FISHES. 533 



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. Lep- 

 tocardia, II. Cyclostomata, III. Selachia, IV. Ganoide.a, V. Teleostea. 

 Agassiz's system of classification of fish, founded on the form of the 

 scales, is perhaps better suited than this to the palaeontologist, 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 Ctenoid, Placoid, Ganoid, and Cycloid. 



I. 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 

 dor salts). 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. 



II. 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 



