310 FISHES. 



indurated and quite stony on their surfaces, but not at all fibrous. 

 On the other hand, the cord passing through the axes of the bodies of 

 its vertebrae are not affected by any shrinking^, and this circumstance 

 approximates it to the lamprey. 



In the latter genus, the whole of the parts of the skeleton are more 

 simple still than in the selacians : its spine especially is much softer, 

 and it is destitute of branchial arches. Its branches adhere to the 

 interior only by the medium of a membranous canal ; on the con- 

 trary, externally, there is an apparatus formed, by these descriptions 

 of ribs, the earliest vestiges of which have been shown in the squalus, 

 and in the present case are branchy, like a sort of cage. 



The ammocetes have even no cartilaginous skeleton. All the parts, 

 of their frame- work permanently remain in the membranous state, 

 and thus in reference to their peculiar condition, will resemble much 

 more the worms, than they will the vertebrated animals. 



The Chimsera likewise has the cord of its spine stronger and better 

 marked than the body of the vertebrae, and on the fore part, a number 

 of annular pieces is superseded by a^ crest of a single piece; in the 

 polyodon too, the cord is exceedingly vigorous. 



All these varieties in the skeleton of the Chondropterygian fishes 

 will be more clearly explained Avhen we come to their particular 

 joints, and can enter into the necessary details. 



CHAPTER IV. 



MYOLOGY OF FISHES. 



Movements of the Skeleton in Fishes. 



The spine composed of an indefinite number of vertebrae, with the 

 motion which their uniting cartilages allow them on each other, 

 bends freely from right to left, in one or more curves alternately 

 convex and concave, according to its length. It would be equally 

 pliant in the vertical direction, did not the superior and inferior spi- 

 nous apophyses limit this movemement in the ratio of their height 

 and their approximation to each other. 



It is therefore chiefly in striking the water laterally by the alter- 

 nate flexions of its body and tail, that the fish moves forward. The 

 surface which thus strikes the water, is augmented or lessened in 

 height, according as the fins of the back, the anus, and the tail, have 

 their rays more spread and erect ; this is effected by the mobility of 

 the rays on the small interspinal bones, with which they are articu- 

 lated, which mobility, according to the form of the articulations, 

 takes place forwards, backwards, or to the sides, at the will of the 

 fish, producing the same effects as a rudder. 



As to the fins which are in pairs, the pectorals in the first place, 

 ].«)sscss the movement of the girdle of the shoulder, which takes place 

 from before backwards, or from behind forwards, to an extent that 

 depends on the freedom of the articulation of the scapula, and on the 



