PEDIGREE OF VERTEBRATES
| 9. Mammals Mammalia | ||||
| 8. Birds Aves | │ │ | |||
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| 7. Reptiles Reptilia | │ │ | |||
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| 5. Sea-dragons Halisauria |
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| Osseous fish Teleostei | 4. Mud-fish Dipneusta | │ │ | Amnion Animals Amniota | |
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| Ganoid fish Ganoidei | │ │ | │ │ | 6. Batrachians Amphibia | |
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| │ │ | Vertebrate animals breathing through lungs Amphipneumones | |||
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| Primæval fish 3. Fishes Double-nostriled | Selachii Pisces Amphirrhina | |||
| 2. Round-mouthed Cyclostoma | ||||
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| Single-nostriled Animals with skulls | Monorrhina Craniota | |||
| 1. Tube-hearted Leptocardia | │ │ | |||
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| Sea-barrels Thaliacea | Ascidiæ │ |
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| │ │ │ │ | │ │ │ │ | Skull-less Animals Acrania Vertebrate Animals Vertebrata | ||
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| Tunicate Animals Tunicata | │ │ | |||
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| 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 canals 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 Cæciliæ 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 assurance 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 primæval fish, or Selachii, to which the Proselachii probably belonged, as a primary group, not only of the Fish class, but of the whole main-class of double-nostriled animals.
The class of Fish (Pisces) with which we accordingly begin the series of Double-nostriled animals, is distinguished from the other six classes 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 classes of double-nostriled animals, both nostrils are changed into air passages 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-classes of Fishes. | Legions of Fishes. | Orders of Fishes. | Examples from the Orders. | ||||
| A. Primæval Fish Selachii |
| I. Transverse mouths Plagiostomi |
| 1. | Sharks Squalacei | Sharks, dog-fish | |
| 2. | Rays Rajacei | Spiked rays, electric rays, etc. | |||||
| II. Sea-Cats Holocephali |
| 3. | Sea-Cats Chimæracei | Chimæra, Calorrhynchias | |||
| B. Ganoid Fish Ganoides |
| III. Mailed Ganoid Fish Tabuliferi |
| 4. | Buckler-heads Pamphracte | Cephalaspidæ, Placoderma, etc. | |
| 5. | Sturgeons Sturiones | Spoon-sturgeons, sterlet, etc. | |||||
| IV. Angular-scaled Fish Rhombiferi |
| 6. | Efulcri | Double-finned | |||
| 7. | Fulcrati | Palæoniscus, bony pike, etc. | |||||
| 8. | Semæopteri | African finny pike, etc. | |||||
| V. Round-scaled Ganoid Fish Cycliferi |
| 9. | Cœloscolopes | Holoptychius, Cœlacanthides, etc. | |||
| 10. | Pycnoscolopes | Coccolepida, Amiadæ, etc. | |||||
| C. Osseous Fish Teleostei |
| VI. Osseous Fish with an air passage to the swimming bladder Physostomi |
| 11. | Herring species Thrissogenes | Herrings, salmon, carp, etc. | |
| 12. | Eel species Enchelygenes | Eels, snake eels, electric eels, etc. | |||||
| VI. Osseous Fish without an air passage to the swimming bladder Physoclisti |
| 13. | Stichobranchii | Perch, wrasse, turbot, etc. | |||
| 14. | Plectognathi | Trunk fish, globe fish, etc. | |||||
| 15. | Lophobranchii | Pipe fish, sea horses, etc. | |||||
