Chapter II: A Branchies En Forme De Houppes

Lophobranches.

B. CARTILAGINEUX OU CHONDROPTÉRYGIENS.

Sturioniens. Plagiostomes. Cyclostomes.

We have only to compare this system with that of Linnaeus if we wish to measure the gigantic stride made by ichthyology during the intervening period of seventy years. The various characters employed for classification have been examined throughout the whole class, and their relative importance has been duly weighed and understood. The important category of "family" appears now in Cuvier's system fully established as intermediate between genus and order. Important changes in Cuvier's system have been made and proposed by his successors, but in the main it is still that of the present day.

Cuvier had extended his researches beyond the living forms, into the field of palaeontology; he was the first to observe the close resemblance of the scales of the fossil _Palaeoniscus_ to those of the living _Polypterus_ and _Lepidosteus_, the prolongation and identity of structure of the upper caudal lobe in _Palaeoniscus_ and the sturgeons, the presence of peculiar "fulcra" on the anterior margin of the dorsal fin in _Palaeoniscus_ and _Lepidosteus_, and inferred from these facts that the fossil genus was allied either to the sturgeons or to _Lepidosteus_. But it did not occur to him that there was a close relationship between those recent fishes. _Lepidosteus_ and, with it, the fossil genus remained in his system a member of the order of _Malacopterygii abdominales_.

Agassiz.

It was left to L. Agassiz (1807-1873) to point out the importance of the structure of the scales as a characteristic, and to open a path towards the knowledge of a whole new subclass of fishes, the _Ganoidei_. Impressed with the fact that the peculiar scales of _Polypterus_ and _Lepidosteus_ are common to all fossil osseous fishes down to the Chalk, he takes the structure of the scales generally as the base for an ichthyological system, and distinguishes four orders:--

1. _Placoids._--Without scales proper, but with scales of enamel,
sometimes large, sometimes small, and reduced to mere points (Rays,
Sharks and Cyclostomi, with the fossil Hybodontes). 2.
_Ganoids._--With angular bony scales, covered with a thick stratum of
enamel: to this order belong the fossil Lepidoides, Sauroides,
Pycnodontes and Coelacanthi; the recent Polypterus, Lepidosteus,
Sclerodermi, Gymnodontes, Lophobranches and Siluroides; also the
Sturgeons. 3. _Ctenoids._--With rough scales, which have their free
margins denticulated: Chaetodontidae, Pleuronectidae, Percidae,
Polyacanthi, Sciaenidae, Sparidae, Scorpaenidae, Aulostomi. 4.
_Cycloids._--With smooth scales, the hind margin of which lacks
denticulation: Labridae, Mugilidae, Scombridae, Gadoidei, Gobiidae,
Muraenidae, Lucioidei, Salmonidae, Clupeidae, Cyprinidae.

If Agassiz had had an opportunity of acquiring a more extensive and intimate knowledge of existing fishes before his energies were absorbed in the study of fossil remains, he would doubtless have recognized the artificial character of his classification. The distinctions between cycloid and ctenoid scales, between placoid and ganoid fishes, are vague, and can hardly be maintained. So far as the living and post-Cretacean forms are concerned, he abandoned the vantage-ground gained by Cuvier; and therefore his system could never supersede that of his predecessor, and finally shared the fate of every classification based on the modifications of one organ only. But Agassiz opened an immense new field of research by his study of the infinite variety of fossil forms. In his principal work, _Recherches sur les poissons fossiles_, Neuchâtel, 1833-1843, 4to, atlas in fol., he placed them before the world arranged in a methodical manner, with excellent descriptions and illustrations. His power of discernment and penetration in determining even the most fragmentary remains is astonishing; and, if his order of Ganoids is an assemblage of forms very different from what is now understood by that term, he was the first who recognized that such an order of fishes exists.

The discoverer of the _Ganoidei_ was succeeded by their explorer Johannes Müller (1801-1858). In his classical memoir _Über den Bau und die Grenzen der Ganoiden_ (Berl., 1846) he showed that the Ganoids differ from all the other osseous fishes, and agree with the Plagiostomes, in the structure of the heart. By this primary character, all heterogeneous elements, as Siluroids, _Osteoglossidae_, &c., were eliminated from the order as understood by Agassiz. On the other hand, he did not recognize the affinity of _Lepidosiren_ to the Ganoids, but established for it a distinct subclass, _Dipnoi_, which he placed at the opposite end of the system. By his researches into the anatomy of the lampreys and _Amphioxus_, their typical distinctness from other cartilaginous fishes was proved; they became the types of two other subclasses, _Cyclostomi_ and _Leptocardii_.

Müller proposed several other modifications of the Cuvierian system; and, although all cannot be maintained as the most natural arrangements, yet his researches have given us a much more complete knowledge of the organization of the Teleostean fishes, and later inquiries have shown that, on the whole, the combinations proposed by him require only some further modification and another definition to render them perfectly natural.

The discovery (in the year 1871) of a living representative of a genus hitherto believed to be long extinct, _Ceratodus_, threw a new light on the affinities of fishes. The writer of the present article, who had the good fortune to examine this fish, was enabled to show that, on the one hand, it was a form most closely allied to _Lepidosiren_, and, on the other, that it could not be separated from the Ganoid fishes, and therefore that _Lepidosiren_ also was a Ganoid,--a relation already indicated by Huxley in a previous paper on "Devonian Fishes."

Having followed the development of the ichthyological system down to this period, we now enumerate the most important contributions to ichthyology which appeared contemporaneously with or subsequently to the publication of the great work of Cuvier and Valenciennes. For the sake of convenience we may arrange these works under two heads.

I. VOYAGES, CONTAINING GENERAL ACCOUNTS OF ZOOLOGICAL COLLECTIONS

A. _French._--1. _Voyage autour du monde sur les corvettes de S. M.
l'Uranie et la Physicienne, sous le commandement de M. Freycinet_,
"Zoologie--Poissons," par Quoy et Gaimard (Paris, 1824). 2. _Voyage de
la Coquille_, "Zoologie," par Lesson (Paris, 1826-1830). 3. _Voyage de
l'Astrolabe, sous le commandement de M. J. Dumont d'Urville_,
"Poissons," par Quoy et Gaimard (Paris, 1834). 4. _Voyage au Pôle Sud
par M. J. Dumont d'Urville_, "Poissons," par Hombron et Jacquinot
(Paris, 1853-1854).

B. _English._--1. _Voyage of H.M.S. Sulphur_, "Fishes," by J.
Richardson (Lond., 1844-1845). 2. _Voyage of H.M.SS. Erebus and
Terror_, "Fishes," by J. Richardson (Lond., 1846). 3. _Voyage of
H.M.S. Beagle_, "Fishes," by L. Jenyns (Lond., 1842).

_C. German._--1. _Reise der österreichischen Fregatte Novara_,
"Fische," von R. Kner (Vienna, 1865).