FISHES. 53 
become diminutive in size, and finally perish of exhaustion. At 
certain seasons an irresistible impulse brings the two sexes together. 
Many species whose ordinary appearance is generally dull and 
unsightly now shine in the most brilliant colours. The female soon 
after lays her eggs, the number of which passes all imagination. 
Nature seems to have accumulated in the body of each female 
myriads of eggs—a wise provision, which is rendered necessary by 
the numerous causes of destruction which threaten them in their 
native element. The eggs, abandoned by the females to the mercy 
of the waves, are fecundated by the milt of the males, after being 
deposited. Such is a very brief summary of the organisation of fishes, 
which may be briefly described as vertebrate, cold-blooded animals, 
breathing by means of gills; living in water, moving through it by 
means of their fins, and reproducing their kind by means of eggs. 
And now a few words on their classification. 
Fishes are naturally divided into two series, according to the 
composition of their internal skeleton. This is usually osseous ; never- 
theless, in a whole group of them the skeleton constantly retains the 
cartilaginous or fibro-cartilaginous state. 
Professor Miiller divides the fishes into five great orders. I. Lef- 
tocardia, II. Cyclostomata, III. Selachia, 1V. Ganoidea, V. Teleostea. 
Agassiz’s system of classification of fish, founded on the form of the 
scales, is perhaps better suited than this to the paleontologist, but 
the one given above, founded as it is principally on the anatomical 
peculiarities of fishes, is better suited to the zoologist. Agassiz’s 
orders are Crenoid, Placoid, Ganoid, and Cycloid. 
J.—LEPTOCARDIA. 
This order includes but a single genus, Amphioxus. A. lanceolatus 
is a little slender transparent fish, rarely attaining two inches in length. 
The vertebral column in it is represented by a gelatinous cord (chorda 
dorsalis). The mouth is quite destitute of jaws, and there is no. trace 
of a true muscular heart. It is common on sandy coasts in various 
parts of the world. 
JI.—CycLosToMATA. 
The fishes of this order are characterised by the singular con- 
formation of their mouth, which is formed for suction. The body is 
elongated, naked, and viscous, reminding us of serpents in their 
external form; they have neither pectoral nor ventral fins; the 
skeleton is cartilaginous, and consists of a dorsal cord and of a 
