PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.
I. ANIMAL-PLANTS AND WORMS.
The Natural System of the Animal Kingdom.—Linnæus and Lamarck’s
Systems.—The Four Types of Bär and Cuvier.—Their Increase to
Seven Types.—Genealogical Importance of the Seven Types as
Independent Tribes of the Animal Kingdom.—Derivation of
Zoophytes and Worms from Primæval Animals.—Monophyletic and
Polyphyletic Hypothesis of the Descent of the Animal
Kingdom.—Common Origin of the Four Higher Animal Tribes out of
the Worm Tribe.—Division of the Seven Animal Tribes into
Sixteen Main Classes, and Thirty-eight Classes.—Primæval
Animals (Monera, Amœbæ, Synamœbæ), Gregarines, Infusoria,
Planæades, and Gastræades (Planula and Gastrula).—Tribe of
Zoophytes.—Spongiæ (Mucous Sponges, Fibrous Sponges, Calcareous
Sponges).—Sea Nettles, or Acalephæ (Corals, Hood-jellies,
Comb-jellies).—Tribe of Worms.
The natural system of organisms which we must employ in the animal as
well as in the vegetable kingdom, as a guide in our genealogical
investigations, is in both cases of but recent origin, and essentially
determined by the progress of comparative anatomy and ontogeny (the
history of individual development) during the present century. Almost
all the attempts at classification made in the last century followed the
path of the artificial system, which was first established in a
consistent manner by Charles Linnæus. The artificial system differs
essentially from the natural one, in the fact that it does not make the
whole organization and the internal structure (depending upon the blood
relationship) the basis of classification, but only employs individual,
and for the most part external, characteristics, which readily strike
the eye. Thus Linnæus distinguished his twenty-four classes of the
vegetable kingdom principally by the number, formation, and combination
of the stamens. In like manner he distinguished six classes in the
animal kingdom principally by the nature of the heart and blood. These
six classes were: (1) Mammals; (2) Birds; (3) Amphibious Animals; (4)
Fishes; (5) Insects; and (6) Worms.
But these six animal classes of Linnæus are by no means of equal value,
and it was an important advance when, at the end of the last century,
Lamarck comprised the first four classes as vertebrate animals
(Vertebrata), and put them in contrast with the remaining animals (the
insects and worms of Linnæus), of which he made a second main
division—the invertebrate animals (Invertebrata). In reality Lamarck
thus agreed with Aristotle, the father of Natural History, who had
distinguished these two main groups, and called the former
_blood-bearing animals_, the latter _bloodless animals_.
The next important progress towards a natural system of the animal
kingdom was made some decades later by two most illustrious zoologists,
Carl Ernst Bär and George Cuvier. As has already been remarked, they
established, almost simultaneously and independently of one another, the
proposition that it was necessary to distinguish several completely
distinct main groups in the animal kingdom, each of which possessed an
entirely peculiar type or structure (compare above, vol. i. p. 53). In
each of these main divisions there is a tree-shaped and branching
gradation from most simple and imperfect forms to those which are
exceedingly composite and highly developed. The _degree of development_
within each type is quite independent of the peculiar _plan of
structure_, which forms the basis of the type and gives it a special
characteristic. The “type” is determined by the peculiar relations in
position of the most important parts of the body, and the manner in
which the organs are connected. The degree of development, however, is
dependent upon the greater or less division of labour among organs, and
on the differentiation of the plastids and organs. This extremely
important and fruitful idea was established by Bär, who relied more
distinctly and thoroughly upon the history of individual development
than did Cuvier. Cuvier based his argument upon the results of
comparative anatomy. But neither of them recognized the true cause of
the remarkable relationships pointed out by them, which is first
revealed to us by the Theory of Descent. It shows us that the common
_type_ or plan of structure is determined by _inheritance_, and the
degree of development or differentiation by _adaptation_. (Gen. Morph.
ii. 10).
Both Bär and Cuvier distinguished four different types in the animal
kingdom, and divided it accordingly into four great main divisions
(branches or circles). The first of these is formed by the vertebrate
animals (Vertebrata), and comprises Linnæus’ first four classes—mammals,
birds, amphibious animals, and fishes. The second type is formed by the
articulated animals (Articulata), containing Linnæus’ insects,
consequently the six-legged insects, and also the myriopods, spiders,
and crustacea, but besides these, a large number of the worms,
especially the ringed worms. The third main division comprises the
molluscous animals (Mollusca)—slugs, snails, mussels, and some kindred
groups. Finally, the fourth and last circle of the animal kingdom
comprises the various radiated animals (Radiata), which at first sight
differ from the three preceding types by their radiated, flower-like
form of body. For while the bodies of molluscs, articulated animals, and
vertebrated animals consist of two symmetrical lateral halves—of two
counterparts or antimera, of which the one is the mirror of the
other—the bodies of the so-called radiated animals are composed of more
than two, generally of four, five, or six counterparts grouped round a
common central axis, as in the case of a flower. However striking this
difference may seem at first, it is, in reality, a very subordinate one,
and the radial form has by no means the same importance in all “radiated
animals.”
The establishment of these natural main groups or types of the animal
kingdom by Bär and Cuvier was the greatest advance in the classification
of animals since the time of Linnæus. The three groups of vertebrated
animals, articulated animals, and molluscs are so much in accordance
with nature that they are retained, even at the present day, little
altered in extent. But a more accurate knowledge soon showed the utterly
unnatural character of the group of the radiated animals. Leuckart, in
1848, first pointed out that two perfectly distinct types were
confounded under the name, namely, the _Star-fishes_ (Echinoderma)—the
sea-stars, lily encrinites, sea-urchins, and sea-cucumbers; and, on the
other hand, the _Animal-plants_, or _Zoophytes_ (Cœlenterata or
Zoophyta)—the sponges, corals, hood-jellies, and comb-jellies. At the
same time, Siebold united the Infusoria with the Rhizopoda, under the
name of Protozoa (lowest animals), into a special main division of the
animal kingdom. By this the number of animal types was increased to six.
It was finally increased to seven by the fact that modern zoologists
separated the main division of the articulated animals into two groups:
(_a_) those possessing _articulated feet_ (Arthropoda), corresponding to
Linnæus’ Insects, namely, the Flies (with six legs), Myriopods, Spiders,
and Crustacea; and (_b_) the footless _Worms_ (Vermes), or those
possessing non-articulated feet. These latter comprise only the real or
genuine Worms (ring-worms, round worms, planarian worms, etc.), and
therefore in no way correspond with the Worms of Linnæus, who had
included the molluscs, the radiates, and many other lower animals under
this name.
Thus, according to the views of modern zoologists, which are given in
all recent manuals and treatises on zoology, the animal kingdom is
composed of seven completely distinct main divisions or types, each of
which is distinguished by a characteristic plan of structure peculiar to
it, and perfectly distinct from every one of the others. In the natural
system of the animal kingdom—which I shall now proceed to explain as its
probable pedigree—I shall on the whole agree with this usual division,
but not without some modifications, which I consider very important in
connection with genealogy, and which are rendered absolutely necessary
in consequence of our view as to the history of the development of
animals.
We evidently obtain the greatest amount of information concerning the
_pedigree of the animal kingdom_ (as well as concerning that of the
vegetable kingdom) from comparative anatomy and ontogeny. Besides
these, palæontology also throws much valuable light upon the historical
succession of many of the groups. From numerous facts in comparative
anatomy, we may, in the first place, infer the _common origin of all
those animals which belong to one of the seven “types.”_ For in spite of
all the variety in the external form developed within each of these
types, the essential relative position of the parts of the body which
determines the type, is so constant, and agrees so completely in all the
members of every type, that on account of their relations of form alone
we are obliged to unite them, in the natural system, into a single main
group. But we must certainly conclude, moreover, that this conjunction
also has its expression in the pedigree of the animal kingdom. For the
true cause of the intimate agreement in structure can only be the actual
blood relationship. Hence we may, without further discussion, lay down
the important proposition that all animals belonging to one and the same
circle or type must be descended from one and the same original primary
form. In other words, the idea of the circle or type, as it is employed
in zoology since Bär and Cuvier’s time to designate the few principal
main groups or “sub-kingdoms” of the animal kingdoms, coincides with the
idea of “tribe” or “phylum,” as employed by the Theory of Descent.
If, then, we can trace all the varieties of animal forms to these seven
fundamental forms, the following question next presents itself to us as
a second phylogenetic problem—Where do these seven animal tribes come
from? Are they seven original primary forms of an entirely independent
origin, or are they also distantly related by blood to one another?
[Illustration: _PL. VI._
Historical Growth of the six great stems of Animals. _See the
Explanation._]
At first we might be inclined to answer this question in a
_polyphyletic_ sense, by saying that we must assume, for each of the
seven great animal tribes, at least one independent primary form
completely distinct from the others. On further considering this
difficult problem, we arrive in the end at the notion of a
_monophyletic_ origin of the animal kingdom, viz., that these seven
primary forms are connected at their lowest roots, and that they are
derived from a single, common primæval form. _In the animal as well as
in the vegetable kingdom, when closely and accurately considered, the
monophyletic hypothesis of descent is found to be more satisfactory than
the polyphyletic hypothesis._
It is _comparative ontogeny_ (embryology) which first and foremost leads
to the assumption of the monophyletic origin of the whole animal kingdom
(the Protista excepted of course). The zoologist who has thoughtfully
compared the history of the individual development of various animals,
and has understood the importance of the biogenetic principle (p. 33),
cannot but be convinced that a common root must be assumed for the seven
different animal tribes, and that all animals, including man, are
derived from a single, common primary form. The result of the
consideration of the facts of embryology, or ontogeny, is the following
genealogical or phylogenetic hypothesis, which I have put forward and
explained in detail in my “Philosophy of Calcareous Sponges” (Monograph
of the Calcareous Sponges, vol. i. pp. 464, 465, etc.,—“the Theory of
the Layers of the Embryo, and the Pedigree of Animals”).
The first stage of organic life in the Animal kingdom (as in the
Vegetable and Protista kingdoms) was formed by perfectly simple
_Monera_, originating by spontaneous generation. The former existence
of this simplest animal form is, even at present, attested by the fact
that the egg-cell of many animals loses its kernel directly after
becoming fructified, and thus relapses to the lower stage of development
of a cytod without a kernel, like a Moneron. This remarkable occurrence
I have interpreted, according to the law of latent inheritance (vol. i.
p. 205), as a phylogenetic _relapse_ of the cellular form into the
original form of a cytod. The _Monerula_, as we may call this egg-cytod
without a kernel, repeats then, according to the biogenetic principle
(vol. ii. p. 33), the most ancient of all animal forms, the common
primary form of the animal kingdom, namely, the Moneron.
The second ontogenetic process consists in a new kernel being formed in
the Monerula, or egg-cytod, which thus returns again to the value of a
true _egg-cell_. According to this, we must look upon the simple animal
cell, containing a kernel, or the single-celled primæval animal—which
may still be seen in a living state in the _Amœbæ_ of the present day—as
the _second_ step in the series of phylogenetic forms of the animal
kingdom. Like the still living simple Amœbæ, and like the naked
egg-cells of many lower animals (for example, of Sponges and Medusæ,
etc.), which cannot be distinguished from them, the remote phyletic
primary Amœbæ also were perfectly simple naked-cells, which moved about
in the Laurentian primæval ocean, creeping by means of the ever-changing
processes of their body-substance, and nourishing and propagating
themselves in the same way as the Amœbæ of the present day. (Compare
vol. i. p. 188, and vol. ii. p. 54.) The existence of this Amœba-like,
_single-celled primary form_ of the whole animal kingdom is unmistakably
indicated by the exceedingly important fact that the egg of all
animals, from those of sponges and worms up to those of the ant and man,
is a simple cell.
Thirdly, from the “single-cell” state arose the _simplest multicellular
state_, namely, a heap or a small community of simple, equi-formal, and
equivalent cells. Even at the present day, in the ontogenetic
development of every animal egg-cell, there first arises a globular heap
of equi-formal naked cells, by the repeated self-division of the primary
cell. (Compare vol. i. p. 190 and the Frontispiece, Fig. 3.) We called
this accumulation of cells the _mulberry state_ (Morula), because it
resembles a mulberry or blackberry. This Morula-body occurs in the same
simple form in all the different tribes of animals, and on account of
this most important circumstance we may infer—according to the
biogenetic principle—that the _most ancient, many-celled, primary form
of the animal kingdom_ resembled a Morula like this, and was in fact a
simple heap of Amœba-like primæval cells, one similar to the other. We
shall call this most ancient community of Amœbæ—this most simple
accumulation of animal cells—which is recapitulated in individual
development by the Morula—the _Synamœba_.
Out of the Synamœbæ, in the early Laurentian period, there afterwards
developed a fourth primary form of the animal kingdom, which we shall
call the ciliated germ (Planæa). This arose out of the Synamœba by the
outer cells on the surface of the cellular community beginning to extend
vibrating fringes called cilia, and becoming “ciliated cells,” and thus
differentiating from the inner and unchanged cells. The Synamœbæ
consisted of completely equi-formed and naked cells, and crept about
slowly, at the bottom of the Laurentian primæval ocean, by means of
movements like those of an Amœba. The Planæa, on the other hand,
consisted of two kinds of different cells—inner ones like the Amœbæ, and
external “ciliated cells.” By the vibrating movements of the cilia the
entire multicellular body acquired a more rapid and stronger motion, and
passed over from the creeping to the swimming mode of locomotion. In
exactly the same manner the _Morula_, in the ontogenesis of lower
animals, still changes into a ciliated form of larva, which has been
known, since the year 1847, under the name of _Planula_. This Planula is
sometimes a globular, sometimes an oval body, which swims about in the
water by means of a vibrating movement; the fringed (ciliated) and
smaller cells of the surface differ from the larger inner cells, which
are unfringed. (Fig. 4 of the Frontispiece.)
Out of this Planula, or fringed larva, there then develops, in animals
of all tribes, an exceedingly important and interesting animal form,
which, in my Monograph of the Calcareous Sponges, I have named
_Gastrula_ (that is, larva with a stomach or intestine). (Frontispiece,
Fig. 5, 6). This Gastrula externally resembles the Planula, but differs
essentially from it in the fact that it encloses a cavity which opens to
the outside by a mouth. The cavity is the “_primary intestine_,” or
“primary stomach,” the _progaster_, the first beginning of the
alimentary canal; its opening is the “_primary mouth_” (prostoma). The
wall of the progaster consists of two layers of cells: an outer layer of
smaller ciliated cells (outer skin, or ectoderm), and of an inner layer
of larger non-ciliated cells (inner skin, or entoderm). This exceedingly
important larval form, the “Gastrula,” makes its appearance in the
ontogenesis of all tribes of animals—in Sponges, Medusæ, Corals, Worms,
Sea-squirts, Radiated animals, Molluscs, and even in the lowest
Vertebrata (Amphioxus: compare p. 200, Plate XII., Fig. _B_ 4; see also
in the same place the Ascidian, Fig. _A_ 4).
Definition of the _forms_ | +Ontogenesis.+ | +Phylogenesis.+
of the five first stages | The five first stages | The five first stages
of the development of | of the individual | of the phyletic or
the animal body. | development. | historical development.
-------------------------------+------------------------------+------------------------
| |
_First Stage of Development._ | 1. | 1.
| +Monerula.+ | +Moneron.+
A simple cytod (a | |
plastid without a kernel.) | Animal egg without a | Most ancient animal
| | kernel (when the egg-kernel | Monera, originating by
| | has disappeared, | spontaneous generation.
| | after being fructified). | |
| | | | |
| | | | |
_Second Stage of Development._ | 2. | 2.
| +Ovulum.+ | +Amœbæ.+
A simple cell (a | |
plastid containing a | Animal egg with kernel | Animal Amœbæ.
kernel.) | (a simple egg-cell). | |
| | | | |
| | | | |
| | | | |
_Third Stage of Development._ | 3. | 3.
| +Morula.+ | +Synamœba.+
A community (an | (_Mulberry form._) |
aggregation of identical | | An aggregation of
simple cells). | Globular heap of homogeneous | Amœbæ.
| | “cleavage spheres.” | |
| | | | |
| | | | |
_Fourth Stage of Development._ | 4. | 4.
| +Planula.+ | +Planæa.+
A solid or bladder-shaped, | (_Ciliated larva_.) |
globular, or oval | | Many-celled primæval
body, _composed of two | Many-celled larva | animal without
kinds of different cells_: | without mouth, composed | mouth, composed of
externally ciliated, | of different cells. | two kinds of different
internally non-ciliated | | | cells.
cells. | | | |
| | | | |
| | | | |
_Fifth Stage of Development._ | 5. | 5.
| +Gastrula.+ | +Gastræa.+
A globular or oval | (_Larva with mouth._) |
_body with simple intestinal | Many-celled with intestines | Many-celled primæval
cavity and mouth-opening. | and mouth; intestinal | animal with intestine
Body wall composed | wall with two | and mouth; intestinal
of two layers_; an | layers. | wall with two
externally ciliated ectoderm | | layers. (Primary form
(dermal layer), an | | of zoophytes and
internally non-ciliated | | worms.)
entoderm (gastral layer). | |
From the ontogenetic occurrence of the Gastrula in the most different
animal classes, from Zoophytes up to Vertebrata, we may, according to
the biogenetic principle, safely draw the conclusion that during the
Laurentian period there existed a common primary form of the six higher
animal tribes, which in all essential points was formed like the
Gastrula, and which we shall call the Gastræa. This Gastræa possessed a
perfectly simple globular or oval body, which enclosed a simple cavity
of like form, namely, the progaster; at one of the poles of the
longitudinal axis the primary intestine opened by a mouth which served
for the reception of nutrition. The body wall (which was also the
intestinal wall) consisted of two layers of cells, the unfringed
entoderm, or intestinal layer, and the fringed ectoderm, or skin-layer;
by the motion of the cilia or fringes of the latter the Gastræa swam
about freely in the Laurentian ocean. Even in those higher animals, in
the ontogenesis of which the original Gastrula form has disappeared,
according to the laws of abbreviated inheritance (vol. i. p. 212), the
composition of the Gastræa body has been transmitted to the phase of
development which directly arises out of the Morula. This phase is an
oval or round disc consisting of two cell-layers or membranes: the outer
cell-layer, the _animal or dermal layer_ (ectoblast), corresponds to the
ectoderm of the Gastræa; out of it develops the external, loose skin
(epidermis), with its glands and appendages, as well as the central
nervous system. The inner cell-layer, the _vegetative or intestinal
layer_ (hypoblast), is originally the entoderm of the Gastræa; out of it
develops the inner membrane (epithelium) of the intestinal canal and its
glands. (Compare my Monograph of the Calcareous Sponges, vol. i. p. 466,
etc.)
By ontogeny we have already gained five primordial stages of development
of the animal kingdom: (1) the Moneron; (2) the Amœba; (3) the Synamœba;
(4) the Planæa; and (5) the Gastræa. The former existence of these five
oldest primary forms, which succeeded one another, and which must have
lived in the Laurentian period, follows as a consequence of the
biogenetic principle; that is to say, from the parallelism and the
mechanico-causal connection of ontogenesis and phylogenesis. (Compare
vol. i. p. 309.) In our genealogical system of the animal kingdom we may
class all these animal forms, long since extinct, and, which on account
of the soft nature of their bodies could leave no fossil remains, among
the tribe of Primæval animals (Protozoa), which also comprises the still
living Infusoria and Gregarinæ.
The phyletic development of the six higher animal tribes, which are all
derived from the Gastræa, deviated at this point in two directions. In
other words, the _Gastræads_ (as we may call the group of forms
characterized by the Gastræa-type of structure), divided into two
divergent lines or branches; the one branch of Gastræads gave up free
locomotion, adhered to the bottom of the sea, and thus, by adopting an
adhesive mode of life, gave rise to the _Protascus_, the common primary
form of the _Animal-plants_ (Zoophyta). The other branch of the
Gastræads retained free locomotion, did not become adherent and later
on developed into the _Prothelmis_, the common primary form of _Worms_
(Vermes). (Compare p. 133.)
This latter tribe (as limited by modern zoology) is of the greatest
interest in the study of genealogy. For among Worms, as we shall see
later, there are, besides very numerous peculiar families, and besides
many independent classes, also very remarkable forms, which may be
considered as _forms of direct transition_ to the four higher animal
tribes. Both comparative anatomy and the ontogeny of these worms enable
us to recognize in them the nearest blood relations of those extinct
animal forms which were the original primary forms of the four higher
animal tribes. Hence these latter, the Molluscs, Star-fishes,
Articulated animals, and Vertebrate animals, do not stand in any close
blood relationship to one another, but have originated independently in
four different places out of the tribe of Worms.
In this way comparative anatomy and phylogeny lead us to the
_monophyletic pedigree of the animal kingdom_, the outlines of which are
given on p. 133. According to it the seven phyla, or tribes, of the
animal kingdom are of different value in regard to genealogy. The
original primary group of the whole animal kingdom is formed by the
Primæval animals (Protozoa), including the Infusoria and Gastræads. Out
of these latter arose the two tribes of Animal-plants (Zoophyta) and
Worms as diverging branches. Out of four different groups of the Worm
tribe, the four higher tribes of the animal kingdom were developed—the
Star-fishes (Echinoderma) and Insects (Arthropoda) on the one hand, and
the Molluscs (Mollusca) and Vertebrated animals (Vertebrata) on the
other.
Having thus sketched out the monophyletic pedigree of the animal kingdom
in its most important features, we must now turn to a closer examination
of the historical course of development which the seven tribes of the
animal kingdom, and the classes distinguished in them, have passed
through (p. 132). There is a much larger number of classes in the animal
than in the vegetable kingdom, owing to the simple reason that the
animal body, in consequence of its more varied and perfect vital
activity, could differentiate and develop in very many more different
directions than could the vegetable body. Thus, while we were able to
divide the whole vegetable kingdom into six main classes and nineteen
classes, we have to distinguish, at least, sixteen main classes and
thirty-eight classes in the animal kingdom. These are distributed among
the seven different tribes of the animal kingdom in the way shown in the
Systematic Survey on pages 132 and 133.
The group of _Primæval animals_ (Protozoa) within the compass which we
here assign to this tribe, comprises the most ancient and the simplest
primary forms of the animal kingdom; for example, the five oldest
phyletic stages of development previously mentioned, and besides these
the Infusoria and Gregarinæ, as well as all those imperfect animal
forms, for which, on account of their simple and indifferent
organization, no place can be found in any of the other six animal
tribes. Most zoologists, in addition to these, include among the
Protozoa a larger or smaller portion of those lowest organisms, which we
mentioned in our neutral kingdom of Protista (in Chapter XVI.). But
these Protista, especially the large division of the Rhizopoda, which
are so rich in forms, cannot be considered as real animals for reasons
previously given. Hence, if we here leave them out of the question, we
may accept two main classes or provinces of real Protozoa, namely, _Egg
animals_ (Ovularia) and _Germ animals_ (Blastularia). To the former
belong the three classes of Archezoa, Gregarinæ, and Infusoria, to the
latter the two classes of Planæads and Gastræads.
SYSTEMATIC SURVEY
_Of the 16 Main Classes and 38 Classes of the Animal Kingdom._
------------------+-----------------------+----------------------+-----------------
_Tribes or Phyla_ | _Main Classes_, | _Classes_ |_Systematic Name_
_of the_ |_Branches or Clades_ | _of the_ | _of the_
_Animal Kingdom._ | _of the_ | _Animal Kingdom._ | _Classes._
| _Animal Kingdom._ | |
------------------+-----------------------+----------------------+------------------
A. {
=Primæval= { I. Egg-animals { 1. Archaic animals 1. Archezoa
=Animals= { _Ovularia_ { 2. Gregarines 2. Gregarinæ
{ { 3. Infusoria 3. Infusoria
+Protozoa+ {
{II. Mulberry animals { 4. Planæads 4. Planæadas
{ _Blastularia_ { 5. Gastræads 5. Gastræadas
B. {
=Animal= { III. Sponges { 6. Sponges 6. Porifera
=Plants= { _Spongiæ_ {
{
+Zoophyta+ { IV. Sea-nettles { 7. Corals 7. Coralla
{ _Acalephæ_ { 8. Hood-jellies 8. Hydromedusæ
{ { 9. Comb-jellies 9. Ctenophora
C. {V. Bloodless worms {10. Planary worms 10. Platyhelminthes
=Worms= { _Acœlomi_
{ {11. Round worms 11. Nemathelminthes
+Vermes+ { VI. Blood-bearing {12. Moss-polyps 12. Bryozoa
{ worms {13. Sac-worms 13. Tunicata
{ _Cœlomati_ {14. Proboscideans 14. Rhynchocœla
{ {15. Star-worms 15. Gephyrea
{ {16. Wheel animalcules 16. Rotatoria
{ {17. Ring-worms 17. Annelida
D. {VII. Headless shellfish {18. Lamp-shells 18. Spirobranchia
=Molluscs= { _Acephala_ {19. Mussels 19. Lamellibranchia
{
+Mollusca+ { VIII. Head-bearing {20. Snails 20. Cochlides
{ _Eucephala_ {21. Cuttles 21. Cephalopoda
E. { IX. Ringed-arms {22. Sea-stars 22. Asterida
=Star-fishes= { _Colobrachia_ {23. Lily-stars 23. Crinoida
{
+Echinoderma+ { X. Armless {24. Sea-urchins 24. Echinida
{ _Lipobrachia_ {25. Sea-cucumbers 25. Holothuriæ
F. { XI. Gill-breathers {26. Crab-fish 26. Crustacea
=Articulated= { _Carides_ {
=Animals= {
{ XII. Tube-breathers {27. Spiders 27. Arachnida
+Arthropoda+ { _Tracheata_ {28. Centipedes 28. Myriopoda
{ {29. Flies 29. Insecta
{ XIII. Skull-less {30. Lancelets 30. Leptocardia
{ _Acrania_ {
G. {
=Vertebrate= { XIV. Single-nostriled {31. Lampreys 31. Cyclostoma
=Animals= { _Monorrhina_ {
{
+Vertebrata+ { XV. Amnion-less {32. Fishes 32. Pisces
{ _Anamnia_ {33. Mud-fish 33. Dipneusta
{ {34. Sea dragons 34. Halisauria
{ {35. Amphibians 35. Amphibia
{
{ XVI. Amnion-bearing {36. Reptiles 36. Reptilia
{ _Amniota_ {37. Birds 37. Aves
{ {38. Mammals 38. Mammalia
----------------------------------------------------------------
+Vertebrata+
(_Vertebrated animals_)
Craniota
|
+Arthropoda+ | +Mollusca+
(_Articulated Animals_) | (_Molluscs_)
+Echinoderma+ Tracheata | Eucephala
(_Star-fishes_) | | |
| | |
Lipobrachia Crustacea Acrania |
| Annelida | | Tunicata Acephala
| | | | | Bryozoa |
Colobrachia | | | | | |
| Gephyrea| | Rotatoria \--v---/ \--v---/
| | | | | | |
| | | | | | |
\---------v-------------------/ \------v-----/
| |
| +Vermes+ |
| (_Worms_) |
\------------------v-------------------/
|
|
CŒLOMATI
(_Worms with a body-cavity_)
| Platyhelminthes
| |
| |
\-----v------/
+Zoophyta+ |
(_Animal-Plants_) +ACŒLOMI+
Spongiæ Acalephæ (_Worms without body-cavity_)
| | |
| | |
\------v-------/ |
| |
Protascus Prothelmis
| |
| |
\-----------v---------------/
|
| +Protozoa+
| (_Primæval animals_)
|
|/-------------^-------------\
+GASTRÆA+
|
| Infusoria
| |
+PLANÆA+ | Gregarinæ
| | |
| | |
+SYNAMŒBÆ+ \----v---/
| |
| |
\---------v--------/
+AMŒBÆ+
|
+MONERA+
The first province of the Protozoa consists of the _Egg animals_
(Ovularia); we include among them all _single-celled animals_, all
animals whose body, in the fully developed state, possesses the
form-value of a _simple plastid_ (of a cytod or a cell), also those
simple animal forms whose body consists of an aggregation of several
cells perfectly similar one to another.
The _Archaic animals_ (Archezoa) form the first class in the series of
Egg animals. It contains only the most simple and most ancient primary
forms of the animal kingdom, whose former existence we have proved by
means of the fundamental law of biogenesis; they are, (1) Animal Monera;
(2) Animal Amœbæ; (3) Animal Synamœbæ. We may, if we choose, include
among them a portion of the still living Monera and Amœbæ, but another
portion (according to the discussion in Chapter XVI.) must on account of
their neutral nature be considered as Protista, and a third portion, on
account of their vegetable nature, must be considered as plants.
A second class of the egg animals consists of the _Gregarines_
(Gregarinæ), which live as parasites in the intestines and body-cavities
of many animals. Some of these Gregarines are perfectly simple cells
like the Amœbæ; some form chains of two or three identical cells, one
lying behind the other. They differ from the naked Amœbæ by possessing
a thick, simple membrane, which surrounds their cell-body; they can be
considered as animal Amœbæ which have adopted a parasitical mode of
life, and in consequence have surrounded themselves with a secreted
covering.
As a third class of egg animals, we adopt the real _Infusoria_
(Infusoria), embracing those forms to which modern zoology almost
universally limits this class of animals. The principal portion of them
consists of the small _ciliated Infusoria_ (Ciliata), which inhabit all
the fresh and salt waters of the earth in great numbers, and which swim
about by means of a delicate garb of vibratile fringes. A second and
smaller division consists of the adherent _sucking Infusoria_ (Acinetæ),
which take their food by means of fine sucking-tubes. Although during
the last thirty years numerous and very careful investigations have been
made on these small animalcules,—which are mostly invisible to the naked
eye,—still we are even now not very sure about their development and
form-value. We do not even yet know whether the Infusoria are single or
many-celled; but as no investigator has as yet proved their body to be a
combination of cells, we are, in the mean time, justified in considering
them as single-celled, like the Gregarines and the Amœbæ.
The second main class of primæval animals consists of the _Germ animals_
(Blastularia). This name we give to those extinct Protozoa which
correspond to the two ontogenetic embryonic forms of the six higher
animal tribes, namely, the Planula and the Gastrula. The body of these
Blastularia, in a perfectly developed state, was composed of many cells,
and these cells moreover differentiated—in two ways at least—into an
external (animal or dermal) and an internal (vegetative or gastral)
mass. Whether there still exist representatives of this group is
uncertain. Their former existence is undoubtedly proved by the two
exceedingly important ontogenetic animal forms which we have already
described as Planula and Gastrula, and which still occur as a transient
stage of development in the ontogeny of the most different tribes of
animals. Corresponding to these, we may, according to the biogenetic
principle, assume the former existence of two distinct classes of
Blastularia, namely, the _Planæada_ and _Gastræada_. The type of the
_Planæada_ is the _Planæa_—long since extinct—but whose historical
portrait is still presented to us at the present day in the widely
distributed _ciliated larva_ (Planula). (Frontispiece, Fig. 4.) The type
of the _Gastræada_ is the _Gastræa_, of whose original nature the
mouth-and-stomach larva (Gastrula), which recurs in the most different
animal tribes, still gives a faithful representation. (Frontispiece Fig.
5, 6.) Out of the Gastræa, as we have previously mentioned, there were
at one time developed two different primary forms, the Protascus and
Prothelmis; the former must be looked upon as the primary form of the
Zoophytes, the latter as the primary form of Worms. (Compare the
enunciation of this hypothesis in my Monograph of the Calcareous
Sponges, vol i. p. 464.)
The _Animal-plants_ (Zoophyta, or Cœlenterata) which constitute the
second tribe of the animal kingdom, rise considerably above the
primitive animals in the characters of their whole organisation, while
they remain far below most of the higher animals. For in the latter
(with the exception only of the lowest forms) the four distinct
functions of nutrition—namely, digestion, circulation of the blood,
respiration, and excretion—are universally accomplished by four
perfectly different systems of organs: by the intestines, the vascular
system, the organs of respiration, and the urinary apparatus. In
Zoophytes, however, these functions and their organs are not yet
separate, and are all performed by a single system of alimentary canals,
by the so-called gastro-vascular system, or the cœlenteric apparatus of
the intestinal cavity. The mouth, which is also the anus, leads into a
stomach, into which the other cavities of the body also open. In
Zoophytes the body-cavity, or “cœloma,” possessed by the four higher
tribes of animals is still completely wanting, likewise the vascular
system and blood, as also the organs of respiration, etc.
All Zoophytes live in water; most of them in the sea, only a very few in
fresh water, such as fresh-water sponges (Spongilla) and some primæval
polyps (Hydra, Cordylophora). A specimen of the pretty flower-like forms
which are met with in great variety among Zoophytes is given on Plate
VII. (Compare its explanation in the Appendix.)
The tribe of animal-plants, or Zoophytes, is divided into two distinct
provinces, the _Sponges_, or _Spongiæ_, and the _Sea-nettles_, or
_Acalephæ_ (p. 144). The latter are much richer in forms and more highly
organized than the former. In all Sponges the entire body, as well as
the individual organs, are differentiated and perfected to a much less
extent than in Sea-nettles. All Sponges lack the characteristic
_nettle-organs_ which all Sea-nettles possess.
The common primary form of all Zoophytes must be looked for in the
_Protascus_, an animal form long since extinct, but whose existence is
proved according to the biogenetic principle by the Ascula. This Ascula
is an ontogenetical development form which, in Sponges as well as in
Sea-nettles, proceeds from the Gastrula. (Compare the Ascula of the
calcareous sponge on the Frontispiece, Fig. 7, 8.) For after the
Gastrula of zoophytes has for a time swum about in the water it sinks to
the bottom, and there adheres by that pole of its axis which is opposite
to the opening of the mouth. The external cells of the ectoderm draw in
their vibrating, ciliary hairs, whereas, on the contrary, the inner
cells of the entoderm begin to form them. Thus the Ascula, as we call
this changed form of larva, is a simple sack, its cavity (the cavity of
the stomach or intestine) opening by a mouth externally, at the upper
pole of the longitudinal axis (opposite the basal point of fixture). The
entire body is here in a certain sense a mere stomach or intestinal
canal, as in the case of the Gastrula. The wall of the sack, which is
both body wall and intestinal wall, consists of two layers or coats of
cells, a fringed _entoderm_, or gastral layer (corresponding with the
inner or vegetative germ-layer of the higher animals), and an unfringed
exoderm or dermal layer (corresponding with the external or animal
germ-layer of the higher animals). The original _Protascus_, a true
likeness of which is still furnished by the Ascula, probably formed
egg-cells and sperm-cells out of its gastral layer.
The Protascads—as we will call the most ancient group of vegetable
animals, represented by the Protascus-type—divided into two lines or
branches, the Spongiæ and the Sea-nettles, or Acalephæ. I have shown in
my Monograph of the Calcareous Sponges (vol. i. p. 485) how closely
these two main classes of Zoophytes are related, and how they must both
be derived, as two diverging forms, from the Protascus-form. The primary
form of Spongiæ, which I have there called Archispongia, arose out of
the Protascus by the formation of pores through its body-wall; the
primary form of Sea-nettles, which I there called Archydra, developed
out of the Protascus by the formation of nettle-organs, as also by the
formation of feelers or tentacles.
The main-class or branch of the _Sponges_, _Spongiæ_, or _Porifera_,
lives in the sea, with the single exception of the green fresh-water
Sponge (Spongilla). These animals were long considered as plants, later
as Protista; in most Manuals they are still classed among the primæval
animals, or Protozoa. But since I have demonstrated their development
out of the Gastrula, and the construction of their bodies of two
cellular germ-layers (as in all higher animals), their close
relationship to Sea-nettles, and especially to the Hydrapolyps, seems
finally to be established. The _Olynthus_ especially, which I consider
as the common primary form of calcareous sponges, has thrown a complete
and unmistakable light upon this point.
The numerous forms comprised in the class of Spongiæ have as yet been
but little examined; they may be divided into three legions and eight
orders. The first legion consists of the soft, gelatinous _Mucous
Sponges_ (Myxospongiæ), which are characterized by the absence of any
hard skeleton. Among them are, on the one hand, the long-since-extinct
primary forms of the whole class, the type of which I consider to be the
Archispongia; on the other hand there are the still living, gelatinous
sponges, of which the _Halisarca_ is best known. We can obtain a notion
of the Archispongia, the most ancient primæval sponge, if we imagine the
Olynthus (see Frontispiece), to be deprived of its radiating calcareous
spiculæ.
The second legion of Spongiæ contains the _Fibrous Sponges_
(Fibrospongiæ), the soft body of which is supported by a firm, fibrous
skeleton. This fibrous skeleton often consists merely of so-called
“horny fibres,” formed of a very elastic, not readily destructible,
organic substance. This is the case for instance in our common bathing
Sponge (Euspongia officinalis), the purified skeleton of which we use
every morning when washing. Blended with the horny, fibrous skeleton of
many of these Sponges, there are numerous flinty spicula; this is the
case for example with the fresh-water Sponge (Spongilla). In others the
whole skeleton consists of only calcareous or silicious spicula which
are frequently interwoven into an extremely beautiful lattice-work, as
in the celebrated Venus’ Flower Basket (Euplectella). Three orders of
fibrous sponges may be distinguished according to the different
formation of the spicula, namely, Chalynthina, Geodina, and
Hexactinella. The natural history of the fibrous sponges is of especial
interest to the Theory of Descent, as was first shown by Oscar Schmidt,
the greatest authority on this group of animals. In no other group,
perhaps, can the unlimited pliability of the specific form, and its
relation to Adaptation and Inheritance, be so clearly followed step by
step; perhaps in no other group is the species so difficult to limit and
define.
[Illustration: Pl. VII.
E. Haeckel del. Lagesse sc.]
This proposition, which applies to the great legion of the Fibrous
Sponges, applies in a still higher degree to the smaller but exceedingly
interesting legion of the calcareous sponges (Calcispongiæ), on which in
1872, after five years’ careful examination, I published a comprehensive
Monograph. The sixty plates of figures accompanying this Monograph
explain the extreme pliability of these small sponges “good species” of
which, in fact, cannot be spoken of in the usual systematic sense. We
find among them only varying series of forms, which do not even
completely transmit their specific form to their nearest descendants,
but by adaptation to subordinate, external conditions of existence,
perpetually change. It frequently occurs here, that there arise out of
one and the same stock different form-species, which according to the
usual system would belong to several quite distinct genera; this is the
case, for instance, with the remarkable Ascometra (Frontispiece, Fig.
10.) The entire external bodily form is much more pliable and protean in
Calcareous Sponges than in the silicious sponges, which are
characterized by possessing silicious spicula, forming a beautiful
skeleton. Through the study of the comparative anatomy and ontogeny of
calcareous sponges, we can recognise, with the greatest certainty, the
common primary form of the whole group, namely, the sack-shaped
_Olynthus_, whose development is represented in the Frontispiece
(compare its explanation in the Appendix). Out of the Olynthus (Fig. 9
on the Frontispiece), the order of the Ascones was the first to develop,
out of which, at a later period, the two other orders of Calcareous
Sponges, the _Leucones_ and _Sycones_, arose as diverging branches.
Within these orders, the descent of the individual forms can again be
followed step by step. Thus the Calcareous Sponges in every respect
confirm the proposition which I have elsewhere maintained: that “the
natural history of sponges forms a connected and striking argument in
favour of Darwin.”
The second main class or branch in the tribe of Zoophytes is formed by
the Sea-nettles (Acalephæ, or Cnidæ). This interesting group of animals,
so rich in forms, is composed of three different classes, namely, the
Hood-jellies (Hydromedusæ), the Comb-jellies (Ctenophora), and the
Corals (Coralla). The hypothetical, extinct Archydra must be looked upon
as the common primary form of the whole group; it has left two near
relations in the still living fresh-water polyps (Hydra and
Cordylophora). The Archydra was very closely related to the simplest
forms of Spongiæ (Archispongia and Olynthus), and probably differed from
them only by possessing nettle organs, and by the absence of cutaneous
pores. Out of the Archydra there first developed the different Hydroid
polyps, some of which became the primary forms of Corals, others the
primary forms of Hydromedusæ. The Ctenophora developed later out of a
branch of the latter.
The Sea-nettles differ from the Spongiæ (with which they agree in the
characteristic formation of the system of the alimentary canal)
principally by the constant possession of nettle organs. These are small
bladders filled with poison, large numbers—generally millions—of which
are dispersed over the skin of the sea nettles, and which burst and
empty their contents when touched. Small animals are killed by this; in
larger animals this nettle poison causes a slight inflammation of the
skin, just as does the poison of our common nettles. Any one who has
often bathed in the sea, will probably have at times come in contact
with large Hood-jellies (Jelly-fish), and become acquainted with the
unpleasant burning feeling which their nettle organs can produce. The
poison in the splendid blue Jelly-fish, Physalia, or Portuguese
Man-of-war, acts so powerfully that it may lead to the death of a human
being.
The class of Corals (Coralla) lives exclusively in the sea, and is more
especially represented in the warm seas by an abundance of beautiful and
highly-coloured forms like flowers. Hence they are also called
_Flower-animals_ (Anthozoa). Most of them are attached to the bottom of
the sea, and contain an internal calcareous skeleton. Many of them by
continued growth produce such immense stocks that their calcareous
skeletons have formed the foundation of whole islands, as is the case
with the celebrated coral reefs and atolls of the South Seas, the
remarkable forms of which were first explained by Darwin.(13) In corals
the counterparts, or antimera—that is, the corresponding divisions of
the body which radiate from and surround the central main axis of the
body—exist sometimes to the number of four, sometimes to the number of
six or eight. According to this we distinguish three legions, the
Fourfold (Tetracoralla), Sixfold (Hexacoralla), and Eightfold corals
(Octocoralla). The fourfold corals form the common primary group of the
class, out of which the sixfold and eightfold have developed as two
diverging branches.
SYSTEMATIC SURVEY
_Of the 4 Classes and 30 Orders of the Animal Plants, or Zoophytes._
----------------+--------------------+--------------------+---------------
_Class of the_ | _Legions of the_ | _Orders of the_ | _A Genus Name_
_Zoophytes._ | _Zoophytes._ | _Zoophytes._ | _as example._
----------------+--------------------+--------------------+---------------
I. { I. Myxospongiæ { 1. Archispongina | Archispongia
{ _Mucous Sponges_ { 2. Halisarcina | Halisarca
=Sponges= { |
{ II. Fibrospongiæ { 3. Chalynthina | Spongilla
+Spongiæ+ {_Fibrous Sponges_ { 4. Geodina | Ancorina
or { { 5. Hexactinella | Euplectella
+Porifera+ { |
{ III. Calcispongiæ { 6. Ascones | Olynthus
{_Calcareous Sponges_ { 7. Leucones | Dyssycus
{ { 8. Sycones | Sycurus
|
|
II. { IV. Tetracoralla { 9. Rugosa | Cyathophyllum
{_Fourfold Corals_ { 10. Paranemeta | Cereanthus
=Corals= { |
{ V. Hexacoralla { 11. Cauliculata | Antipathes
+Coralla+ {_Sixfold Corals_ { 12. Madreporaria | Astræa
or { { 13. Halirhoda | Actinia
+Anthozoa+ { |
{ VI. Octocoralla { 14. Alcyonida | Lobularia
{_Eightfold Corals_ { 15. Gorgonida | Isis
{ { 16. Pennatulida | Veretillum
|
|
III. { VII. Archydræ } 17. Hydraria | Hydra
{_Primæval Polyps_ } |
=Jelly-polyps= { |
{ VIII. Leptomedusæ { 18. Vesiculata | Sertularia
+Hydromedusæ+ {_Soft Jelly-fish_ { 19. Ocellata | Tubularia
{ { 20. Siphonophora | Physophora
or { |
{ IX. Trachymedusæ { 21. Marsiporchida| Trachynema
=Hood-jellies= { _Hard Jelly-fish_ { 22. Phyllorchida | Geryonia
{ { 23. Elasmorchida | Charybdæ
+Medusa+ { |
{ X. Calycozoa } 24. Podactinaria | Lucernaria
{ _Stalked Jellies_ } |
{ |
{ XI. Discomedusæ { 25. Semæostomeæ | Aurelia
{ _Disc-jellies_ { 26. Rhizostomeæ | Crambessa
|
|
IV. { XII. Eurystoma } 27. Beroida | Beroe
{ _Wide-mouthed_ } |
=Comb-jellies= { |
{ XIII. Stenostoma { 28. Saccata | Cydippe
+Ctenophora+ { _Narrow-mouthed_ { 29. Lobata | Eucharis
{ { 30. Tæniata | Cestum
----------------------------------------------------------------
Ctenophora Hydromedusæ
Tæniata Lobata Rhizostomeæ
| | |
| | |
\---v----/ Semæostomeæ
Saccata DISCOMEDUSÆ
STENOSTOMA |
| Trachymedusæ | Siphonophora
| | | |
| | Lucernaria | |
EURYSTOMA> | Calycozoa | |
| | | | |
| | | | |
\-------------------------v----------------/
LEPTOMEDUSÆ
Coralla |
Octocoralla |
Hexacoralla | |
| | |
| | |
\----v----/ |
Tetracoralla |
Spongiæ | |
Fibrospongiæ Calcispongiæ | |
Chalynthina Leucones Sycones| |
| | | | |
| | | | |
| | | | |
Hexactinella|Geodina Dyssycus Sycurus | Hydroida
| | | | | | | Cordylophora
| | | | | | | | Hydra
\------v---/ \-----v--/ | | | |
| Ascones | | | |
| Myxospongia | | \-----v-------/
| Halisarcina | | HYDROIDA
| | | Procorallum |
| | | | |
CHALYNTHUS | OLYNTHUS | |
| | | \-----v--------/
\------v----------/ Hydroida
Archispongiæ |
| |
| Archydra
| |
\--------------v--------/
Protascus
|
Gastræa
The second class of Sea-nettles is formed by the _Hood-jellies_ (Medusæ)
or _Polyp-jellies_ (Hydromedusæ). While most corals form stocks like
plants, and are attached to the bottom of the sea, the Hood-jellies
generally swim about freely in the form of gelatinous bells. There are,
however, numbers of them, especially the lower forms, which adhere to
the bottom of the sea, and resemble pretty little trees. The lowest and
simplest members of this class are the little fresh-water polyps (Hydra
and Cordylophora). We may look upon them as but little changed
descendants of those _Primæval polyps_ (Archydræ), from which, during
the primordial period, the whole division of the Sea-nettles originated.
Scarcely distinguishable from the Hydra are the adherent Hydroid polyps
(Campanularia, Tubularia), which produce freely swimming medusæ by
budding, and out of the eggs of these there again arise adherent polyps.
These freely swimming Hood-jellies are mostly of the form of a mushroom,
or of an umbrella, from the rim of which many long and delicate
tentacles hang. They are among the most beautiful and most interesting
inhabitants of the sea. The remarkable history of their lives, and
especially the complicated alternation of generation of polyps and
medusæ, are among the strongest proofs of the truth of the theory of
descent. For just as Medusæ still daily arise out of the Hydroids, did
the freely swimming medusa-form originally proceed, phylogenetically,
out of the adherent polyp-form. Equally important for the theory of
descent is the remarkable _division of labour_ of the individuals, which
among some of them is developed to an astonishingly high degree, more
especially in the splendid _Siphonophora_.(37) (Plate VII. Fig. 13.)
The third class of Sea-nettles—the peculiar division of Comb-jellies
(Ctenophora), probably developed out of a branch of the Hood-jellies.
The Ctenophora, which are also called Ribbed-jellies, possess a body of
the form of a cucumber, which, like the body of most Hood-jellies, is as
clear and transparent as crystal or cut glass. Comb or Ribbed-jellies
are characterized by their peculiar organs of motion, namely, by eight
rows of paddling, ciliated leaflets, which run in the form of eight ribs
from one end of the longitudinal axis (from the mouth) to the opposite
end. Those with narrow mouths (Stenostoma) probably developed later out
of those with wide mouths (Eurystoma). (Compare Plate VII. Fig. 16.)
The third tribe of the animal kingdom, the phylum of _Worms_ or
worm-like animals (Vermes, or Helminthes), contains a number of
diverging branches. Some of these numerous branches have developed into
well-marked and perfectly independent classes of Worms, but others
changed long since into the original, radical forms of the four higher
tribes of animals. Each of these four higher tribes (and likewise the
tribe of Zoophytes) we may picture to ourselves in the form of a lofty
tree, whose branches represent the different classes, orders, families,
etc. The phylum of Worms, on the other hand, we have to conceive as a
low bush or shrub, out of whose root a mass of independent branches
shoot up in different directions. From this densely branched shrub, most
of the branches of which are dead, there rise four high stems with many
branches. These are the four lofty trees just mentioned as representing
the higher phyla—the Echinoderma, Articulata, Mollusca, and Vertebrata.
These four stems are directly connected with one another at the root
only, to wit, by the common primary group of the Worm tribe.
The extraordinary difficulties which the systematic arrangement of Worms
presents, for this reason merely, are still more increased by the fact
that we do not possess any fossil remains of them. Most of the Worms had
and still have such soft bodies that they could not leave any
characteristic traces in the neptunic strata of the earth. Hence in this
case again we are entirely confined to the records of creation furnished
by ontogeny and comparative anatomy. In making then the exceedingly
difficult attempt to throw a few hypothetical rays of light upon the
obscurity of the pedigree of Worms, I must therefore expressly remark
that this sketch, like all similar attempts possesses only a provisional
value.
The numerous classes distinguished in the tribe of Worms, and which
almost every zoologist groups and defines according to his own personal
views, are, in the first place, divided into two essentially different
groups or branches, which in my Monograph of the Calcareous Sponges I
have termed Acœlomi and Cœlomati. For all the lower Worms which are
comprised in the class of Flat-worms (Platyhelminthes), (the
Gliding-worms, Sucker-worms, Tape-worms), differ very strikingly from
other Worms, in the fact that they possess neither blood nor body-cavity
(no cœlome); they are, therefore, called Acœlomi. The true cavity, or
cœlome, is completely absent in them as in all the Zoophytes; in this
important respect the two groups are directly allied. But _all other
Worms_ (like the four higher tribes of animals) possess a genuine
body-cavity and a vascular system connected with it, which is filled
with blood; hence we class them together as _Cœlomati_.
The main division of _Bloodless Worms_ (Acœlomi) contains, according to
our phylogenetic views, besides the still living Flat-worms, the unknown
and extinct primary forms of the whole tribe of Worms, which we shall
call the Primæval Worms (Archelminthes). The type of these _Primæval
Worms_, the ancient Prothelmis, may be directly derived from the Gastræa
(p. 133). Even at present the Gastrula-form—the faithful historical
portrait of the Gastræa—recurs in the ontogenesis of the most different
kinds of worms as a transient larva-form. The ciliated Gliding-worms
(Turbellaria), the primary group of the present Planary or Flat-worms
(Platyhelminthes), are the nearest akin to the Primæval Worms. The
parasitical Sucker-worms (Trematoda) arose out of the Gliding-worms,
which live freely in water, by adaptation to a parasitical mode of life;
and out of them later on—by an increasing parasitism—arose the
Tape-worms (Cestoda).
Out of a branch of the Acœlomi arose the second main division of the
Worm tribe, the Worms with blood and body-cavity (Cœlomati): of these
there are seven different classes.
The Pedigree on p. 151 shows how the obscure phylogeny of the seven
classes of Cœlomati may be supposed to stand. We shall, however, mention
these classes here quite briefly, as their relationships and derivation
are, at present, still very complicated and obscure. More numerous and
more accurate investigations of the ontogeny of the different Cœlomati
will at some future time throw light upon their phylogenesis.
The Round Worms (Nemathelminthes) which we mention as the first class of
the Cœlomati, and which are characterized by their cylindrical form,
consist principally of parasitical Worms which live in the interior of
other animals. Of human parasites, the celebrated Trichinæ, the
Maw-worms, Whip-worms, etc., for example, belong to them. The Star-worms
(Gephyrea) which live exclusively in the sea are allied to round worms,
and the comprehensive class of Ring-worms (Annelida) are allied to the
former. To the Ring-worms, whose long body is composed of a number of
segments, all alike in structure, belong the Leeches (Hirudinea),
Earth-worms (Lumbricina), and all the marine bristle-footed Worms
(Chætopoda). Nearly akin to them are the Snout-worms (Rhynchocœla), and
the small microscopic Wheel-worms (Rotifera). The unknown, extinct,
primary forms of the tribe of Sea-stars (Echinoderma), and of the tribe
of the articulated animals (Arthropoda), were nearest akin to the
Ring-worms. On the other hand, we must probably look for the primary
forms of the great tribe of Molluscs in extinct Worms, which were very
closely related to the Moss-polyps (Bryozoa) of the present day; and for
the primary forms of the Vertebrata in the unknown Cœlomati, whose
nearest kin of the present day are the Sea-sacs, especially the Ascidia.
SYSTEMATIC SURVEY
_Of the 8 Classes and 22 Orders of the Worm Tribe._
(Compare Gen. Morph. ii. Plate V. pp. 75-77.)
------------------+-------------------------+----------------------+------------------
_Classes_ | | _Systematic_ |
_of the_ | _Orders of the_ | _Name of the_ | _Name of a Genus_
_Worm Tribe._ | _Worm Tribe._ | _Orders of Worms_. | _as example._
| | |
------------------+-------------------------+----------------------+------------------
1. _Flat_ { 1. Primæval worms | 1. Archelminthes | Prothelmis
_Worms_ { 2. Gliding-worms | 2. Turbellaria | Planaria
Platyhel- { 3. Sucker-worms | 3. Trematoda | Distoma
minthes { 4. Tape-worms | 4. Cestoda | Tænia
| |
2. _Round_ { 5. Arrow-worms | 5. Chætognatha | Sagitta
_Worms_ { 6. Thread-worms | 6. Nematoda | Trichina
Nemathel- { 7. Hook-headed | 7. Acanthocephala | Echinorhynchus
minthes { worms | |
| |
3. _Moss_ } 8. Horse-shoe-lipped | 8. Lophopoda | Alcyonella
_Polyps_ } 9. Circle-lipped | 9. Stelmopoda | Retepora
Bryozoa } | |
| |
4. _Sea-sacs_ { 10. Sea-squirts | 10. Ascidia | Phallusia
Tunicata { 11. Sea-barrels | 11. Thaliacea | Salpa
| |
5. _Proboscideans_ } 12. Tongue-worms | 12. Enteropneusta | Balanoglossus
Rhynchocœla } 13. Cord-worms | 13. Nemertina | Borlasia
| |
{ 14. Star-worms without | 14. Sipunculida | Sipunculus
6. _Star-Worms_ { bristles | |
Gephyrea { 15. Star-worms with | 15. Echiurida | Echiurus
{ bristles | |
| |
7. _Wheel_ } | |
_Animalcule_ } 16. Wheel-worms | 16. Rotatoria | Hydatina
Rotifera } | |
| |
{ 17. Bear-worms | 17. Arctisca | Macrobiotus
{ 18. Worms with claws | 18. Onychophora | Peripatus
8. _Ring_ { 19. Leeches | 19. Hirudinea | Hirudo
_Worms_ { 20. Land-worms | 20. Drilomorpha | Lumbricus
Annelida { 21. Mailed worms | 21. Phracthelminthes | Crossopodia
{ 22. Bristle-footed | 22. Chætopoda | Aphrodite
{ worms | |
----------------------------------------------------------------
Chætopoda
| Drilomorpha
| |
\-------v-------/
|
Phracthelminthes |
| |
Echiurida| | Hirudinea
Sipunculida | | | |
| | | | | Onychophora
\----v---/ | | | |
+Gephyrea+ | | | | Arctisca
| | | | | |
| | | | \--v-—/
Chætognatha | \--v-----/ | |
| | | \-----v-----/
Nematoda | | | |
| | | \---------v---------/
\--v--/ | +Annelida+
| Acantho- | |
| cephala | Stelmopoda |
| | | | | Enteropneusta
\--v--/ | | | |
+Nemathelminthes+ | Lophopoda | Ascidia |
| | +Bryozoa+ | Thalicea | | Nemertina
| | | | | | | |
| | | +Rotifera+ | | | | |
| | | | | | | \---v-/
| | | | | | | +Rhynchocœla+
| | | | | \--v---/ |
| | | \----v---/ +Tunicata+ |
| | | | | |
\-----------------------v-----------------------------/
+Cœlomati+ (_worms with body-cavity_)
|
Cestoda |
| |
| |
Trematoda |
| |
| |
Turbellaria |
+Platyhelminthes+ |
| |
| |
\-------v---------/
+Acœlomi+ (_worms without body-cavity_)
|
|
Archelminthes
+Prothelmis+
|
|
+Gastræa+
The class of Sea-sacs (Tunicata) is one of the most remarkable among
Worms. They all live in the ocean, where some of the Ascidiæ adhere to
the bottom, while others (the sea-barrels, or Thaliacea) swim about
freely. In all of them the non-jointed body has the form of a simple
barrel-shaped sack, which is surrounded by a thick cartilaginous mantle.
This mantle consists of the same non-nitrogenous combination of carbon,
which, under the name of cellulose, plays an important part in the
Vegetable Kingdom, and forms the largest portion of vegetable cellular
membranes, and consequently also the greater part of wood. The
barrel-shaped body generally possesses no external appendages. No one
would recognise in them a trace of relationship to the highly
differentiated vertebrate animals. And yet this can no longer be
doubted, since Kowalewsky’s investigations, which in the year 1867
suddenly threw an exceedingly surprising and unmistakable light upon
them. From these investigations it has become clear that the individual
development of the adherent simple Ascidian Phallusia agrees in most
points with that of the lowest vertebrate animal, namely, the Lancelet
(Amphioxus lanceolatus). The early stages of the Ascidia possess the
beginnings of the _spinal marrow_ and the _spinal column_ (chorda
dorsalis) lying beneath it, which are the two most essential and most
characteristic organs of the vertebrate animal. Accordingly, of all
invertebrate animals known to us, the _Tunicates are without doubt the
nearest blood relations of the Vertebrates_, and must be considered as
the nearest relations of those Worms out of which the vertebrate tribe
has developed. (Compare Plates XII. and XIII.)
While thus different branches of the Cœlomatous group of the Worms
furnish us with several genealogical links leading to the four higher
tribes of animals, and give us important phylogenetic indications of
their origin, the lower group of Acœlomi, on the other hand, show close
relationships to the Zoophytes, and to the Primæval animals. The great
phylogenetic interest of the Worm tribe rests upon this peculiar
intermediate position.