annelid have independent external openings,
the nephridia in the Elasmobranch have not. These discharge their
secretions into certain general tubes of exit common to them all;
showing that each of them, instead of being a member of a partially
independent structure, is united with others in subordination to a
general structure. That is to say, the segmentations are far from being
parallel in their essential natures. The assertion accompanying these
criticisms, that there is “no difference _in principle_ between the
segmentation of _Amphioxus_ and Annelid” is difficult to reconcile
with the visible contrast between the two. Whatever local segmentations
there are in an _Amphioxus_ appear to me quite unlike “in principle”
to those which an Annelid exhibits. Could its portion of gut be duly
supplied with nutriment, the segment of a low Annelid could carry on
its vital functions independently. In the parts of the _Amphioxus_ we
see nothing approaching to this. Cut it into transverse sections and
no one of them contains anything like the assemblage of structures
required for living. The _Amphioxus_ is a physiological whole, and
in that respect differs radically from the Annelid, each segment of
which is in chief measure a physiological whole. No occurrence of
local segmentation in the _Amphioxus_ can obliterate this fundamental
contrast.
An accompanying contrast tells the same story. On ascending from the
lowest to the highest annulose types we see a progressing integration,
morphological and physiological; so that whereas in a low annelid
the successive parts are in large measure independent in their
structures and in their lives, in a high arthropod, as a crab, most
of the parts have lost their individualities and have become merged
in a consolidated organism with a single life. Quite otherwise is it
in the vertebrate series. Its lowest member is at the very outset a
complete morphological and physiological whole, and the formation of
those serial parts which some think analogous to the serial parts of an
Annelid, begins at a later stage and becomes gradually pronounced. That
is to say, the course of transformation is reversed.]