THE MORPHOLOGICAL COMPOSITION OF ANIMALS.
§ 199. What was said in § 180, respecting the ultimate structure of
organisms, holds more manifestly of animals than of plants. That
throughout the vegetal kingdom the cell is the morphological unit, is
a proposition admitting of a better defence, than the proposition that
the cell is the morphological unit throughout the animal kingdom. The
qualifications with which, as we saw, the cell-doctrine must be taken,
are qualifications thrust upon us more especially by the facts which
zoologists have brought to light. It is among the _Protozoa_ that
there occur numerous cases of vital activity displayed by specks of
protoplasm; and from the minute anatomy of all creatures above these,
are drawn the numerous proofs that non-cellular tissues may arise by
direct metamorphosis of mixed colloidal substances.[16]
Our survey of morphological composition throughout the animal kingdom,
must therefore begin with those undifferentiated aggregates of
physiological units [or constitutional units], out of which are formed
what we call, with considerable license, morphological units.
§ 200. In that division of the _Protozoa_ distinguished as _Rhizopoda_,
are presented, under various modifications, these minute portions of
living organic matter, so little differentiated, if not positively
undifferentiated, that animal individuality can scarcely be claimed
for them. Figs. 131, 132, and 133, represent certain nearly-allied
types of these--_Amœba_, _Actinophrys_, and _Lieberkühnia_. The
viscid jelly or sarcode, comparable in its physical properties to
white of egg, out of which one of these creatures is mainly formed,
shows us in various ways, the feebleness with which the component
physiological units are integrated--shows us this by its very slight
cohesion, by the extreme indefiniteness and mutability of its form,
and by the absence of a limiting membrane. It is no longer held even
by unqualified adherents of the cell-doctrine that the _Amœba_ has an
investment. Its outer surface, compared to the film which forms on
the surface of paste, does not prevent the taking of solid particles
into the mass of the body, and does not, in such kindred forms as Fig.
133, prevent the pseudopodia from coalescing when they meet. Hence it
cannot properly have the name of a cell-wall. A considerable portion
of the body, however, in _Difflugia_, Fig. 134, has a denser coating
formed of agglutinated foreign particles; so that the protrusion of
the pseudopodia is limited to one part of it. And in the solitary
_Foraminifera_, like _Gromia_, the sarcode is covered over most of its
surface by a delicate calcareous shell, pierced with minute holes,
through which the slender pseudopodia are thrust. The _Gregarina_
exhibits an advance in integration, and a consequent greater
definiteness. Figs. 135 and 136, exemplifying this type, show the
complete membrane in which the substance of the creature is contained.
Here there has arisen what may be properly called a cell: under its
solitary form this animal is truly unicellular. Its embryology has
considerable significance. After passing through a certain quiescent,
“encysted” state, its interior breaks up into small portions, which,
after their exit, assume forms like that of the _Amœba_; and from
this young condition in which they are undifferentiated, they pass
into that adult condition in which they have limiting membranes. If
this development of the individual _Gregarina_ typifies the mode of
evolution of the species, it yields further support to the belief, that
fragments of sarcode existed earlier than any of the structures which
are called cells. Among aggregates of the first order, there are some
much more highly developed. These are the _Infusoria_, constituting
the most numerous of the _Protozoa_, in species as in individuals.
Figs. 137, 138, and 139, are examples. In them we find, along with
greater definiteness, a considerable heterogeneity. The sarcode of
which the body consists, has an indurated outer layer, bearing cilia
and sometimes spines; there is an opening serving as mouth, a permanent
œsophagus, and a cavity or cavities, temporarily formed in the interior
of the sarcode, to serve as one or more stomachs; and there is a
comparatively specific arrangement of these and various minor parts.
[Illustration: Figs. 131–139.]
Thus in the animal kingdom, as in the vegetal kingdom, there exists
a class of minute forms having this peculiarity, that no one of them
is separable into a number of visible components homologous with one
another--no one of them can be resolved into minor individualities.
Its proximate units are those physiological units of which we conclude
every organism consists. The aggregate is an aggregate of the first
order.
§ 201. Among plants are found types indicating a transition from
aggregates of the first order to aggregates of the second order; and
among animals we find analogous types. But the stages of progressing
integration are not here so distinct. The reason probably is, that
the simplest animals, having individualities much less marked than
those of the simplest plants, do not afford us the same facilities for
observation. In proportion as the limits of the minor individualities
are indefinite, the formation of major individualities out of them,
naturally leaves less conspicuous traces.
[Illustration: Figs. 140–145.]
Be this as it may, however, in such types of _Protozoa_ as the compound
_Radiolaria_, we find that though there is reason to regard the
aggregate as an aggregate of the second order, yet its divisibility
into minor individualities like those just described, is less manifest.
Fig. 140 representing _Sphærozoum punctatum_, one of the group,
illustrates this. The sceptically-minded may perhaps doubt whether we
can regard the “cellæform bodies” contained in it, as the morphological
units of the animal. The jelly-like mass in which they are imbedded, is
but indefinitely divisible into portions having each a cell or nucleus
for its centre.[17] Among the _Foraminifera_, we find only indefinite
evidence of the coalescence of aggregates of the first order, into
aggregates of the second order. There are solitary Foraminifers,
allied to the creature represented in Fig. 134. Certain ideal types of
combination among them, are shown in Fig. 141. And setting out from
these, we may ascend in various directions to kinds compounded to
an immense variety of degrees in an immense variety of ways. In all
of them, however, the separability of the major individuality into
minor individualities, is very incomplete. The portion of sarcode
contained in one of these calcareous chambers, gives origin to an
external bud; and this presently becomes covered, like its parent,
with calcareous matter: the position in which each successive chamber
is so produced, determining the form of the compound shell. But the
portions of sarcode thus budded out one from another, do not become
distinctly individualized. Fig. 142, representing the living network
which remains when the shell of an Orbitolite has been dissolved,
shows the continuity that exists among the occupants of its aggregated
chambers.[18] In the compound _Infusoria_, the component units remain
quite distinct. Being, as aggregates of the first order, much more
definitely organized, their union into aggregates of the second
order does not destroy their original individualities. Among the
_Vorticellæ_, of which two kinds are delineated in Figs. 144 and 145,
there are various illustrations of this: the members of the community
being sometimes appended to a single stem; sometimes attached by long
separate stems to a common base; and sometimes massed together.
[Illustration: Figs. 146–147.]
Thus far, these aggregates of the second order exhibit but indefinite
individualities. The integration is physical; but not physiological.
Though, in the _Polycytharia_, there is a shape that has some symmetry;
and though, in the _Foraminifera_, the formation of successive chambers
proceeds in such methodic ways as to produce quite-regular and
tolerably-specific shells; yet no more in these than in the Sponges or
the compound _Vorticellæ_, do we find such co-ordination as gives the
whole a life predominating over the lives of its parts. We have not
yet reached an aggregate of the second order, so individuated as to be
capable of serving as a unit in still higher combinations. But in the
class _Cœlenterata_, this advance is displayed. The common _Hydra_,
habitually taken as the type of the lowest division of this class,
has specialized parts performing mutually-subservient functions, and
thus exhibiting a total life distinct from the lives of the units.
Fig. 146 represents one of these creatures in its contracted state
and in its expanded state; while Fig. 147 is a diagram showing the
wall of this creature’s sac-like body as seen in section under the
microscope: _a_ and _b_ being the outer and inner cellular layers;
while between them is the “mesoglœa” or “structureless lamella,” the
supporting or skeletal layer. But this lowly-organized tissue of the
Hydra, illustrates a phase of integration in which the lives of the
minor aggregates are only partially-subordinated to the life of the
major aggregate formed by them. For a _Hydra’s_ substance is separable
into _Amœba_-like portions, capable of moving about independently.
If we bear in mind how analogous are the extreme extensibility and
contractility of a _Hydra’s_ body and tentacles, to the properties
displayed by the sarcode among Rhizopods; we may infer that probably
the movements and other actions of a _Hydra_, are due to the
half-independent co-operation of the _Amœba_-like individuals composing
it.
§ 202. A truth which we before saw among plants, we here see repeated
among animals--the truth that as soon as the integration of aggregates
of the first order into aggregates of the second order, produces
compound wholes so specific in their shapes and sizes, and so mutually
dependent in their parts, as to have distinct individualities; there
simultaneously arises the tendency in them to produce, by gemmation,
other such aggregates of the second order. The approach towards
definite limitation in an organism, is, by implication, an approach
towards a state in which growth passing a certain point, results, not
in the increase of the old individual, but in the formation of a new
individual. Thus it happens that the common polype buds out other
polypes, some of which very shortly do the like, as shown in Fig. 148:
a process paralleled by the fronds of sundry _Algæ_, and by those of
the lower _Jungermanniaceæ_. And just as, among these last plants,
the proliferously-produced fronds, after growing to certain sizes
and developing rootlets, detach themselves from their parent fronds;
so among these animals, separation of the young ones from the bodies
of their parents ensues when they have acquired tolerably complete
organizations.
[Illustration: Figs. 148–150.]
There is reason to think that the parallel holds still further.
Within the limits of the _Jungermanniaceæ_, we found that while some
genera exhibit this discontinuous development, other genera exhibit
a development that is similar to it in all essential respects, save
that it is continuous. And here within the limits of the _Hydrozoa_,
we find, along with this genus in which the gemmiparous individuals
are presently cast off, other genera in which they are not cast off,
but form a permanent aggregate of the third order. Figs. 149 and 150,
exemplify these compound _Hydrozoa_--one of them showing this mode
of growth so carried out as to produce a single axis; and the other
showing how, by repetitions of the process, lateral axes are produced.
Integrations characterizing certain higher genera of the _Hydrozoa_
which swim or float instead of being fixed, are indicated by Figs. 151
and 152: the first of them representing the type of a group in which
the polypes growing from an axis, or cœnosarc, are drawn through the
water by the rhythmical contractions of the organs from which they
hang; and the second of them representing a _Physalia_ the component
polypes of which are united into a cluster, attached to an air-vessel.
[Illustration: Figs. 151–152.]
A parallel series of illustrations might be drawn from that second
division of the _Cœlenterata_, known as the _Actinozoa_. Here, too,
we have a group of species--the Sea-anemones--the individuals of
which are solitary. Here, too, we have agamogenetic multiplication:
occasionally by gemmation, but more frequently by that modified process
called spontaneous fission. And here, too, we have compound forms
resulting from the arrest of this spontaneous fission before it is
complete. To give examples is needless; since they would but show, in
more varied ways, the truth already made sufficiently clear, that the
compound _Cœlenterata_ are aggregates of the third order, produced by
integration of aggregates of the second order such as we have in the
_Hydra_. As before, it is manifest that on the hypothesis of evolution,
these higher integrations will insensibly arise, if the separation
of the gemmiparous polypes is longer and longer postponed; and that
an increasing postponement will result by survival of the fittest,
if it profits the group of individuals to remain united instead of
dispersing.[19]
§ 203. The like relations exist, and imply that the like processes
have been gone through, among those more highly organized animals
called _Polyzoa_ and _Tunicata_. We have solitary individuals, and we
have variously-integrated groups of individuals: the chief difference
between the evidence here furnished, and that furnished in the last
case, being the absence of a type obviously linking the solitary state
with the aggregated state.
[Illustration: Figs. 153–155.]
This integration of aggregates of the second order, is carried on
among the _Polyzoa_ in divers ways, and with different degrees of
completeness. The little patches of minute cells, shown as magnified
in Fig. 153, so common on the fronds of sea-weeds and the surfaces of
rocks at low-water mark, display little beyond mechanical combination.
The adjacent individuals, though severally originated by gemmation
from the same germ, have but little physiological dependence. In
kindred kinds, however, as shown in Figs. 154 and 155, one of which
is a magnified portion of the other, the integration is somewhat
greater: the co-operation of the united individuals being shown in the
production of those tubular branches which form their common support,
and establish among them a more decided community of nutrition.
[Illustration: Figs. 156–159.]
Among the Ascidians this general law of morphological composition is
once more displayed. Each of these creatures subsists on the nutritive
particles contained in the water which it draws in through one orifice
and sends out through another; and it may thus subsist either alone, or
in connexion with others that are in some cases loosely aggregated and
in other cases closely aggregated. Fig. 156, _Phallusia mentula_, is
one of the solitary forms. A type in which the individuals are united
by a stolon that gives origin to them by successive buds, is shown in
_Perophora_, Fig. 157. Among the _Botryllidæ_, of which one kind is
drawn on a small scale in Fig. 159, and a portion of the same on a
larger scale in Fig. 158, there is a combination of the individuals
into annular clusters, which are themselves imbedded in a common
gelatinous matrix. And in this group there are integrations even a
stage higher, in which several such clusters of clusters grow from
a single base. Here the compounding and recompounding appears to be
carried further than anywhere else in the animal kingdom.
Thus far, however, among these aggregates of the third order, we see
what we before saw among the simpler aggregates of the second order--we
see that the component individualities are but to a very small extent
subordinated to the individuality made up of them. In nearly all the
forms indicated, the mutual dependence of the united animals is so
slight, that they are more fitly comparable to societies, of which
the members co-operate in securing certain common benefits. There is
scarcely any specialization of functions among them. Only in the last
type described do we see a number of individuals so completely combined
as to simulate a single individual. And even here, though there appears
to be an intimate community of nutrition, there is no physiological
integration beyond that implied in several mouths and stomachs having a
common vent.[20]
§ 204. We come now to an extremely interesting question. Does there
exist in other sub-kingdoms composition of the third degree, analogous
to that which we have found so prevalent among the _Cœlenterata_ and
the _Polyzoa_ and _Tunicata_? The question is not whether elsewhere
there are tertiary aggregates produced by the branching or clustering
of secondary aggregates, in ways like those above traced; but whether
elsewhere there are aggregates which, though otherwise unlike in the
arrangement of their parts, nevertheless consist of parts so similar to
one another that we may suspect them to be united secondary aggregates.
The various compound types above described, in which the united animals
maintain their individualities so distinctly that the individuality
of the aggregate remains vague, are constructed in such ways that the
united animals carry on their several activities with scarcely any
mutual hindrance. The members of a branched _Hydrozoon_, such as is
shown in Fig. 149 or Fig. 150, are so placed that they can all spread
their tentacles and catch their prey as well as though separately
attached to stones or weeds. Packed side by side on a flat surface or
forming a tree-like assemblage, the associated individuals among the
_Polyzoa_ are not unequally conditioned: or if one has some advantage
over another in a particular case, the mode of growth and the relations
to surrounding objects are so irregular as to prevent this advantage
re-appearing with constancy in successive generations. Similarly
with the Ascidians growing from a stolon or those forming an annular
cluster: each of them is as well placed as every other for drawing in
the currents of sea-water from which it selects its food. In these
cases the mode of aggregation does not expose the united individuals
to multiform circumstances; and therefore is not calculated to produce
among them any structural multiformity. For the same reason no marked
physiological division of labour arises among them; and consequently no
combination close enough to disguise their several individualities. But
under converse conditions we may expect converse results. If there is a
mode of integration which necessarily subjects the united individuals
to unlike sets of incident forces, and does this with complete
uniformity from generation to generation, it is to be inferred that
the united individuals will become unlike. They will severally assume
such different functions as their different positions enable them
respectively to carry on with the greatest advantage to the assemblage.
This heterogeneity of function arising among them, will be followed by
heterogeneity of structure; as also by that closer combination which
the better enables them to utilize one another’s functions. And hence,
while the originally-like individuals are rendered unlike, they will
have their homologies further obscured by their progressing fusion into
an aggregate individual of a higher order.
These converse conditions are in nearly all cases fulfilled where
the successive individuals arising by continuous development are so
budded-off as to form a linear series. I say in nearly all cases,
because there are some types in which the associated individuals,
though joined in single file, are not thereby rendered very unlike
in their relations to the environment; and therefore do not become
differentiated and integrated to any considerable extent. I refer
to such Ascidians as the _Salpidæ_. These creatures float passively
in the sea, attached together in strings. Being placed side by side
and having mouths and vents that open laterally, each of them is as
well circumstanced as its neighbours for absorbing and emitting the
surrounding water; nor have the individuals at the two extremities any
marked advantages over the rest in these respects. Hence in this type,
and in the allied type _Pyrosoma_, which has its component individuals
built into a hollow cylinder, linear aggregation may exist without the
minor individualities becoming obscured and the major individuality
marked: the conditions under which a differentiation and integration
of the component individuals may be expected, are not fulfilled.
But where the chain of individuals produced by gemmation, is either
habitually fixed to some solid body by one of its extremities or moves
actively through the water or over submerged stones and weeds, the
several members of the chain become differently conditioned in the way
above described; and may therefore be expected to become unlike while
they become united. A clear idea of the contrast between these two
linear arrangements and their two diverse results, will be obtained by
considering what happens to a row of soldiers, when changed from the
ordinary position of a single rank to the position of Indian file. So
long as the men stand shoulder to shoulder, they are severally able
to use their weapons in like ways with like efficiency; and could,
if called on, similarly perform various manual processes directly
or indirectly conducive to their welfare. But when, on the word of
command “right face,” they so place themselves that each has one of
his neighbours before him and another behind him, nearly all of them
become incapacitated for fighting and for many other actions. They can
walk or run one after another, so as to produce movement of the file
in the direction of its length; but if the file has to oppose an enemy
or remove an obstacle lying in the line of its march, the front man
is the only one able to use his weapons or hands to much purpose. And
manifestly such an arrangement could become advantageous only if the
front man possessed powers peculiarly adapted to his position, while
those behind him facilitated his actions by carrying supplies, &c. This
simile, grotesque as it seems, serves to convey better perhaps than
any other could do, a clear idea of the relations that must arise in a
chain of individuals arising by gemmation, and continuing permanently
united end to end. Such a chain can arise only on condition that
combination is more advantageous than separation; and for it to be more
advantageous, the anterior members of the series must become adapted to
functions facilitated by their positions, while the posterior members
become adapted to functions which their positions permit. Hence, direct
or indirect equilibration or both, must tend continually to establish
types in which the connected individuals are more and more unlike
one another, at the same time that their several individualities are
more and more disguised by the integration consequent on their mutual
dependence.
Such being the anticipations warranted by the general laws of
evolution, we have now to inquire whether there are any animals which
fulfil them. Very little search suffices; for structures of the kind
to be expected are abundant. In that great division of the animal
kingdom at one time called _Annulosa_, but now grouped into _Annelida_
and _Arthropoda_, we find a variety of types having the looked-for
characters. Let us contemplate some of them.
§ 205. An adult Chætopod is composed of segments which repeat one
another in their details as well as in their general shapes. Dissecting
one of the lower orders, such as is shown in Fig. 160, proves that
the successive segments, besides having like locomotive appendages,
like branchiæ, and sometimes even like pairs of eyes, also have like
internal organs. Each has its enlargement of the alimentary canal;
each its contractile dilatation of the great blood-vessel; each its
portion of the double nervous cord, with ganglia when these exist;
each its branches from the nervous and vascular trunks answering to
those of its neighbours; each its similarly answering set of muscles;
each its pair of openings through the body-wall; and so on throughout,
even to the organs of reproduction. That is to say, every segment is
in great measure a physiological whole--every segment contains most
of the organs essential to individual life and multiplication: such
essential organs as it does not contain, being those which its position
as one in the midst of a chain, prevents it from having or needing. If
we ask what is the meaning of these homologies, no adequate answer is
supplied by any current hypothesis. That this “vegetative repetition”
is carried out to fulfil a predetermined plan, was shown to be quite
an untenable notion (§§ 133, 134). On the one hand, we found nothing
satisfactory in the conception of a Creator who prescribed to himself
a certain unit of composition for all creatures of a particular class,
and then displayed his ingenuity in building up a great variety of
forms without departing from the “archetypal idea.” On the other hand,
examination made it manifest that even were such a conception worthy
of being entertained, it would have to be relinquished; since in each
class there are numerous deviations from the supposed “archetypal
idea.” Still less can these traits of structure be accounted for
teleologically. That certain organs of nutrition and respiration and
locomotion are repeated in each segment of a dorsibranchiate annelid,
may be regarded as functionally advantageous for a creature following
its mode of life. But why should there be a hundred or even two
hundred pairs of ovaries? This is an arrangement at variance with that
physiological division of labour which every organism profits by--is
a less advantageous arrangement than might have been adopted. That
is to say, the hypothesis of a designed adaptation fails to explain
the facts. Contrariwise, these structural traits are just such as
might naturally be looked for, if these annulose forms have arisen by
the integration of simpler forms. Among the various compound animals
already glanced at, it is very general for the united individuals to
repeat one another in all their parts--reproductive organs included.
Hence if, instead of a clustered or branched integration, such as
the _Cœlenterata_, _Polyzoa_ and _Tunicata_ exhibit, there occurs a
longitudinal integration; we may expect that the united individuals
will habitually indicate their original independence by severally
bearing germ-producing or sperm-producing organs.
[Illustration: Figs. 160–161.]
The reasons for believing one of these creatures to be an aggregate of
the third order, are greatly strengthened when we turn from the adult
structure to the mode of development. Among the _Dorsibranchiata_
and _Tubicolæ_, the embryo leaves the egg in the shape of a ciliated
gemmule, not much more differentiated than that of a polype. As shown
in Fig. 162, it is a nearly globular mass; and its interior consists
of untransformed cells. The first appreciable change is an elongation
and a simultaneous commencement of segmentation. The segments multiply
by a modified gemmation, which takes place from the hinder end of the
penultimate segment. And considerable progress in marking out these
divisions is made before the internal organization begins. Figs. 163,
164, 165, represent some of these early stages. In annelids of other
orders, the embryo assumes the segmented form while still in the
egg. But it does this in just the same manner as before. Indeed, the
essential identity of the two modes of development is shown by the fact
that the segmentation within the egg is only partially carried out: in
all these types the segments continue to increase in number for some
time after hatching. Now this process is as like that by which compound
animals in general are formed, as the different conditions of the case
permit. When new individuals are budded-out laterally, their unfolding
is not hindered--there is nothing to disguise either the process or the
product. But gemmæ produced one from another in the same straight line,
and remaining connected, restrict one another’s developments; and that
the resulting segments are so many gemmiparously-produced individuals,
is necessarily less obvious.
[Illustration: Figs. 162–165.]
§ 206. Evidence remains which adds very greatly to the weight of
that already assigned. Thus far we have studied only the individual
segmented animal; considering what may be inferred from its mode of
evolution and final organization. We have now to study segmented
animals in general. Comparison of different groups of them and of
kinds within each group, will disclose various phases of progressive
integration of the nature to be anticipated.
[Illustration: Figs. 166–169.]
Among the simpler _Platyhelminthes_, as in some kinds of _Planaria_,
transverse fission occurs. A portion of a _Planaria_ separated by
spontaneous constriction, becomes an independent individual. Sir J.
G. Dalyell found that in some cases numerous fragments artificially
separated, grew into perfect animals.[21] In these creatures which thus
remind us of the lowest _Hydrozoa_ in their powers of agamogenetic
multiplication, the individuals produced one from another do not
continue connected. As the young ones laterally budded-off by the
_Hydra_ separate when complete, so do the young ones longitudinally
budded-off by the _Planaria_. Fig. 166 indicates this. But there
are allied types which show us a more or less persistent union of
homologous parts, or individuals, similarly arising by longitudinal
gemmation.[22] The cestoid _Entozoa_ furnish illustrations. Without
dwelling on the fact that each segment of a _Tænia_, like each separate
_Planaria_, is an independent hermaphrodite; and without specifying the
sundry common structural traits which add probability to the suspicion
that there is some kinship between the individuals of the one order
and the segments of the other; it will suffice to point out that the
two types are so far allied as to demand their union under the same
sub-class title. And recognizing this kinship, we see significance
in the fact that in the one case the longitudinally-produced gemmæ
separate as complete individuals, and in the other continue united
as segments in smaller or larger numbers and for shorter or longer
periods. In _Tænia echinococcus_, represented in Fig. 167, we have
a species in which the number of segments thus united does not
exceed four. In _Echinobothrium typus_ there are eight or ten; and
in cestoids generally they are numerous.[23] A considerable hiatus
occurs between this phase of integration and the next higher phase
which we meet with; but it is not greater than the hiatus between the
types of the _Platyhelminthes_ and the _Chætopoda_, which present
the two phases. Though it is doubtful whether separation of single
segments occurs among the Annelida,[24] yet very often we find strings
of segments, arising by repeated longitudinal budding, which after
reaching certain lengths undergo spontaneous fission: in some cases
doing this so as to form two or more similar strings of segments
constituting independent individuals; and in other cases doing it so
that the segments spontaneously separated are but a small part of the
string. Thus a _Syllis_, Fig. 168, after reaching a certain length,
begins to transform itself into two individuals: one of the posterior
segments develops into an imperfect head, and simultaneously narrows
its connexion with the preceding segments, from which it eventually
separates. Still more remarkable is the extent to which this process
is carried in certain kindred types; which exhibit to us several
individuals thus being simultaneously formed out of groups of segments.
Fig. 169, copied (omitting the appendages) from one contained in a
memoir by M. Milne-Edwards, represents six worms of different ages
in course of development: the terminal one being the eldest, the
one having the greatest number of segments, and the one that will
first detach itself; and the successively anterior ones, with their
successively smaller numbers of segments, being successively less
advanced towards fitness for separation and independence. Here among
groups of segments we see repeated what in the previous cases occurs
with single segments. And then in other annelids we find that the
string of segments arising by gemmation from a single germ becomes a
permanently united whole: the tendency to any more complete fission
than that which marks out the segments, being lost; or, in other words,
the integration having become relatively complete. Leaving out of sight
the question of alliance among the types above grouped together, that
which it here concerns us to notice is, that longitudinal gemmation
does go on; that it is displayed in that primitive form in which the
gemmæ separate as soon as produced; that we have types in which such
gemmæ hang together in groups of four, or in groups of eight and ten,
from which however the gemmæ successively separate as individuals;
that among higher types we have long strings of similarly-formed
gemmæ which do not become individually independent, but separate into
organized groups; and that from these we advance to forms in which all
the gemmæ remain parts of a single individual. One other significant
fact must be added. There are cases in which annelids multiply by
lateral gemmation.[25] That the longitudinally-produced gemmæ which
compose an annelid, should thus have, one of them or several of them,
the power of laterally budding-off gemmæ, from which other annelids
arise, gives further support to the hypothesis that, primordially, the
segments were independent individuals. And it suggests this belief
the more strongly because, in certain types of _Cœlenterata_, we see
that longitudinal and lateral gemmation _do_ occur together, where the
longitudinally-united gemmæ are demonstrably independent individuals.
§ 207. Though it seems next to impossible that we shall ever be able
to find a type such as that which is here supposed to be the unit of
composition of the annulose type, since we must assume such a type
to have been long since extinct, yet the foregoing evidence goes
far towards showing that an annulose animal is an aggregate of the
third order. This repetition of segments, sometimes numbering several
hundreds, like one another in all their organs even down to those of
reproduction, while it is otherwise unaccountable, is fully accounted
for if these segments are homologous with the separate individuals of
some lower type. The gemmation by which these segments are produced, is
as similar as the conditions allow, to the gemmation by which compound
animals in general are produced. As among plants, and as among
demonstrably-compound animals, we see that the only thing required
for the formation of a permanent chain of gemmiparously-produced
individuals, is that by remaining associated such individuals will
have advantages greater than are to be gained by separation. Further,
comparisons of the annuloid and lower annulose forms, disclose
a number of those transitional phases of integration which the
hypothesis leads us to expect. And, lastly, the differences among these
united individuals or successive segments, are not greater than the
differences in their positions and functions explain--not greater than
such differences are known to produce among other united individuals:
witness sundry compound _Hydrozoa_.
Indirect evidence of much weight has still to be given. Thus far
we have considered only the less developed _Annulosa_. The more
integrated and more differentiated types of the class remain. If in
them we find a carrying further of the processes by which the lower
types are here supposed to have been evolved, we shall have additional
reason for believing them to have been so evolved. If we find that in
these superior orders, the individualities of the united segments are
much less pronounced than in the inferior, we shall have grounds for
suspecting that in the inferior the individualities of the segments are
less pronounced than in those lost forms which initiated the annulose
sub-kingdom.
* * * * *
[NOTE.--Partly from the wish to incorporate further evidence, and
partly from the wish to present the evidence, old and new, in a more
effective order, I decide here to recast the foregoing exposition.
Significant traits of development are exhibited in common by two groups
otherwise unallied--certain of the _Platyhelminthes_ and certain of
the lower _Annulosa_. Of the _Platyhelminthes_ the ordinary type is
an unsegmented creature: a Planarian or a Trematode exemplifying it.
Among the free forms, as in some Planarians, there occurs transverse
fission, and prompt separation of the segments; while among some
other free forms, as the _Microstomida_, the two segments first
produced, themselves become segmented while still adherent, and this
process is repeated until a string is formed. Another group of the
_Platyhelminthes_, the Cestoid _Entozoa_, exhibit analogous processes.
There are unsegmented forms, as the _Caryophyllæus_, and there are
forms in which the segments, now few now many, adhere together in
chains; the terminal members of which, however, eventually separate,
and having before separation approached the trematode structure,
become independent individuals which grow, creep about, and continue
the race. In both of these types the condition under which the
gemmiparously-produced members remain connected, is that they shall
be able to feed individually: in the one case by lateral mouths, in
the other case by absorption through the integument. It is further
observable that in both cases separation of the component individuals
occurs at sexual maturity, when advantage in nutrition has ceased
to be the dominant need and dispersion of the species has taken its
place in degree of importance. Among Annelids, higher though they are
in type, we find parallelisms. Usually in its first stage an annelid
is unsegmented, but as fast as it elongates lines of segmentation
indent its surface. This segmentation proceeds in various ways, and
the segments exhibit various degrees of dependence. In some low types,
spontaneous fission goes on to the extent of producing single segments,
each of which has such vitality that it buds out anterior and posterior
parts at its two ends. Thus alike in the simple form which exists
before segmentation and in the form exhibited by a detached segment, we
have a unit analogous to each of the units which are joined together in
certain free _Turbellaria_ and in the Cestoids: the difference being
that in the Annelids the sexually mature units do not individually
disunite. But though there does not take place separation of single
completed segments, there takes place separation of groups of segments,
which are either sexually mature at the time they drop off or presently
become so. And the groups of segments which have become sexually mature
before they drop off, have simultaneously acquired swimming organs
and developed eyes, enabling them to spread and diffuse the species.
Sundry biologists recognize a parallelism between that detachment of
developed segments which goes on in the cestoid _Entozoa_, and that
which goes on in the _Scyphomedusæ_. The successively detached members
of the strobila are sexually matured or maturing individuals which,
as medusæ, are fitted for swimming about, multiplying, and reaching
other habitats; while each detached proglottis of the cestoid is, by
the nature of its medium, limited to creeping about. Clearly this
fissiparous process in such Annelids as the _Syllidæ_, which has
similarly been compared to the strobilization of the _Scyphomedusæ_,
differs simply in the respect that single segments are not adapted for
locomotion, and it therefore profits the species to separate in groups.
All these facts and analogies point to the conclusion that the remote
ancestor of the Annelids was an unsegmented creature homologous with
each of the segments of an existing Annelid.
This conclusion is supported by other kinds of evidence here to
be added. The larvæ of Annelids are very various; but amid their
differences there is a recognizable type. “The Trochophore is the
typical larval form of the Annelid stem”: a trochophore being a curious
spheroidal ciliated structure suggestive of cœlenterate affinities. And
this unsegmented larva, representing the remote ancestor from which the
many Annelid types diverged, is similar to the larvæ of the _Rotifera_
and the _Mollusca_: a trochophore is common to all these great classes.
Moreover since, among the _Rhizota_ (a sub-class of the _Rotiferæ_),
there is a species, _Trochosphæra_, solitary and free-swimming,
resembling in form and structure a trochophore, though it is not a
larva but an adult, we get further evidence that there was a primitive
creature of this general character, of which the trochophores of
_Mollusca_, _Rotifera_, and _Annelida_ are divergent modifications,
and which was unsegmented: the implication being that the segmentation
of the _Annelida_ was superinduced. That this segmentation resulted
from gemmation is implied by what are called polytrochal larvæ. These
“sometimes appear as a stage succeeding other larval types. Thus those
of _Arenicola marina_ arise from larvæ which at first were monotrochal,
later became telotrochal, and finally, by the appearance of new
ciliated rings between those already present, assumed the stage of
polytrochal larvæ.... This condition warrants the assumption that the
segmented forms are to be looked upon as the younger, the unsegmented,
on the other hand, as the phylogenetically older.” (Korschelt and
Heider, i, 278.) And that the above-described rings of cilia mark
off segments is shown by the case of _Ophryotrocha puerilis_, which
“remains, as it were, in a larval condition, since the segments
retain their ciliation throughout life.” (_Ib._, 277.) Yet one more
significant fact must be named. In early stages of development each
segment of an archiannelidan has cœlomic spaces separate from those
of neighbouring segments, but in the adult the septa “generally break
down either partially or completely, so that the peri-visceral cavity
becomes a continuous space from end to end of the animal.” (Sedgwick,
_Text Book_, 449.) While this fact is congruous with the hypothesis
here maintained, it is incongruous with the hypothesis that the annelid
was originally an elongated creature which afterwards became segmented;
since in that case the implication would be that the cœlomic septa, not
arising from recapitulation of an ancestral structure, but originated
by the process of segmentation, were first superfluously formed and
then destroyed.
Various lines of evidence thus converge to the conclusion that an
annulose animal is an aggregate of the third order.
In June, 1865, when No. 14 of my serial containing the foregoing
chapter was issued, I supposed myself to be alone in holding this
belief respecting the annulose type, and long continued to suppose so.
Over thirty years later, however, in M. Edmond Perrier’s work, _La
Philosophie Zoologique avant Darwin_, I found mention of a lecture
delivered by M. Lacaze-Duthiers at the École Normale Supérieure in
Paris, and reported in the _Revue des Cours Scientifiques_ for January
28, 1865, in which he enunciated a like belief. Judging, however, by
the account of this lecture which M. Perrier gives (he was present), it
appears that M. Lacaze-Duthiers simply contended that this view of the
annulose structure as arising by union of once-independent units, is
suggested by certain _à priori_ considerations. There is no indication
that he assigned any of the classes of facts above given, which go to
show that it _has_ thus arisen.
For further facts and arguments concerning the genesis of the annulose
type, see Appendix D 2.]