PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.
II. MOLLUSCA, STAR-FISHES, AND ARTICULATED ANIMALS.
Tribe of Molluscs.—Four Classes of Molluscs: Lamp-shells
(Spirobranchia); Mussels (Lamellibranchia); Snails (Cochlides);
Cuttle-fish (Cephalopoda).—Tribe of Star-fishes, or
Echinoderma.—Their Derivation from Ringed Worms (Mailed Worms,
or Phracthelminthes).—The Alternation of Generation in the
Echinoderma.—Four Classes of Star-fish: Sea-stars (Asteridea);
Sea-lilies (Crinoidea); Sea-urchins (Echinidea); Sea-cucumbers
(Holothuridea).—Tribe of Articulated Animals, or
Arthropoda.—Four Classes of Articulated Animals: Branchiata, or
Crustacea, breathing through gills; Jointed Crabs; Mailed
Crabs; Articulata Tracheata, breathing through Air
Tubes.—Spiders (Long Spiders, Round
Spiders).—Myriopods.—Insects.—Chewing and Sucking
Insects.—Pedigree and History of the Eight Orders of Insects.
The great natural main groups of the animal kingdom, which we have
distinguished as TRIBES, or PHYLA (“types” according to Bär and Cuvier),
are not all of equal systematic importance for our phylogeny or history
of the pedigree of the living world. They can neither be classed in a
single series of stages, one above another, nor be considered as
entirely independent stems, nor as equal branches of a single
family-tree. It seems rather (as we saw in the last chapter) that the
tribe of Protozoa, the so-called primæval animals, is the common radical
group of the whole animal kingdom. Out of the Gastræada—which we class
among the Protozoa—the Zoophytes and the Worms have developed, as two
diverging branches. We must now in turn look upon the varied and
much-branching tribe of Worms as the common primary group, out of which
(from perfectly distinct branches) arose the remaining tribes, the four
higher phyla of the animal kingdom. (Compare the Pedigree, p. 133.)
Let us now take a genealogical look at these four higher tribes of
animals, and try whether we cannot make out the most important outlines
of their pedigree. Even should this attempt prove defective and
imperfect, we shall at all events have made a beginning, and paved the
road for subsequent and more satisfactory attempts.
It does not matter in what succession we take up the examination of the
four higher tribes. For these four phyla have no close relationship
whatever among one another, but have grown out from entirely distinct
branches of the group of Worms (p. 133). We may consider the tribe of
Molluscs as the most imperfect and the lowest in point of morphological
development. We nowhere meet among them with the characteristic
articulation or segmented formation of the body, which distinguishes
even the Ring-worms, and which in the other three higher tribes—the
Echinoderma, Articulata, and Vertebrata—is most essentially connected
with the high development of their forms, their differentiation, and
perfection. The body in all Molluscs—in mussels, snails, etc.—is a
simple non-jointed sack, in the cavity of which lie the intestines. The
nervous system consists not of a cord but of several distinct (generally
three) pairs of knots loosely connected with one another. For these and
many other anatomical reasons, I consider the tribe of Molluscs (in
spite of the high physiological development of its most perfect forms)
to be morphologically the lowest among the four higher tribes of
animals.
Whilst, for reasons already given, we exclude the Moss-polyps, and
Tunicates—which have hitherto been generally classed with the tribe of
Molluscs—we retain as genuine Molluscs the following four classes:
Lamp-shells, Mussels, Snails, and Cuttles. The two lower classes of
Molluscs, the Lamp-shells and Mussels, possess neither head nor teeth,
and they can therefore be comprised under one main class, or branch, as
headless animals (Acephala), or toothless animals (Anodontoda). This
branch is also frequently called that of the clam-shells (Conchifera, or
Bivalvia), because all its members possess a two-valved calcareous
shell. In contrast to these the two higher classes of Molluscs, the
snails and cuttles, may be represented as a second branch with the name
of Head-bearers (Cephalophora), or Tooth-bearers (Odontophora), because
both head and teeth are developed in them.
The soft, sack-shaped body in most Molluscs is protected by a calcareous
shell or house, which in the Acephala (lamp-shells and mussels) consists
of two valves, but in the Cephalophora (snails and cuttles) is generally
a spiral tube (the so-called snail’s house). Although these hard
skeletons are found in large quantities in a petrified state in all the
neptunic strata, yet they tell us but little of the historical
development of the tribe, which must have taken place for the most part
in the primordial period. Even in the Silurian strata we find fossil
remains of all the four classes of Molluscs, one beside the other, and
this, conjointly with much other evidence, distinctly proves that the
tribe of Molluscs had then obtained a strong development, when the
higher tribes, especially the Articulates and Vertebrates, had scarcely
got beyond the beginning of their historical development. In subsequent
periods, especially in the primary and secondary periods, these higher
tribes increased in importance more and more at the expense of Molluscs
and Worms, which were no match for them in the struggle for life, and
accordingly decreased in number. The still living Molluscs and Worms
must be considered as only a proportionately small remnant of the vast
molluscan fauna, which greatly predominated in the primordial and
primary periods over the other tribes. (Compare Plate VI. and
explanation in the Appendix.)
No tribe of animals shows more distinctly than do the Molluscs, how very
different the value of fossils is in geology and in phylogeny. In
geology the different species of the fossil shells of Molluscs are of
the greatest importance because they serve as excellent marks whereby to
characterize the different groups of strata, and to fix their relative
ages. As far as relates to the genealogy of Molluscs, however, they are
of very little value, because, on the one hand, the shells are parts of
quite subordinate morphological importance, and because the actual
development of the tribe belongs to the earlier primordial period, from
which no distinct fossils have been preserved. If therefore we wish to
construct the pedigree of Molluscs, we are mainly dependent upon the
records of ontogeny and comparative anatomy from which we obtain
something like the following result. (Gen. Morph. ii. Plate VI. pp.
102-116.)
The lowest stage of the four classes of genuine Molluscs known to us, is
occupied by the Lamp-shells or Spiral-gills (Spirobranchia), frequently
but inappropriately called Arm-footers (Brachiopoda), which have become
attached to the bottom of the sea. There now exist but few forms of
this class; for instance, some species of Lingula, Terebratula, and
others akin to them, which are but feeble remnants of the great variety
of forms which represented the Lamp-shells in earlier periods of the
earth’s history. In the Silurian period they constituted the principal
portion of the whole Mollusc tribe. From the agreement which, in many
respects, their early stage of development presents with the Moss
animals, it has been concluded that they have developed out of Worms,
which were nearly related to this class. Of the two sub-classes of
Lamp-shells, the Hinge-less (Ecardines) must be looked upon as the lower
and more imperfect, the Hinged (Testicardines) as the higher and more
fully developed group.
The anatomical difference between the Lamp-shells and the three other
classes of Molluscs is so considerable that the latter may be
distinguished from the former by the name of Otocardia. All the
Otocardia have a heart with chamber (ventricle) and ante-chamber
(auricle), whereas Lamp-shells do not possess the ante-chamber.
Moreover, the central nervous system is developed only in the former
(and not in the latter) in the shape of a complete pharyngeal ring.
Hence the four classes of Molluscs may be grouped in the following
manner:—
{ 1. Lamp-shells } I. Haplocardia
I. Molluscs { (Spirobranchia). } (with simple heart).
without head. {
_Acephala._ { 2. Mussels }
{ (Lamellibranchia). } II. Otocardia
} (with chamber
II. Molluscs { 3. Snails } and ante-chamber
with head. { (Cochlides). } to the heart).
_Cephalophora._ { 4. Cuttles }
{ (Cephalopoda). }
The result of these structural dispositions for the history of the
pedigree of Molluscs, which is confirmed by palæontology, is that
Lamp-shells stand much nearer to the primæval root of the whole tribe of
Molluscs than do the Otocardia. Probably Mussels and Snails developed as
two diverging branches out of Molluscs, which were nearly akin to the
Lamp-shells.
Mussels, or Plate-gills (Lamellibranchia), possess a bivalved shell like
the Lamp-shells. In the latter, one of the two valves covers the back,
the other the belly of the animal; whereas in Mussels the two valves lie
symmetrically on the right and left side of the body. Most Mussels live
in the sea, only a few in fresh water. The class is divided into two
sub-classes, Asiphonia and Siphonida, of which the latter were developed
at a later period out of the former. Among the Asiphonia are Oysters,
mother-of-pearl Shells, and fresh water Mussels; among the Siphonida,
which are characterized by a respiratory tube, are the Venus-shells,
Razor-shells, and Burrowing Clams. The higher Molluscs seem to have
developed at a later period out of those without head and teeth; they
are distinguished from the latter by the distinct formation of the head,
and more especially by a peculiar kind of tooth apparatus. Their tongue
presents a curious plate, armed with a great number of teeth. In our
common Vineyard Snail (Helix pomatia) the number of teeth amount to
21,000, and in the large Garden Slug (Limax maximus) to 26,800.
SYSTEMATIC SURVEY
_Of the 4 Classes, 8 Sub-classes, and 21 Orders of Molluscs._
------------------+-----------------------+------------------------+-------------------
_Classes of_ | _Sub-classes of_ | _Orders of_ | _Systematic Name_
_Molluscs._ | _Molluscs._ | _Molluscs._ | _of the Orders._
------------------+-----------------------+------------------------+-------------------
I. _Molluscs without head or teeth_: ACEPHALA _or_ ANODONTODA.
---------------------------------------------------------------------------------------
I. { I. Ecardines { 1. Stalked 1. Lingulida
=Lamp-shells= { _Hinge-less_ { 2. Flattened 2. Craniada
{
+Spirobranchia+ {
or {
+Brachiopoda+ { II. Testicardines { 3. Fleshy armed 3. Sarcobrachia
{ _Hinge-less_ { 4. Calcareous-armed 4. Sclerobrachia
II. { III. Asiphonia { 5. One-muscled 5. Monomya
=Mussels= { _Mussels without_ { 6. Uneven-muscled 6. Heteromya
or { _respiratory tubes_ { 7. Even-muscled 7. Isomya
=Plate-gills= {
{
+Lamellibranchia+ { IV. Siphonida { 8. Round-mantled 8. Integripallia
or { _Mussels with_ { 9. Ray-mantled 9. Sinupalliata
+Phyllobranchia+ { _respiratory tubes_ { 10. Tube-mussels 10. Inclusa
---------------------------------------------------------------------------------------
II. _Molluscs with head and teeth_: CEPHALOPHORA _or_ ODONTOPHORA.
---------------------------------------------------------------------------------------
{ V. Stump-headed { 11. Tube-snails 11. Scaphopoda
III. { _Perocephala_ { 12. Butterfly-snails 12. Pteropoda
=Snails= {
{ { 13. With hind gills 13. Opisthobranchia
+Cochlides+ { { 14. With fore gills 14. Prosobranchia
or { VI. Large-headed { 15. Swimming-snails 15. Heteropoda
+Gasteropoda+ { _Delocephala_ { 16. Beetle-snails 16. Chitonoida
{ { 17. Snails with lungs 17. Pulmonata
VII. Chamber-Poulps { 18. Pearl boats 18. Nautilida
IV. with four gills { 19. Ammon’s horns 19. Ammonitida
=Cuttles= } _Tetrabranchia_ {
or }
=Poulps= } VIII. Ink-Poulps with { 20. Ten-armed 20. Decabrachiones
} two gills {
+Cephalopoda+ } _Dibranchia_ { 21. Eight-armed 21. Octobrachiones
----------------------------------------------------------------
Dibranchia
Heteropoda |
| _Prosobranchia_ |
_Pulmonata_ | | Tetrabranchia
| Lipobranchia | | Cephalopoda
| | | | (=Cuttles= or =Poulps=)
| Gymnobranchia | | |
| | \-------v------/ |
| | | |
| Pleurobranchia | Chitonides |
| _Opisthobranchia_ | | |
| | | | |
\---------------------------------v---------/ |
| |
_Delocephala_ |
| |
| |
\--------v----------/
|
| _Pteropoda_
Inclusa | |
| | |
| \------v-----/
Sinupalliata |
| Scaphopoda |
| | |
Integripalliata \---v--/
Sclerobrachia _Siphoniata_ |
| | _Perocephala_
| | Cochlides
| | (=Snails=)
Sarcobrachia _Asiphonia_ |
_Testicardines_ Lamellibranchia |
| (=Mussels=) |
| | |
_Ecardines_ \----------v-------------/
Spirobranchia |
(=Lamp-shells=) Otocardia
| (Molluscs with chamber and ante-chamber
| to the heart)
| |
| |
\-----------------v--------------/
|
Promollusca (Primæval Molluscs)
Molluscs with simple heart
|
(Worms)
|
Gastræa
We distinguish two sub-classes among the Snails (Cochlides, or
Gasteropoda), namely, the Stump-headed and the Large-headed Snails. The
Stump-headed Snails (Perocephala) are very closely allied to Mussels
(through the Tooth-shells), and also to the Cuttle-fish (through the
Butterfly-snails). The more highly developed Snails, with large heads
(Delocephala), can be divided into Snails with gills (Branchiata) and
Snails with lungs (Pulmonata). Among the latter are the Land-snails, the
only Molluscs which have left the water and become habituated to a life
on land. The great majority of Snails live in the sea, only a few live
in fresh water. Some River-snails in the tropics (the Ampullaria) are
amphibious, living sometimes on land, sometimes in water, and at one
time they breathe through gills, at another through lungs. They have
both kinds of respiratory organs, like the Mud-fish and Gilled Newts
among the Vertebrata.
The fourth and last class, and at the same time the most highly
developed class of Molluscs, is that of the Cuttles, or Poulps, also
called Cephalopoda (foot attached to the head). They all live in the
sea, and are distinguished from Snails by eight, ten, or more long arms,
which surround the mouth in a circle. The Cuttles existing in our recent
oceans—the Sepia, Calamary, Argonaut, and Pearly Nautilus—are, like the
few Spiral-gill Lamp-shells of the present time, but a poor remnant of
the host which represents this class in the oceans of the primordial,
primary, and secondary periods. The numerous fossil “Ammon’s horns”
(Ammonites), “pearl boats” (Nautilus), and “thunderbolts” (Belemnites)
are evidences of the long since extinct splendour of the tribe. The
Poulps, or Cuttles, have probably developed out of a low branch of the
snail class, out of the Butterfly-snails (Pteropoda) or kindred forms.
The different sub-classes and orders, distinguished in the four classes
of Molluscs, whose systematic succession is given on the Table (p.
160), furnish various proofs of the validity of the law of progress by
their historical development and by the systematic development
corresponding to it. As however these subordinate groups of Molluscs are
in themselves of no further special interest, I must refer to the sketch
of their pedigree on p. 161, and to the detailed pedigree of Molluscs
which I have given in my General Morphology, and I shall now at once
turn to the consideration of the tribe of Star-fishes.
The Star-fishes (Echinoderma, or Estrellæ) among which are the four
classes of Sea-stars, Sea-lilies, Sea-urchins, and Sea-cucumbers are one
of the most interesting divisions of the animal kingdom, and yet we know
less about them than about any. They all live in the sea. Every one who
has been at the sea shore must have seen at least two of their forms,
the Sea-stars and the Sea-urchins. The tribe of Star-fishes must be
considered as a completely independent tribe of the animal kingdom on
account of its very peculiar organization, and must be carefully
distinguished from the Animal-plants—Zoophytes, or Cœlenterata, with
which it is still frequently but erroneously classed under the name
Radiata (as for example, by Agassiz, who even to this day defends this
error of Cuvier’s, together with many others).
All Echinoderma are characterized, and at the same time distinguished
from all other animals, by a very remarkable apparatus for locomotion,
which consists of a complicated system of canals or tubes, filled with
sea water from without. The sea water in these aqueducts is moved partly
by the strokes of the cilia, or vibratile hairs lining their walls, and
partly by the contractions of the muscular walls of the tubes
themselves, which resemble india-rubber bags. The water is pressed from
the tubes into a number of little hollow feet, which thereby become
widely distended, and are then employed for walking and suction. The
Sea-stars are moreover characterized by a peculiar calcareous formation
in the skin, which in most cases forms a firm, well-closed coat of mail,
composed of a number of plates. In almost all Echinoderma the body
consists of five radii (counterparts, or antimera) standing round the
main axis of the body, where they meet. It is only in some species of
Sea-stars that the number of these radii amount to more than five—to
6-9, 10-12, or even to 20-40; and in this case the number of radii is
generally not constant, but varies in different individuals of one
species.
The historical development and the pedigree of the Echinoderma are
completely revealed to us by their numerous and, in most cases,
excellently preserved fossil remains, by their very remarkable
individual developmental history, and by their interesting comparative
anatomy; this is the case with no other tribe of animals, even the
Vertebrata themselves are not to be excepted. By a critical use of those
three archives, and by a careful comparison of the results derived from
their study, we obtain the following genealogy of the Star-fishes, which
I have already published in my General Morphology (vol. ii. Plate IV.
pp. 62-77.)
The most ancient and original group of the Star-fishes, the primary form
of the whole phylum, consists of the class of the true Sea-stars
(Asterida). This is established by numerous and important arguments in
anatomy and the history of development, but above all by the irregular
and varying number of the radii, or antimera, which in all other
Echinoderma is limited, without exception, to five. Every Star-fish
consists of a central, small, body-disc, all round the circumference of
which are attached five or several long articulated arms. _Each arm of
the Star-fish essentially corresponds in its organisation with an
articulated worm_ of the class of Ring-worms, or Annelida (p. 149). I
therefore consider the Star-fish as a genuine _stock or cormus of five
or more articulated worms_, which have arisen by the star-wise growth of
a number of buds out of a central mother-worm. The connected members,
thus grouped like the rays of a star, have inherited from the
mother-worm the common opening of the mouth, and the common digestive
cavity (stomach) lying in the central body-disc. The end by which they
have grown together, and which fuses in the common central disc,
probably corresponds to the posterior end of the original independent
worms.
In exactly the same way several individuals of certain kinds of worms
are united so as to form a star-like cormus. This is the case in the
_Botryllidæ_, compound Ascidians, belonging to the class of the
Tunicata. Here also the posterior ends of the individual worms have
grown together, and have formed a common outlet for discharges, a
central cloaca; whereas at the anterior end each worm still possesses
its own mouth. In Star-fishes the original mouths have probably become
closed in the course of the historical development of the cormus, or
colony, whereas the cloaca has developed into a common mouth for the
whole cormus.
SYSTEMATIC SURVEY
_Of the 4 Classes, 9 Sub-classes, and 20 Orders of Star-fishes._
(Compare Gen. Morph. II. Plate IV. pp. 62-67.)
----------------+-------------------------+---------------------------+------------------
_Classes of the | _Sub-classes of the | _Orders of the | _Systematic Name
Star-fishes._ | Star-fishes._ | Star-fishes._ | of the Orders._
----------------+-------------------------+---------------------------+------------------
|
{ I. { 1. Primary Stars | 1. Tecastra
{ Sea Stars with radiated { 2. Articulated Stars | 2. Colastra
I. { stomach { 3. Brisinga Stars | 3. Brisingastra
=Sea Stars= { _Actinogastra_ |
+Asterida+ {
{ II. { 4. Serpent Stars | 4. Ophiastra
{ Sea Stars with disc- { 5. Tree Stars | 5. Phytastra
{ shaped stomach { 6. Lily Stars | 6. Crinastra
{ _Discogastra_ { |
|
{ III. { 7. Plated Lilies with | 7. Phatnocrinida
{ Lilies with arms { arms |
{ _Brachiata_ { 8. Articulated Lilies | 8. Colocrinida
{ { with arms |
|
II. { IV. { 9. Regularly budding | 9. Pentremitida
=Sea Lilies= { Lilies with buds { Lilies |
+Crinoida+ { _Blastoidea_ { 10. Lilies budding on | 10. Eleutherocrina
{ { two sides |
|
{ V. { 11. Bladder Lilies | 11. Agelacrinida
{ Bladder Lilies { without stalks |
{ _Cystidea_ { 12. Bladder Lilies | 12. Sphæronitida
{ with stalks |
|
{ VI. { 13. Palechinida with | 13. Melonitida
{ Older Sea Urchins { more than 10 |
{ (with more than { rows of ambulacral |
{ 20 rows of plates) { plates |
III. { _Palechinida_ { 14. Palechinida with | 14. Eocidaria
=Sea Urchins= { { 10 rows of |
+Echinida+ { VII. { ambulacral plates |
{ More recent Sea { 15. Autechinida with | 15. Desmosticha
{ Urchins (with 20 { band-like ambulacra |
{ rows of plates) { 16. Autechnidia with | 16. Petalosticha
{ _Autechinida_ { leaf-like ambulacra |
|
{ VIII. { 17. Eupodia with scuti- | 17. Aspidochirota
{ Sea Cucumbers { form tentacles |
{ with aquatic feet { 18. Eupodia with | 18. Dendrochirota
IV. { _Eupodia_ { branching tentacles |
=Sea Cucumbers= { { |
{ IX. { |
+Holothuriæ+ { Sea Cucumbers { 19. Apodia with water- | 19. Liodermatida
{ without aquatic { lungs |
{ feet { 20. Apodia without | 20. Synaptida
{ _Apodia_ { water-lungs |
----------------------------------------------------------------
Clypeastridæ
|
Spatangidæ |
| |
Dysasteridæ | |
| | |
Aspidochirota | | |
| | \------v------/
Synaptida | | |
| | | Cassidulidæ
| | | +Petalosticha+
| | | |
| | | |
Liodermatida | | |
+Apodia+ | \------v-------/ Echinonidæ
| | | |
| | Galeritidæ |
| | | Echinometridæ |
\---------v-----------/ | | |
| \------v---------/
Dendrochirota |
+Eupodia+ Latistellæ
+Holothuriæ+ Salenidæ |
\------v------/ | |
| | |
| \------v------------/
| |
| Angustistellæ
| +Desmosticha+
| +Autechinida+
| |
| | Colocrinæ
| | |
| | Sphæronitidæ |
Phytastra | | | |
| | | | |
| | Eocidaridæ | | Eleutherocrina
| | | | | |
| | | | | |
Ophiastra | | Agelacrinæ | |
+Discogastra+ | | +Cystidea+ | |
| | | | | |
| | | | | |
| | Melonitida | Phatnocrinæ |
Brisingastra | Palechinida | +Brahiata+ Pentremitida
| | +Echinida+ | | +Blastoidea+
| | | | | |
| | | | | |
Colastra \----------v-----------------/ \-------v-----------/
| | +Brachiata+
| | Crinoida
| | Crinastra
| | |
\-------------------------v----------------------------------/
|
Tocastra
+Actinogastra+
+Asterida+
|
Phracthelminthes
|
Cœlomati
|
Gastræa
Hence the Star-fishes would be compound stocks of worms which, by the
radial formation of buds, have developed out of true articulated worms,
or Annelids. This hypothesis is most strongly supported by the
comparative anatomy, and by the ontogeny of some Star-fishes (Colastra),
and of segmented worms. The many-jointed Ring-worms (Annelida) in their
inner structure are closely allied to the individual arms or radii of
the Star-fishes, that is to the original single worms, which each arm
represents. Each of the five worms of the Star-fish is a chain composed
of a great number of equi-formal members, or metamera, lying one behind
the other, like every segmented Worm, and every Arthropod. As in the
latter a central nervous cord, the ventral nerve cord runs along the
central line of the ventral wall of each segment. On each metameron
there is a pair of non-jointed feet, and besides these, in most cases,
one or more hard thorns or bristles similar to those of many Ring-worms.
A detached arm of a Star-fish can lead an independent life, and can
then, by the radially-directed growth of buds at one end, again become a
complete star.
The most important proofs, however, of the truth of my hypothesis are
furnished by the ontogeny or the individual development of the
Echinoderma. The most remarkable facts of this ontogeny were first
discovered in the year 1848 by the great zoologist, Johannes Müller of
Berlin. Some of its most important stages are represented on Plates
VIII. and IX. (Compare their explanation in the Appendix.) Fig. _A_ on
Plate IX. shows us a common Sea-star (Uraster), Fig. _B_, a Sea-lily
(Comatula), Fig. _C_, a Sea-urchin (Echinus), and Fig. _D_, a
Sea-cucumber (Synapta). In spite of the extraordinary difference of form
manifested by these four representatives of the different classes of
Star-fishes, yet the beginning of their development is identical in all
cases. Out of the egg an animal-form develops which is utterly
different from the fully developed Star-fish, but very like the ciliated
larvæ of certain segmented Worms (Star-worms and Ring-worms). This
peculiar animal-form is generally called the “larva,” but more correctly
the “nurse” of these Star-fish. It is very small and transparent, swims
about by means of a fringe of cilia, and is always composed of two equal
symmetrical halves or sides. The fully grown Echinoderm, however—which
is frequently more than a hundred times larger, and quite opaque—creeps
at the bottom of the sea, and is always composed of at least five
co-ordinate pieces, or antimera, in the form of radii. Plate VIII. shows
the development of the “nurses” of the four Echinoderms represented on
Plate IX.
The fully developed Echinoderm arises by a very remarkable process of
budding in the interior of the “nurse,” of which it retains little more
than the stomach. The nurse, erroneously called the “larva,” of the
Echinoderm, must accordingly be regarded as a solitary worm, which by
internal budding produces a second generation, in the form of a stock of
star-shaped and connected worms. The whole of this process is a genuine
alternation of generations, or metagenesis, not a “metamorphosis,” as is
generally though erroneously stated. A similar alternation of
generations also occurs in many other worms, especially in some star
worms (Sipunculidæ), and cord worms (Nemertinæ). Now if, bearing in mind
the fundamental law of biogeny, we refer the ontogeny of Echinoderma to
their phylogeny, then the whole historical development of the
Star-fishes suddenly becomes clear and intelligible to us, whereas
without this hypothesis it remains an insoluble mystery. (Compare Gen.
Morph. ii. pp. 95-99.)
Besides the reasons mentioned, there are many other facts (principally
from the comparative anatomy of Echinoderma) which most distinctly prove
the correctness of my hypothesis. I established this hypothesis in 1866,
without having any idea that _fossil articulated worms_ still existed,
apparently answering to the hypothetical primary forms. Such have in the
mean time, however, really been discovered. In a treatise “On the
Equivalent of the North American Taconic Schist in Germany,”[3] Geinitz
and Liebe, in 1867, have described a number of articulated Silurian
worms, which completely confirm my suppositions. Numbers of these very
remarkable worms are found in an excellent state of preservation in the
slates of Würzbach, in the upper districts of Reusz. They are of the
same structure as the articulated arm of a Star-fish, and evidently
possessed a hard coat of mail, a much denser, more solid cutaneous
skeleton than other worms in general. The number of body-segments, or
metamera, is very considerable, so that the worms, although no more than
a quarter or half an inch in breadth, attained a length of from two to
three feet. The excellently preserved impressions, especially those of
the Phyllodocites thuringiacus and Crossopodia Henrici, are so like the
arms of many Star-fish (Colastra) that their true blood relationship
seems very probable. This primæval group of worms, which are most
probably the ancestors of Star-fish, I call Mailed worms
(Phracthelminthes, p. 150.)
[Illustration: Pl. viii.
STAR FISHES. FIRST GENERATION. WORM PERSON.]
[Illustration: Pl. ix.
STAR FISHES. SECOND GENERATION. WORM STOCK.]
The three other classes of Echinoderma evidently arose at a later period
out of the class of Sea-stars which have most faithfully retained the
original form of the stellate colony of worms. The Sea-lilies, or
Crinoida, differ least from them, but having given up the free, slow
motion possessed by other Sea-stars, they have become adherent to rocks,
etc., and form for themselves a long stalk. Some Encrinites, however
(for example, the Comatulæ, Fig. _B_, on Plates VIII. and IX.),
afterwards detach themselves from their stalk. The original worm
individuals in the Crinoida are indeed no longer preserved in the same
independent condition as in the case of the common star-fish; but they
nevertheless always possess articulated arms extending from a common
central disc. Hence we may unite the Sea-lilies and Sea-stars into a
main-class, or branch, characterized as possessing articulated arms
(Colobrachia).
In the other two classes of Echinoderma, the Sea-urchins and
Sea-cucumbers, the articulated arms are no longer present as independent
parts, but, by the increased centralization of the stock, have
completely fused so as to form a common, inflated, central disc, which
now looks like a simple box or capsule without arms. The original stock
of five individuals has apparently degenerated to the form-value of a
simple individual, a single person. Hence we may represent these two
classes as a branch characterized as being without arms (Lipobrachia),
equivalent to those which possess articulated arms. The first of these
two classes, that of Sea-urchins (Echinida) takes its name from the
numerous and frequently very large thorns which cover the hard shell,
which is itself artistically built up of calcareous plates. (Fig. _C_,
Plates VIII. and IX.) The fundamental form of the shell itself is a
pentagonal pyramid. The Sea-urchins probably developed directly out of
the group of Sea-stars. The different classes and orders of marine
lilies and stars which are given in the following table, illustrate the
laws of progress and differentiation in a striking manner. In each
succeeding period of the earth’s history we see the individual classes
continually increasing in variety and perfection. (Gen. Morph. ii. Plate
IV.)
The history of three of these classes of Star-fish is very minutely
recorded by numerous and excellently preserved fossils, but on the other
hand, we know almost nothing of the historical development of the fourth
class, that of the Sea-cucumbers (Holothuriæ). These curious
sausage-shaped Star-fish manifest externally a deceptive similarity to
worms. (Fig. _D_, Plates VIII. and IX.) The skeletal structures in their
skin are very imperfect, and hence no distinct remains of their
elongated, cylindrical, worm-like body could be preserved in a fossil
state. However, from the comparative anatomy of the Holothuriæ, we can
infer that they have arisen, by the softening of the cutaneous skeleton,
from members of the class of Sea-urchins.
From the Star-fish we turn to the fifth and most highly developed tribe
of the invertebrate animals, namely, the phylum of Articulata, or those
with _jointed feet_ (Arthropoda). As has already been remarked, this
tribe corresponds to Linnæus’ class of Insects. It contains four
classes: (1) the genuine six-legged Insects, or Flies; (2) the
eight-legged Spiders; (3) the Centipedes, with numerous pairs of legs;
and (4) the Crabs, or Crustacea, whose legs vary in number. The last
class breathe water through gills, and may therefore be contrasted as
the main-class of gill-breathing Arthropoda, or Gilled Insects
(Carides), with the three first classes. The latter breathe air by means
of peculiar wind-pipes, or tracheæ, and may therefore appropriately be
united to form the main-class of the trachea-breathing Arthropoda, or
Tracheate Insects (Tracheata).
In all animals with articulated feet, as the name indicates, the legs
are distinctly articulated, and by this, as well as by the strong
differentiation of the separate parts of the body, or metamera, they are
sharply distinguished from Ringed worms, with which Bär and Cuvier
classed them. They are, however, in every respect so like the Ringed
worms that they can scarcely be considered altogether distinct from
them. They, like the Ringed worms, possess a very characteristic form of
the central nervous system, the so-called ventral marrow, which
commences in a gullet-ring encircling the mouth. From other facts also,
it is evident that the Arthropoda developed at a late period out of
articulated worms. Probably either the Wheel Animalcules or the Ringed
worms are their nearest blood relations in the Worm tribe. (Gen. Morph.
ii. Plate V. pp. 85-102.)
Now, although the derivation of the Arthropoda from ringed Worms may be
considered as certain, still it cannot with equal assurance be
maintained that the whole tribe of the former has arisen out of one
branch of the latter. For several reasons seem to support the
supposition that the Gilled Arthropods have developed out of a branch of
articulated worms, different from that which gave rise to the Tracheate
Arthropods. But on the whole it remains more probable that both
main-classes have arisen out of one and the same group of Worms. In this
case the Tracheate Insects—Spiders, Flies, and Centipedes—must have
branched off at a later period from the gill-breathing Insects, or
Crustacea.
The pedigree of the Arthropoda can on the whole be clearly made out from
the palæontology, comparative anatomy, and ontogeny of its four
classes, although here, as everywhere else, many details remain very
obscure. Not until the history of the individual development of all the
different groups has become more accurately known than it is at present,
can this obscurity be removed. The history of the class of Gilled
Insects, or Crabs (Carides), is at present that best known to us; they
are also called encrusted animals (Crustacea), on account of the hard
crust or covering of their body. The ontogeny of these animals is
extremely interesting and, like that of Vertebrate animals, distinctly
reveals the essential outlines of the history of their tribe, that is,
their phylogeny. Fritz Müller, in his work, “Für Darwin,”(16) which has
already been referred to, has explained this remarkable series of facts
in a very able manner.
[Illustration: _Nauplius. Youth-form of six Crab-fish._ _Pl. X._
A. Limnetis.
B. Cyclops.
C. Lernacocera.
D. Lepas.
E. Sacculina.
F. Peneus.]
[Illustration: _Adult form of the same six Crab-fish._ _Pl. XI._
A. Limnetis.
B. Cyclops.
C. Lernacocera.
D. Lepas.
E. Sacculina.
F. Peneus.]
The common primary form of all Crabs, which in most cases is even now
the first to develop out of the egg, is originally one and the same, the
so-called _Nauplius_. This remarkable primæval crab represents a very
simple form of articulated animal, the body of which in general has the
form of a roundish, oval, or pear-shaped disc, and has on its ventral
side only three pairs of legs. The first of these is uncloven, the two
subsequent pairs are forked. In front, above the mouth, lies a simple,
single eye. Although the different orders of the Crustacean class differ
very widely from one another in the structure of their body and its
appendages, yet the early Nauplius form always remains essentially the
same. In order to be convinced of this, let the reader look attentively
at Plates X. and XI., a more detailed explanation of which is given in
the Appendix. On Plate XI. we see the fully developed representatives of
six different orders of Crabs, a Leaf-footed Crab (Limnetis, Fig. _A
c_); a Stalked Crab (Lepas, Fig. _D c_); a Root Crab (Sacculina, Fig.
_E c_); a Boatman Crab (Cyclops, Fig. _B c_); a Fish Louse (Lernæocera,
Fig. _C c_); and, lastly, a highly developed Shrimp (Peneus, Fig. _F
c_). These six crabs vary very much, as we see, in the entire form of
body, in the number and formation of the legs, etc. When, however, we
look at the earliest stages, or “nauplius,” of these six different
classes, after they have crept out of the egg—those marked with
corresponding letters on Plate X. (Fig. _A n-F n_)—we shall be surprised
to find how much they agree. The different forms of Nauplius of these
six orders differ no more from one another than would six different
“good species” of one genus. Consequently, we may with assurance infer a
common derivation of all those orders from a common Primæval Crab, which
was essentially like the Nauplius of the present day.
SYSTEMATIC SURVEY
_Of the 7 Legions and 20 Orders of Crabs, or Crustacea._
-------------------------+--------------------------+----------------------+--------------
_Legions of the_ | _Orders of the_ | _Systema Name_ | _Name of a_
_Crustaceæ._ | _Crustaceæ._ | _of the Orders._ | _Genus as an_
| | | _example._
-------------------------+--------------------------+----------------------+--------------
I. ENTOMOSTRACA, _Lower Crustacea_, or Segmented Crabs (not passing through the
actual Zoëa form in youth).
------------------------------------------------------------------------------------------
+I. Branchiopoda+ { 1. Primæval Crabs 1. Archicarida Nauplius
Gill-footed Crabs { 2. Leaf-foot Crabs 2. Phyllopoda Limnetis
{ 3. Trilobites 3. Trilobita Paradoxides
{ 4. Water Fleas 4. Cladocera Daphnia
{ 5. Bivalve Crabs 5. Ostracoda Cypris
+II. Pectostraca+ { 6. Barnacle Crabs 6. Cirripedia Lepas
Fixed Crabs { 7. Root Crabs 7. Rhizocephala Sacculina
+III. Copepoda+ { 8. Boatmen Crabs 8. Eucopepoda Cyclops
Oar-footed Crabs { 9. Fish Lice 9. Siphonostoma Lernæocera
+IV. Pantopoda+ {10. No-body Crabs 10. Pycnogonida Nymphon
No-body Crabs {
+V. Pœcilopoda+ {11. Spear-tails 11. Xiphosura Limulus
Shield Crabs {12. Giant Crabs 12. Gigantostraca Eurypterus
------------------------------------------------------------------------------------------
II. MALACOSTRACA, _Higher Crustacea_, or Mailed Crabs (passing through the Zoëa form
in youth).
------------------------------------------------------------------------------------------
+VI. Podophthalma+ {13. Zoëa Crabs 13. Zoëpoda Zoëa
Stalk-eyed Mailed {14. Split-legged Crabs 14. Schizopoda Mysis
Crabs {15. Mouth-footed Crabs 15. Stomatopoda Squilla
{16. Ten-footed Crabs 16. Decapoda Peneus
+VII. Edriophthalma+ {17. Cuma Crabs 17. Cumacea Cuma
Mailed Crabs with {18. Flea Crabs 18. Amphipoda Gammarus
sessile eyes {19. Wizard Crabs 19. Læmodipoda Caprella
{20. Louse Crabs 20. Isopoda Oniscus
----------------------------------------------------------------
Brachyura Isopoda
| |
| Læmodipoda |
| | |
Anomura | |
| | |
| Amphipoda |
| | |
| \-----v----/
Macrura |
Decapoda Stomatopoda Cumacea
| | +Edriophthalma+
| | |
\----v----/ |
| |
| |
\---------------------------v------------/
|
Schizopoda
+Podophthalma+
|
Zoëpoda
+Malacostraca+
Gigantostraca | Rhizocephala
| | Siphonostoma |
| Xiphosuræ Zoëa | Cirripediæ
| | | | +Pectostraca+
| | | | |
\------v------/ | | |
| | | |
+Pœcilopoda+ | | |
| Nebaliæ | | |
| | | Eucopepoda |
Belinurœ | | +Copepoda+ | Pycnogonida
| | | | | +Pantopoda+
Trilobita | | | | |
| Phyllopoda | | Ostracoda | |
Cladoceræ | | | | | | |
| | | | | | | |
\-----------------v-------/ | | \---v---/ |
| | | | |
+Branchiopoda+ | | | |
| | | | |
| | | | |
\----------------------v-------------------------/
|
Nauplius
|
+Archicaridæ+
(Articulated Worms)
The pedigree on p. 177 will show how we may at present approximately
conceive the derivation of the twenty orders of Crustacea enumerated on
p. 176, from the common primary form of the Nauplius. Out of the
Nauplius form—which originally existed as an independent genus—the five
legions of lower Crabs developed as diverging branches in different
directions, which in the systematic survey of the class are united as
Segmented Crabs (Entomostraca). The higher division of Mailed Crabs
(Malacostraca) have likewise originated out of the common Nauplius form.
The Nebalia is still a direct form of transition from the Phyllopods to
the Schizopods, that is, to the primary form of the stalk-eyed and
sessile-eyed Mailed Crabs. The Nauplius at this stage gives rise to
another larva form, the so-called Zoëa, which is of great importance.
The order of Schizopoda, those with cloven feet (Mysis, etc.), probably
originated from this curious Zoëa; they are at present still directly
allied, through the Nebalia to the Phyllopoda, those with foliaceous
feet. But of all living crabs the Phyllopods are the most closely allied
to the original primary form of the Nauplius. Out of the Schizopoda the
stalk-eyed and sessile-eyed Mailed Crabs, or Malacostraca, developed as
two diverging branches in different directions: the former through
shrimps (Peneus, etc.), the latter through the Cumacea (Cuma, etc.),
which are still living and closely allied to the Schizopoda. Among those
with stalked eyes is the river crab (cray-fish), the lobster, and the
others with long tails, or the Macrura, out of which, in the chalk
period, the short-tailed crabs, or Brachyura, developed by the
degeneration of the tail. Those with sessile eyes divide into the two
branches of Flea-crabs (Amphipoda) and Louse-crabs (Isopoda); among the
latter are our common Rock-slaters and Wood-lice.
The second main-class of Articulated animals, that of the Tracheata, or
air-breathing Tracheate Insects[4] (Spiders, Centipedes, and Flies) did
not develop until the beginning of the palæolithic era, after the close
of the archilithic period, because all these animals (in contrast with
the aquatic crabs) are originally inhabitants of land. It is evident
that the Tracheata can have developed only after the lapse of the
Silurian period when terrestrial life first began. But as fossil remains
of spiders and insects have been found, even in the carboniferous beds,
we can pretty accurately determine the time of their origin. The
development of the first Tracheate Insects out of gill-bearing
Zoëa-crabs, must have taken place between the end of the Silurian and
the beginning of the coal period, that is, in the Devonian period.
Gegenbaur, in his excellent “Outlines of Comparative Anatomy,”(21) has
lately endeavoured to explain the origin of the Tracheata by an
ingenious hypothesis. The system of tracheæ, or air pipes, and the
modifications of organization dependent upon it, distinguish Flies,
Centipedes, and Spiders so much from other animals, that the conception
of its first origin presents no inconsiderable difficulties to
phylogeny. According to Gegenbaur, of all living Tracheate Insects, the
Primæval Flies, or Archiptera, are most closely allied to the common
primary form of the Tracheata. These insects—among which we may
especially mention the delicate Day flies (Ephemera), and the agile
dragon-flies (Libellula)—in their earliest youth, as larvæ, frequently
possess _external tracheate gills_ which lie in two rows on the back of
the body, and are shaped like a leaf or paint-brush. Similar leaf or
paint-brush shaped organs are met with as real water-breathing organs or
gills, in many crabs and ringed worms, and, moreover, in the latter as
real dorsal appendages or limbs. The “tracheate gills,” found in the
larvæ of many primæval winged insects, must in all probability be
explained as “_dorsal limbs_,” and as having developed out of the
corresponding appendages of the Annelida, or possibly as having really
arisen out of similar parts in Crustacea long since extinct. The present
tracheal respiration of the Tracheata developed at a later period out of
respiration through the “tracheate gills.” The tracheate gills
themselves, however, have in some cases disappeared, and in others
become transformed into the _wings_ of the Flies. They have disappeared
entirely in the classes of Spiders and Centipedes, and these groups must
accordingly be conceived of as degenerated or peculiarly developed
lateral branches of the Fly class, which at an early period branched off
from the common primary form of Flies; Spiders probably did so at an
earlier period than Centipedes. Whether that common primary form of all
Tracheata, which in my General Morphology I have named Protracheata, did
develop directly out of genuine Ringed worms, or at first out of
Crustacea of the Zoëa form (Zoëpoda, p. 177) will probably be settled at
some future time by a more accurate knowledge and comparison of the
ontogeny of the Tracheata, Crustacea, and Annelida. However, the root of
the Tracheata, as well as that of the Crustacea, must in any case be
looked for in the group of Ringed worms.
The genuine Spiders (Arachnida) are distinguished from Flies by the
absence of wings, and by four pairs of legs; but, as is distinctly seen
in the Scorpion-spiders and Tarantulæ, they, like Flies, possess in
reality only three pairs of genuine legs. The apparent “fourth pair of
legs” in spiders (the foremost) are in reality a pair of feelers. Among
the still existing Spiders, there is a small group which is probably
very closely allied to the common primary form of the whole class; this
is the order of Scorpion-spiders, or Solifugæ, (Solpuga, Galeodes), of
which several large species live in Africa and Asia, and are dreaded on
account of their poisonous bite. Their body consists—as we suppose to
have been the case in the common ancestor of the Tracheata—of a head
possessing several pairs of feelers like legs, of a thorax, to the three
rings of which are attached three pairs of legs, and of a hinder body,
or abdomen, consisting of many distinct rings. In the articulation of
their body, the Solifugæ are therefore in reality more closely related
to flies than to other spiders. Out of the Devonian Primæval Spiders,
which were nearly related to the Solifugæ of the present day, the Long
Spiders, the Tailor Spiders, and the Round Spiders probably developed as
three diverging branches.
The _Long Spiders_ (Arthrogastres), in which the earlier articulation of
body has been better preserved than in Round Spiders, appear to be the
older and more original forms. The most important members of this
sub-class are the scorpions, which are connected with the Solifugæ
through the Tarantella (or Phrynidæ). The small book scorpions, which
inhabit our libraries and herbariums, appear as a degenerate lateral
branch from the true scorpions. Midway between the Scorpions and Round
Spiders are the long-legged Tailor-spiders (Opiliones) which have
possibly arisen out of a special branch of the Solifugæ. The
Pycnogonida, or No-body Crabs, and the Arctisca, or Bear Worms—still
generally included among Long Spiders—must be completely excluded from
the class of Spiders; the former belong to the Crustacea, the latter to
Ringed worms.
SYSTEMATIC SURVEY
_Of the 3 Classes and 17 Orders of the Tracheata._
------------------+---------------------+----------------------+----------------------
_Classes of the_ | _Sub-Classes of the_| _Order of the_ | _Two Names of_
_Tracheata._ | _Tracheata._ | _Tracheata._ |_Genera as examples._
------------------+---------------------+----------------------+----------------------
{ { 1. Scorpion spiders { Solpuga
{ { _Solifugæ_ { Galeodes
{ {
{ { 2. Tarantella { Phrynus
{ { _Phrynida_ { Thelyphonus
{ I. {
I. { Long spiders { 3. Scorpions { Scorpio
=Spiders= { _Arthrogastres_ { _Scorpioda_ { Buthus
{ {
+Arachnida+ { { 4. Book scorpions { Obisium
{ { _Pseudoscorpioda_ { Chelifer
{ {
{ { 5. Tailor spiders { Phalangium
{ { _Opilionida_ { Opilio
{
{
{ II. { 6. Spinning spiders { Epeira
{ Round spiders { _Araneæ_ { Mygale
{ { {
{ _Sphærogastres_ { 7. Mites { Sarcoptes
{ { _Acarida_ { Demodex
II. { III.
=Centipedes= { Simple-footed { 8. Simple-footed { Scolopendra
{ _Chilopoda_ { _Chilopoda_ { Geophilus
{ {
+Scolopendria+ { IV. { 9. Double-footed { Julus
or { Double-footed { _Diplopoda_ { Polydesmus
+Myriapoda+ { _Diplopoda_ {
{ { 10. Primitive flies { Ephemera
{ { _Archiptera_ { Libellula
{ {
{ { 11. Gauze-wings { Hemerobius
{ { _Neuroptera_ { Phryganea
{ V. {
{ Chewing { 12. Straight-wings { Locusta
{ _Masticantia_ { _Orthoptera_ { Forficula
{ {
III. { { 13. Beetles { Cicindela
=Flies= { { _Coleoptera_ { Melolontha
{ {
+Hexapoda+ { { 14. Bee-wings { Apis
{ { _Hymenoptera_ { Formica
{
{
{ { 15. Bugs { Aphis
{ VI. { _Hemiptera_ { Cimex
{ Sucking {
{ _Sugentia_ { 16. Two-wings { Culex
{ { _Diptera_ { Musca
{ {
{ { 17. Butterflies { Bombyx
----------------------------------------------------------------
Butterflies
_Lepidoptera_
|
Bees | Two-wings
_Hymenoptera_ | _Diptera_
| | |
| | |
Beetles | | | Bugs
_Coleoptera_ \-------v-----/ | _Hemiptera_
| | | |
| Gauze wings | |
Straight-wings _Neuroptera_ \-------v------/
_Orthoptera_ | |
| | |
| | |
\----------------------v-----------------------------/
Primæval Flies
_Archiptera_
|
Scorpions | Double-footed
_Scorpioda_ | _Diplopoda_
Tailor Spiders | | |
_Opiliones_ | | |
| Book Scorpions | | |
Mites | _Pseudoscorpioda_ | | |
_Acarida_ | | | | |
| | \-----v-----/ | |
| | | | |
| | Tarantella | |
Weaving Spiders | _Phrynida_ | Simple-footed
_Araneæ_ | | | _Chilopoda_
| | | | =Centipedes=
| | | | +Myriapoda+
\-------------v--------/ | |
| | |
Scorpion Spiders | |
_Solifugæ_ | |
=Spiders.= | |
+Arachnida+ | |
| | |
| =Flies= |
| +Insecta Hexapoda+ |
| | |
| | |
\--------------------v---------------------/
|
Primary Air-breathing Arthropods
_Protracheata_
|
|
Articulated Worms
_Coelminthes_
Fossil remains of Long Spiders are found in the Coal. The second
sub-class of the Arachnida, the _Round Spiders_ (Sphærogastres), first
appear in the fossil state in the Jura, that is, at a very much later
period. They have developed out of a branch of the Solifuga, by the
rings of the body becoming more and more united with one another. In the
true _Spinning Spiders_ (Araneæ), which we admire on account of their
delicate skill in weaving, the union of the joints of the trunk, or
metamera, goes so far, that the trunk now consists of only two pieces,
of a head-breast (cephalo-thorax) with jaws, feelers, and four pairs of
legs, and of a hinder body without appendages, where the spinning warts
are placed. In _Mites_ (Acarida), which have probably arisen by
degeneration (especially by parasitism) out of a lateral branch of
Spinning Spiders, even these two trunk pieces have become united and now
form an unsegmented mass.
The class of _Scolopendria_, _Myriapoda_, or Centipedes, the smallest
and poorest in forms of the four classes of Arthropoda, is characterized
by a very elongated body, like that of a segmented Ringed worm, and
often possesses more than a hundred pairs of legs. But these animals
also originally developed out of a six-legged form of Tracheata, as is
distinctly proved by the individual development of the millipede in the
egg. Their embryos have at first only three pairs of legs, like genuine
insects, and only at a later period do the posterior pairs of legs bud,
one by one, from the growing rings of the hinder body. Of the two orders
of Centipedes (which in our country live under barks of trees, in moss,
etc.) the round, _double-footed_ ones (Diplopoda) probably did not
develop until a later period out of the older flat, _single-footed_ ones
(Chilopoda), by successive pairs of rings of the body uniting together.
Fossil remains of the Chilopoda are first met with in the Jura period.
The third and last class of the Arthropoda breathing through tracheæ, is
that of the _Flies_, or _Insects_, in the narrow sense of the word
(Insecta, or Hexapoda), the largest of all classes of animals, and next
to that of Mammalia, also the most important. Although Flies develop a
greater variety of genera and species than all other animals taken
together, yet these are all in reality only superficial variations of a
single type, which is entirely and constantly preserved in its essential
characteristics. In all Flies the three divisions of the trunk—head,
breast (thorax), and hinder body are quite distinct. The _hinder body_,
or _abdomen_, as in the case of spiders, has no articulated appendages.
The central division, the _breast_ or _thorax_, has on its ventral side
three pairs of legs, on its back _two pairs of wings_. It is true that,
in very many Flies, one or both pairs of wings have become reduced in
size or have even entirely disappeared; but the comparative anatomy of
Flies distinctly shows that this deficiency has arisen only gradually by
the degeneration of the wings, and that all the Flies existing at
present are derived from a common, primary Fly, which possessed three
pairs of legs and two pairs of wings. (Compare p. 256.) These wings,
which so strikingly distinguish Flies from all other Arthropoda,
probably arose, as has been already shown, out of the tracheate gills
which may still be observed in the larvæ of the ephemeral flies
(Ephemera) which live in water.
The head of Flies universally possesses, besides the eyes, a pair of
articulated feelers, or antennæ, and also three jaws upon each side of
the mouth. These _three pairs of jaws_, although they have arisen in all
Flies from the same original basis, by different kinds of adaptation,
have become changed to very varied and remarkable forms in the various
orders, and are therefore employed for distinguishing and characterizing
the main divisions of the class. In the first place, we may distinguish
two main divisions, namely, Flies with _chewing_ mandibles (Masticantia)
and Flies with _sucking_ mouths (Sugentia). On a closer examination each
of these two divisions may again be divided into two sub-groups. Among
chewing Flies, or Masticantia, we may distinguish the biting and the
licking ones. _Biting flies_ (Mordentia) comprise the most ancient and
primæval winged Flies, the gauzy-winged (Neuroptera), straight-winged
(Orthoptera), and beetles (Coleoptera). _Licking flies_ (Lambentia) are
represented by the one order of skin-winged (Hymenoptera) Flies. We
distinguish two groups of _Sucking Flies_, or Sugentia, namely, those
which prick and those which sip. There are two orders of pricking Flies
(Pungentia), those with half wings (Hemiptera) and gnats and blow-flies
(Diptera); butterflies are the only _sipping_ Flies (Sorbentia),
Lepidoptera.
Biting Flies, and indeed the order of _Primæval Flies_ (Archiptera, or
Pseudoneuroptera) are nearest akin to the still living Flies, and
include the most ancient of all Flies, the primary forms of the whole
class (hence also those of all Tracheata). Among them are, first of all,
the Ephemeral Flies (Ephemera) whose larvæ which live in water, in all
probability still show us in their tracheæ-gills the organs out of which
the wings of Flies were originally developed. This order further
contains the well known dragon-flies, or Libellula, the wine-glass sugar
mites (Lepisma), the hopping Flies with bladder-like feet (Physopoda),
and the dreaded Termites, fossil remains of which are found even in
coal. The order of Gauze-winged Flies (Neuroptera), probably developed
directly out of the primæval Flies, which differ from them only by their
perfect series of transformations. Among them are the gauze-flies
(Planipennia), caddis-flies (Phryganida), and fan-flies (Strepsiptera).
Fossil Flies, which form the transition from the primæval Flies
(Libellula) to the gauze-winged (Sialidæ), are found even in coal
(Dictyophylebia).
The order of _Straight-winged Flies_ (Orthoptera) developed at an early
period out of another branch of the primæval Flies by differentiation of
the two pairs of wings. This division is composed of one group with a
great variety of forms—cockroaches, grasshoppers, crickets, etc.
(Ulonata)—and of a smaller group consisting only of the well-known
earwigs (Labidura), which are characterised by nippers at the hinder end
of their bodies. Fossil remains of cockroaches, as well as of crickets
and grasshoppers, have been found in coal.
Fossil remains of the fourth order of Biting Flies, _beetles_
(Coleoptera) likewise occur in coal. This extremely comprehensive
order—the favourite one of amateurs and collectors—shows more clearly
than any other what infinite variety of forms can be developed
externally by adaptation to different conditions of life, without the
internal structure and the original form of the body being in any way
essentially changed. Beetles have probably developed out of a branch of
the straight-winged Flies, from which they differ only in their
transformations (larva, pupa, etc.).
The one order of _Licking Flies_, namely, the interesting group of the
_Bees_, or _Skin-winged Flies_ (Hymenoptera), is closely allied to the
four orders of biting Flies. Among them are those Flies which have risen
to such an astonishing degree of mental development, of intellectual
perfection, and strength of character, by their extensive division of
labour, formation of communities and states, and surpass in this not
merely most invertebrate animals, but even most animals in general. This
may be said especially of all ants and bees, also of wasps, leaf-wasps,
wood-wasps, gall-wasps, etc. They are first met with in a fossil state
in the oolites, but they do not appear in greater numbers until the
tertiary period. Probably these insects developed either out of a branch
of the primæval Flies or the gauze-winged Flies.
Of the two orders of _Pricking Flies_ (Hemiptera and Diptera), that
containing the _Half-winged Flies_ (Hemiptera), also called Beaked Flies
(Rhynchota), is the older of the two. It includes three sub-orders,
viz., the leaf-lice (Homoptera), the bugs (Heteroptera), and lice
(Pediculina). Fossil remains of the first two classes are found in the
oolites; but an ancient Fly (Eugereon) is found in the Permian system,
and seems to indicate the derivation of the Hemiptera from the
Neuroptera. Probably the most ancient of the three sub-orders of the
Hemiptera are the Homoptera, among which, besides the actual leaf-lice,
are the shield-lice, leaf-fleas, and leaf-crickets, or Cicadæ. Lice have
probably developed out of two different branches of Homoptera, by
continued degeneration (especially by the loss of wings); bugs, on the
other hand, by the perfecting and differentiation of the two pairs of
wings.
The second order of _pricking flies_, namely, the _Two-winged Flies_
(Diptera), are also found in a fossil state in the oolites, together
with Half-winged Flies; but they probably developed out of the Hemiptera
by the degeneration of the hind wings. In Diptera the fore wings alone
have remained perfect. The principal portion of this order consists of
the elongated gnats (Nemocera) and of the compact blow-flies and
house-flies (Brachycera), the former of which are probably the older of
the two. However, remains of both are found in the oolitic period. The
two small groups of lice-flies (Pupipara) forming chrysales, and the
hopping-fleas (Aphaniptera), probably developed out of the Diptera by
degeneration resulting from parasitism.
The eighth and last order of Flies, and at the same time the only one
with mouth-parts adapted to sipping liquids, consists of _moths_ and
_butterflies_ (Lepidoptera). This order appears, in several
morphological respects, to be the most perfect class of Flies, and
accordingly was the last to develop. For we only know of fossil remains
of this order from the tertiary period, whereas the three preceding
orders extend back to the oolites, and the four biting orders even to
the coal period. The close relationship between some moths (Tineæ) and
(Noctuæ), and some caddis-flies (Phryganida) renders it probable that
butterflies have developed from this group, that is, out of the order of
Gauze-winged Flies, or Neuroptera.
The whole history of Flies, and, moreover, the history of the whole
tribe of Arthropoda, essentially confirms the great laws of
differentiation and perfecting which, according to Darwin’s theory of
selection, must be considered as the necessary results of Natural
Selection. The whole tribe, so rich in forms, begins in the Archilithic
period with the class of _Crabs_ breathing by gills, and with the lowest
_Primæval Crabs_, or Archicaridæ. The form of these Primæval Crabs,
which were developed out of segmented worms, is still approximately
preserved by the remarkable _Nauplius_, in the common larval stage of so
many Crabs. Out of the Nauplius, at a later period, the curious Zoëa was
developed, which is the common larval form of all the higher or mailed
crabs (Malacostraca), and, at the same time, possibly of that Arthopod
which at first breathed through tracheæ, and became the common ancestor
of all _Tracheata_. This Devonian ancestor, which must have originated
between the end of the Silurian and the beginning of the Coal period,
was probably most closely related to the still living Primæval Flies, or
_Archiptera_. Out of these there developed, as the main tribe of the
Tracheata, the class of Flies, from the lowest stage of which the
_spiders_ and _centipedes_ separated as two diverging branches.
Throughout a long period there existed only the four biting orders of
Flies—the Primæval flies, Gauze-wings, Straight-wings, and the Beetles,
the first of which is probably the common primary form of the three
others. It was only at a much later period that the Licking, Pricking,
and Sipping flies developed out of the Biting ones, which retained the
original form of the three pairs of jaws most distinctly. The following
table will show once more how these orders succeeded one another in the
history of the earth.
CLASSIFICATION OF FLIES.
{ { 1. Primæval winged { M.I. }
{ { _Archiptera_ { A.A. }
{ { }
{ I. { 2. Gauze-winged { M.C. }
A. { Biting Flies { _Neuroptera_ { A.A. }
{ { }
=Flies= { _Mordentia_ { 3. Straight-winged { M.I. }
=with Chewing= { { _Orthoptera_ { A.D. }
=Mouths= { { }
{ { 4. Beetles { M.C. }
+Masticantia+ { { _Coleoptera_ { A.D. }
{
{
{ II. { 5. Skin-winged { M.C. }
{ Licking Flies { _Hymenoptera_ { A.A. }
{ _Lambentia_ { }
}
}
{ { 6. Half-winged { M.I. }
{ III. { _Hemiptera_ { A.A. }
B. { Stinging Flies { }
=Flies= { { 7. Tway-flies { M.C. }
=with Sucking= { _Pungentia_ { _Diptera_ { A.D. }
=Mouths= {
{
+Sugentia+ { IV. { 8. Butterflies { M.C. }
{ Sipping Flies { _Lepidoptera_ { A.A. }
{ { { }
{ _Sorbentia_ { { }
_Note._—The difference in the metamorphosis or transformation and in
the development of the wings of the eight individual orders of Flies
is also specified by the following letters: M.I. = Imperfect
Metamorphosis. M.C. = Perfect Metamorphosis. (Compare Gen. Morph. ii.
p. 99.) A.A. = Equal wings (fore and hinder wings are the same, or
differ but little). A.D. = Unequal wings (fore and hinder wings very
different in structure and texture, occasioned by strong
differentiation).