484 
VERTEBRATA. 
to be one. Tlie head bears a pair of eyes ; tlie mouth is destitute of jaws, but is surrounded by 
a number of cartilaginous points. The circulation of the blood is effected entirely by the con- 
tractile force of the arteries ; the blood itself^ unlike that of all other vertebrata, is perfectly color- 
less. This creature lives in sandy ground at a depth of between ten and twenty fathoms water. 
It is very tenacious of life, subsisting for hours out of water ; it dislikes the light, and bears 
handling without injury. It has been frequently taken on the British coasts, and has excited 
great curiosity by its anomalous structure. 
Fossil Fishes. — The remains of extinct fishes are exceedingly abundant in various parts of 
the world. These, with the existing races, have been grouped by M. Agassiz, in his celebrated 
work, Recherches suj- les Poissons Fossiles^ in four divisions, the Placoides, Gfanoides, Otenoi- 
des, and Cy chides, these being severally distinguished by the structure of their scales. The 
term Placoides, including the sharks, rays, &c., is derived from the Greek plax, a plate or slab, 
and is applied to the first of these groups on account of the irregularity which the solid tegu- 
mentary parts present. The Ganoides, from the Greek ganos^ splendor, and embracing the silur- 
idfB, sturgeons, &c., are distinguished by the angular form of their scales. In the Ctenoides, in- 
cluding the flat fishes, percidae, &c., named from the Greek ctenos, a comb, the scales consist of 
laminae, Avhose posterior and free margin is pectinated. In the Cycloides, including the bass, 
cod, salmon, carp, &c., and named from the Greek cycles, a circle, the scales consist of simple 
laminae, with the posterior margin smooth. 
The number of species of extinct fishes in the British Islands alone, noted by M. Agassiz, is 
over five hundred. Some existing species have no representatives in the older strata. The most 
ancient of the finny races, in a geological sense, are the Placoid fishes. 
The Artificial Propagation of Fishes. — It has been long known that the eggs of fishes could 
be taken and transported from one place to another, and there, being ]5laced in water suited 
to them, would hatch and produce thrifty oftspring. The Chinese have practiced this for ages, 
and the ancient Romans conducted it on an extensive scale, in this manner not only stocking 
natural lakes and streams with various kinds of fish, but also their vast artificial reservoirs. 
Modern naturalists had gone somewhat farther, having discovered the mode by which the 
eggs of fishes are fecundated, that is, by strewing the ova of the female with the milt of 
the male. No attempt, however, appears to have been made to take practical advantage of this 
fact, and to render the artificial breeding of fishes a general system of national economy, till 
within a few years. In 18-30 the attention of the French Government was called to the opera- 
tions of two illiterate but ingenious fishermen, Messrs. Gehin and Remy,* in the little village of 
Bresse, on the eastern borders of France, among the Vosges Mountains, who, as it appears, of 
themselves, and without instruction, conceived and put in practical operation the artificial breed- 
* The history of the proceedings of Gehin and Remy is thus given in Mr. Fry's excellent and interesting work : "As 
long ago as 1841, they commenced to observe carefully the habits of the trout, and in the month of November of that 
year, during a full moon, they passed night and day on the bank of a river, never for an instant losing sight of these 
fish, and watching most intently all their preparations for laying and preserving their eggs. 
" The results of their observations were these : 
"The trout come together in a shoal, and choose a current with a gravelly bottom as the best place to lay their 
eggs. They dig in it a round hole, sometimes of the depth of several inches by three feet in diameter: they place 
in the middle of this space, parallel with the current, a line of stones, the size of which varies with the size of the 
fish. 
" The female then passes over the lino of stones, gliding over, rubbing against or resting upon them. This she does 
again and again, some twenty or thirty times, till her eggs are all laid in the crevices of the gravel. 
" When the female has done this, the male, in the same manner, by passing over and pressing upon the gravel, emits 
the milt, or soft roe, which covers and fecundates the eggs; then with tail, fins, head, and belly he works away till 
he manages to cover the eggs with gravel. 
"Nowa second female commences, and in the same manner layshcr eggs in a parallel line with and against the first 
row. When the fecundation is complete, which is generally in about fifteen days, according to the number of fish, all 
unite in heaping up stones and gravel ia mounds upon the eggs, in a manner resembling the great ant-hills that may 
be found near by. 
"Mr. Gehin believes that their mason-work is, in a manner, cemented by a slimy secretion, with which they cover 
the stones, while incessantly rubbing over and pressing against them in heaping them up ; for he found it difficult to 
destroy the mounds so formed by scratching apart the material with his fingers. 
" The eggs remain in this way for a month or two, while the process of incubation goes on ; at the end of a time, 
