cla.s.s of Amphibious animals was an unnatural union of two very different cla.s.ses. These two cla.s.ses were separated as early as 1820, by Merrin, as two main groups of Amphibious animals, under the names of Pholidota and Batrachia. The _Batrachia_, which are at present (in a restricted sense) called Amphibious animals, comprise Frogs, Salamanders, gilled Salamanders, Caecilia, and the extinct Labyrinthodonta. Their entire organization is closely allied to that of Fishes. The _Pholidota_, or Reptiles, on the other hand, are much more closely allied to Birds. They comprise lizards, serpents, crocodiles, and tortoises, and the groups of the mesolithic Dragons, Flying reptiles, etc.

In conformity with this natural division of Amphibious animals into two cla.s.ses, the whole tribe of Vertebrate animals was divided into two main groups. The first main group, containing Amphibious animals and Fishes, breathe throughout their lives, or in early life, by means of gills, and are therefore called _gilled Vertebrata_ (Branchiata, or a.n.a.llantoida).

The second main group-Reptiles, Birds, and Mammals-breathe at no period of their lives through gills, but exclusively through lungs, and hence may appropriately be called Gill-less, or _Vertebrata with lungs_ (Abranchiata, or Allantoida). However correct this distinction may be, still we cannot remain satisfied with it if we wish to arrive at a true natural system of the vertebrate tribe, and at a right understanding of its pedigree. In this case, as I have shown in my General Morphology, we are obliged to distinguish three other cla.s.ses of Vertebrate animals, by dividing what has. .h.i.therto been regarded as the cla.s.s of fishes into four distinct cla.s.ses. (Gen. Morph. vol. ii. Plate VII. pp. 116-160.)

The first and lowest of these cla.s.ses comprises the _Skull-less_ animals (Acrania), or animals with _tubular hearts_ (Leptocardia), of which only one representative now exists, namely, the remarkable little Lancelet (Amphioxus lanceolatus). Nearly allied to this is the second cla.s.s, that of the _Single-nostriled_ animals (Monorrhina), or _Round-mouthed_ animals (Cyclostoma), which includes the Hags (Myxinoida) and Lampreys (Petromyzonta). The third cla.s.s contains only the genuine Fish (Pisces): the Mud-fishes (Dipneusta) are added to these as a fourth cla.s.s, and form the transition from Fish to Amphibious animals. This distinction, which, as will be seen immediately, is very important for the genealogy of the Vertebrate animals, increases the original number of Vertebrate cla.s.ses from four to eight.

In most recent times a ninth cla.s.s of Vertebrata has been added to these eight cla.s.ses. Gegenbaur's recently published investigations in comparative anatomy prove that the remarkable cla.s.s of _Sea-dragons_ (Halisauria), which have hitherto been included among Reptiles, must be considered quite distinct from these, and as a separate cla.s.s which branched off from the Vertebrate stock, even before the Amphibious animals. To it belong the celebrated large Ichthyosauri and Plesiosauri of the oolitic and chalk periods, and the older Simosauri of the Trias period, all of which are more closely allied to Fish than to Amphibious animals.

These nine cla.s.ses of Vertebrate animals are, however, by no means of the same genealogical value. Hence we must divide them, as I have already shown in the Systematic Survey on p. 133, into four distinct main-cla.s.ses or tribes. In the first place, the three highest cla.s.ses, Mammals, Birds, and Reptiles, may be comprised as a natural main-cla.s.s under the name of _Amnion animals_ (Amnionata). The _Amnion-less animals_ (Anamnionata), naturally opposed to them as a second main-cla.s.s, include the four cla.s.ses of Batrachians, Sea-dragons, Mud-fish, and Fishes. The seven cla.s.ses just named, the Anamnionata as well as the Amnionata, agree among one another in numerous characteristics, which distinguish them from the two lowest cla.s.ses (the single-nostriled and tubular-hearted animals). Hence we may unite them in the natural main group of _Double-nostriled_ animals (Amphirrhina). Finally, these Amphirrhina on the whole are much more closely related to those animals with round mouths or single nostrils than to the skull-less or tube-hearted animals. We may, therefore, with full justice cla.s.s the single and double-nostriled animals into one princ.i.p.al main group, and contrast them as _animals with skulls_ (Craniota), or _bulbular hearts_ (Pachycardia), to the one cla.s.s of _skull-less animals_, or animals with _tubular hearts_. This cla.s.sification of the Vertebrate animals proposed by me renders it possible to obtain a clear survey of the nine cla.s.ses in their most important genealogical relations. The systematic relationship of these groups to one another may be briefly expressed by the following table.

A.

=Skull-less Animals= 1. Tubular hearts 1. Leptocardia (+Acrania+)

{ _a._ Single nostriled { animals { 2. Round-mouths 2. Cyclostoma B. { _Monorrhina_ { =Animals with= { =Skulls= { b. Double { { 3. Fish 3. Pisces (+Craniota+) { nostriled { I. Non-Amnionate { 4. Mud-fish 4. Dipneusta { animals { Anamnia { 5. Sea-dragons 5. Halisauria or { { { 6. Batrachians 6. Amphibia { _Amphirrhina_ { =Thick Hearts= { { II. Amnionate. { 7. Reptiles 7. Reptilia (+Pachycardia+) { { Amniota { 8. Birds 8. Aves { { { 9. Mammals 9. Mammalia

The only one representative of the first cla.s.s, the small _lanceolate fish_, or Lancelet (Amphioxus lanceolatus) (Plate XIII. Fig. _B_), stands at the lowest stage of organization of all the Vertebrate animals known to us. This exceedingly interesting and important animal, which throws a surprising light upon the older roots of our pedigree, is evidently the last of the Mohicans-the last surviving representative of a lower cla.s.s of Vertebrate animals, very rich in forms, and very highly developed during the primordial period, but which unfortunately could leave no fossil remains on account of the absence of all solid skeleton.

The Lancelet still lives widely distributed in different seas; for instance, in the Baltic, North Sea, and Mediterranean, where it generally lies buried in the sand on flat sh.o.r.es. The body, as the name indicates, has the form of a narrow lanceolate leaf, pointed at both extremities. When full grown it is about two inches long, of a white colour and semi-transparent. Externally, the little lanceolate animal is so little like a vertebrate animal that Pallas, who first discovered it, regarded it as an imperfect naked snail. It has no legs, and neither head, skull, nor brain. Externally, the fore end of the body can be distinguished from the hinder end only by the open mouth. But still the Amphioxus in its internal structure possesses those most important features, which distinguish all Vertebrate animals from all Invertebrate animals, namely, the spinal rod and spinal marrow. The _spinal rod_ (Chorda dorsalis) is a straight, cylindrical, cartilaginous staff, pointed at both ends, forming the central axis of the internal skeleton, and the basis of the vertebral column. Directly above the spinal rod, on its dorsal side, lies the _spinal marrow_ (medulla spinalis), likewise originally a straight but internally hollow cord, pointed at both ends.

This forms the princ.i.p.al piece and centre of the nervous system in all Vertebrate animals. (Compare above vol. i. p. 303.) In all Vertebrate animals without exception, man included, these important parts of the body during the embryological development out of the egg, originally begin in the same simple form, which is retained throughout life by the Amphioxus. It is only at a later period that the brain develops by the expansion of the fore end of the spinal marrow, and out of the spinal rod the skull which encloses the brain. As these two important organs do not develop at all in the Amphioxus, we may justly call the cla.s.s represented by it, _Skull-less animals_ (Acrania), in opposition to all the others, namely, to the _animals with skulls_ (Craniota). The Skull-less animals are generally called _tubular-hearted_ (Leptocardia), because a centralized heart does not as yet exist, and the blood is circulated in the body by the contractions of the tubular blood-vessels themselves. The Skulled animals, which possess a centralized, thick-walled, bulb-shaped heart, ought then by way of contrast to be called _bulbular-hearted_ animals (Pachycardia).

[Ill.u.s.tration: _Ascidia (A.) and Amphioxus (B.)_ Pl. XII.

E. Haeckel del. Legesse sc.]

Animals with skulls and central hearts evidently developed gradually in the later primordial period out of those without skulls and with tubular hearts. Of this the ontogeny of skulled animals leaves no doubt. But whence are these same skull-less animals derived? It is only very lately that an exceedingly surprising answer has been given to this important question. From Kowalewsky's investigations, published in 1867, on the individual development of the Amphioxus and the adhering Sea-squirts (Ascidia) belonging to the cla.s.s of mantled animals (Tunicata), it has been proved that the ontogenies of these two entirely different looking animal-forms agree in the first stage of development in a most remarkable manner. The freely swimming larvae of the Ascidians (Plate XII. Fig. _A_) develop the undeniable beginning of a spinal marrow (Fig.

5 _g_) and of a spinal rod (Fig. 5 _c_), and this moreover in entirely the same way as does the Amphioxus. (Plate XIII. Fig. _B_.) It is true that in the Ascidians these most important organs of the Vertebrate animal-body do not afterwards develop further. The Ascidians take on a retrograde transformation, become attached to the bottom of the sea, and develop into shapeless lumps, which when looked upon externally would scarcely be supposed to be animals. (Plate XIII. Fig. _A_.) But the spinal marrow, as the beginning of the central nervous system, and the spinal rod, as the first basis of the vertebral column, are such important organs, so exclusively characteristic of Vertebrate animals, that we may from them with cert.i.tude infer the true blood relationship of Vertebrate with Tunicate animals. Of course we do not mean to say by this, that Vertebrate animals are derived from Tunicate animals, but merely that both groups have arisen out of a common root, and that the Tunicates, of all the Invertebrata, are the nearest blood relations of the Vertebrates. It is quite evident that genuine Vertebrate animals developed progressively during the primordial period (and the skull-less animals first) out of a group of worms, from which the degenerate Tunicate animals arose in another and a retrograde direction. (Compare the more detailed explanation of Plates XII. and XIII. in the Appendix.)

Out of the Skull-less animals there developed, in the first instance, a second low cla.s.s of Vertebrate animals, which still stands far below that of fish, and which is now represented only by the Hags (Myxinoida) and Lampreys (Petromyzonta). This cla.s.s also, on account of the absence of all solid parts, could, unfortunately, as little as the Skull-less animals leave fossil remains. From its whole organization and ontogeny it is quite evident that it represents a very important intermediate stage between the Skull-less animals and Fishes, and that its few still existing members are only the last surviving remains of a probably very highly developed animal group which existed towards the end of the primordial period. On account of the curious mouth possessed by the Hags and Lampreys, which they use for sucking, the whole cla.s.s is usually called _Round-mouthed_ animals (Cyclostoma). The name of _Single-nostriled_ animals (Monorrhina) is still more characteristic.

For all Cyclostoma possess a simple, single nasal tube, whereas, in all other Vertebrate animals (with the exception of the Amphioxus) the nose consists of two lateral halves, a right and a left nostril. We are therefore enabled to comprise these latter (Anamnionata and Amnionata) under the heading, _double-nostriled_ animals (Amphirrhina). All the Amphirrhina possess a fully developed jaw-skeleton (upper and under jaw), whereas it is completely wanting in the Monorrhina.

Apart also from the peculiar nasal formation, and the absence of jaws, the Single-nostriled animals are distinguished from those with double nostrils by many peculiarities. Thus they want the important sympathetic nervous system, and the spleen which the Amphirrhina possess. Of the swimming bladder, and the two pairs of legs-which all double-nostriled animals have, at least in their embryonic conditions-not a trace exists in the Single-nostriled animals, which is the case also in the Skull-less animals. Hence, we are surely justified in completely separating the Monorrhina, as we have separated the Skull-less animals, from the Fishes, with which they have hitherto been erroneously cla.s.sed.

[Ill.u.s.tration: _Ascidia (A.) and Amphioxus (B.)_ Pl. XIII.

E. Haeckel del. Lagesse sc.]

We owe our first accurate knowledge of the Monorrhina, or Cyclostoma, to the great zoologist, Johannes Muller of Berlin; his cla.s.sical work on the "Comparative Anatomy of the Myxinoida" forms the foundation of our modern views on the structure of the Vertebrate animals. He distinguished two distinct groups among the Cyclostoma, which we shall consider as sub-cla.s.ses.

The first sub-cla.s.s consists of the Hags (Hyperotreta, or Myxinoida).

They live in the sea as parasites upon other fish, into whose skin they penetrate (Myxine, Bdellostoma). Their organ of hearing has only one annular ca.n.a.l, and their single nasal tube penetrates the palate. The second sub-cla.s.s, that of Lampreys, or Prides (Hyperoartia, or Petromyzontia) is more highly developed. It includes the well-known Lamperns, or Nine-eyes, of our rivers (Petromyzon fluviatilis), with which most persons are acquainted. They are represented in the sea by the frequently larger marine or genuine Lampreys (Petromyzon marinus).

The nasal tube of these single-nostriled animals does not penetrate the palate, and in the auricular organ there are two annular ca.n.a.ls.

SYSTEMATIC SURVEY

_Of the 4 Main-cla.s.ses, 9 Cla.s.ses, and 26 Sub-cla.s.ses of Vertebrata._

Gen. Morph. vol. ii. Plate VII. pp. 116-160.

====================================================================================== I. =Skull-less= (+Acrania+), or =Tube-hearted= (+Leptocardia+).

Vertebrata without head, without skull and brain, without centralized heart.

-------------------------------------------------------------------------------------- 1. =Skull-less= I. Tube-hearted { 1. Lancelet 1. Amphioxus +Acrania+ _Leptocardia_ { --------------------------------------------------------------------------------------

II. =Animals with skulls= (+Craniota+) and with =thick-walled hearts= (+Pachycardia+).

Vertebrata with head, with skull and brain, with centralized heart.

====================================================================================== _Main-cla.s.ses_ | _Cla.s.ses_ | _Sub-cla.s.ses_ | _Systematic Name_ _of the Skulled_ | _of the_ | _of the_ | _of the_ _Animals._ | _Skulled Animals._ | _Skulled Animals._ | _Sub-cla.s.ses._ -------------------------------------------------------------------------------------- 2. =Single-Nostriled= { II. Round mouths { 2. Hags, or Mucous 2. Hyperotreta +Monorrhina+ { _Cyclostoma_ { Fish (Myxinoida) { { 3. Lampreys, or 3. Hyperoartia { { Pride (Petromyzontia)

{ III. Fish { 4. Primaeval fish 4. Selachii { _Pisces_ { 5. Ganoid fish 5. Ganoides { { 6. Osseous fish 6. Teleostei { { IV. Mud-fish { 7. Mud-fish 7. Protopteri 3. =Non-amnionate= { _Dipneusta_ { { +Anamnionata+ { { 8. Primaeval 8. Simosauria { V. Sea-dragons { dragons { _Halisauri_ { 9. Snake-dragons 9. Plesiosauria { { 10. Fish-dragons 10. Ichthyosauria { { VI. Batrachians { 11. Mailed Batrachians 11. Phractamphibia { _Amphibia_ { 12. Naked Batrachians 12. Lissamphibia

4. =Amnion= { VII. Reptiles { 13. Primary reptiles 13. Tocosauria =Animals= { _Reptilia_ { 14. Lizards 14. Lacertilia +Amnionata+ { { 15. Serpents 15. Ophidia { { 16. Crocodiles 16. Crocodilia { { 17. Tortoises 17. Chelonia { { 18. Flying reptiles 18. Pterosauria { { 19. Dragons 19. Dinosauria { { 20. Beaked reptiles 20. Anomodontia { { VIII. Birds { 21. Long-tailed 21. Saururae { _Aves_ { 22. Fan-tailed 22. Carinatae { { 23. Bush-tailed 23. Rat.i.tae { { IX. Mammals { 24. Cloacal animals 24. Monotrema { _Mammalia_ { 25. Pouched animals 25. Marsupialia { { 26. Placental animals 26. Placentalia

9. Mammals _Mammalia_ 8. Birds | _Aves_ | | | 7. Reptiles | _Reptilia_ | 5. Sea-dragons | | _Halisauria_ -------v--------/ | | Osseous fish | =Amnion Animals= _Teleostei_ 4. Mud-fish | +Amniota+ | _Dipneusta_ | | | | | | | | | 6. Batrachians Ganoid fish | | _Amphibia_ _Ganoidei_ | | | | | | | | --------------v--------------/ | | | Vertebrate animals breathing through lungs | _Amphipneumones_ | | | | ------------v-----------------/ Primaeval fish Selachii 3. Fishes _Pisces_ =Double-nostriled= +Amphirrhina+ | 2. Round-mouthed | _Cyclostoma_ | | | | --------------------v-------------/ =Single-nostriled= Monorrhina =Animals with skulls= +Craniota+

1. Tube-hearted | _Leptocardia_ | | | | | ---------v------/ _Ascidiae_ | Sea-barrels | =Skull-less Animals= _Thaliacea_ | +Acrania+ | | =Vertebrate Animals= | | +Vertebrata+ ----v----/ | | | =Tunicate Animals= | +Tunicata+ | | | | | -----------v---------/ | Worms _Vermes_

All existing Vertebrate animals, with the exception of the Monorrhina and Amphioxus just mentioned, belong to the group which we designate as Double-nostriled animals (Amphirrhina). All these animals possess (in spite of the great variety in the rest of their forms) a nose consisting of two lateral halves, a jaw-skeleton, a sympathetic nervous system, three annular ca.n.a.ls connected with the auricular sac, and a spleen.

Further, all Double-nostriled animals possess a bladder-shaped expansion of the gullet, which, in Fish, has developed into the swimming bladder, but in all other Double-nostriled animals into lungs. Finally, in all Double-nostriled animals there exist in the youngest stage of growth the beginnings of two pairs of extremities, or limbs, a pair of fore legs, or breast fins, and a pair of hinder legs, or ventral fins. One of these pairs of legs sometimes degenerates (as in the case of eels, whales, etc.), or both pairs of legs (as in Caeciliae and serpents) either degenerate or entirely disappear; but even in these cases there exists some trace of their original beginning in an early embryonic period, or the useless remains of them may be found in the form of rudimentary organs. (Compare above, vol. i. p. 13.)

From all these important indications we may conclude with full a.s.surance that all double-nostriled animals are derived from a single common primary form, which developed either directly or indirectly during the primordial period out of the Monorrhina. This primary form must have possessed the organs above mentioned, and also the beginning of a swimming bladder and of two pairs of legs or fins. It is evident, that of all still living double-nostriled animals, the lowest forms of sharks are most closely allied to this long since extinct, unknown, and hypothetical primary form, which we may call the Primary Double-nostriled animals (Proselachii). We may therefore look upon the group of primaeval fish, or Selachii, to which the _Proselachii_ probably belonged, as a primary group, not only of the Fish cla.s.s, but of the whole main-cla.s.s of double-nostriled animals.

The cla.s.s of _Fish_ (Pisces) with which we accordingly begin the series of Double-nostriled animals, is distinguished from the other six cla.s.ses of the series by the swimming bladder never developing into lungs, but acting only as a hydrostatic apparatus. Agreeing with this, we find that in fish the nose is formed by two blind holes in front of the mouth, which never pierce the palate so as to open into the cavity of the mouth. In the other six cla.s.ses of double-nostriled animals, both nostrils are changed into air pa.s.sages which pierce the palate, and thus conduct air to the lungs. Genuine fish (after the exclusion of the Dipneusta) are accordingly the only double-nostriled animals which exclusively breathe through gills and never through lungs. In accordance with this, they all live in water, and both pairs of their legs have retained the original form of paddling fins.

SYSTEMATIC SURVEY

_Of the 7 Legions and 15 Orders of the Fishes._

--------------------------------------------------------------------------------- _Sub-cla.s.ses_ | _Legions_ | _Orders_ | _Examples_ _of_ | _of_ | _of_ | _from_ _Fishes._ | _Fishes._ | _Fishes._ | _the Orders._ --------------------------------------------------------------------------------- { { { 1. Sharks Sharks, dog-fish { I. Transverse { _Squalacei_ A. { mouths { 2. Rays Spiked rays, electric =Primaeval= { _Plagiostomi_ { _Rajacei_ rays, etc.

=Fish= { +Selachii+ { II. Sea-Cats { 3. Sea-Cats Chimaera, Calorrhynchias { _Holocephali_ { _Chimaeracei_

{ { 4. Buckler-heads Cephalaspidae, Placoderma, { III. Mailed Ganoid { _Pamphracte_ etc.

{ Fish { 5. Sturgeons Spoon-sturgeons, sturgeons, { _Tabuliferi_ { _Sturiones_ sterlet, etc.

{ { { 6. _Efulcri_ Double-finned B. { IV. Angular-scaled { 7. _Fulcrati_ Palaeoniscus, bony pike, =Ganoid= { Ganoid Fish { etc.

=Fish= { _Rhombiferi_ { 8. _Semaeopteri_ African finny pike, etc.

+Ganoides+ { { V. Round-scaled { 9. _Closcolopes_ Holoptychius, Clacanthides, { Ganoid Fish { etc.

{ _Cycliferi_ { 10. _Pycnoscolopes_ Coccolepida, Amiadae, etc.

{ VI. Osseous Fish { { with an air { 11. Herring species Herrings, salmon, carp, { pa.s.sage to the { _Thrissogenes_ etc.