LOCOMOTION OF TISHES. 255 



ratio of the distance from the centre of gra-^'ity, and the centre of 

 force is one-half tlie distance from tlie centre of motion ; conse- 

 quently the fislies so endowed have the greatest velocity. It is 

 such in the Sword-fish as to enable it to drive its rostral weapon 

 through a ship's timliers with the force of a cannon ball — for 

 example, through fourteen inches of oak, after penetrating the 

 copper sheathing, four inches of deal, and a layer of felt.' 



As most fishes require to sustain themselves above the bed or 

 bottom of their rivers, lakes, or seas, and as their specific gravity 

 is greater than that of water, they are couimonly provided with 

 an air-bladder, situated immediately under the spine, and above 

 the centre of gravity, and usually accompanied with powers of 

 renewing, expelling, compressing, and dilating its gaseous contents. 

 This hydrostatic apparatus thus becomes an important auxiliary 

 organ of locomotion. 



The Diodons and Tetrodons fill an immense expansion of the 

 ccso])hagus by swallowing air ; and as this lies below the centre of 

 gravity, the body, so inflated, rolls over, and they are drifted, 

 passively, back downward, by the winds and currents. 



The air-bladder is absent in Dermopteri, Plagiostomi, Ploii.ro- 

 nretidce ; and such fishes, unless endowed with compensating 

 powers and propiirtions of body and fins, as in the Sharks, 

 habitually grovel at the bottom, and exhibit flattened forms of 

 body, as in the Rays and Flounders. 



With the exception of the above-described modifications of a 

 few terminal A'ertebraj, those of the trunk remain throughout life 

 distinct from one another in Fishes, as they originally are in the 

 embryos of all liigher Vertebrates. The confluence of vertebra 

 at the base of tlie tail would have been a hindrance to the 

 required movements of such part of the spijie in creatiu-es which 

 progress by alternate vigorous inflections of a muscular tail. A 

 sacrum is a consolidation of a greater or less proportion of the 

 vertebral axis for the transference of more or less of the weight 

 of the body upon limljs organised for its support on dry land ; 

 such a modification would have been viseless to the fish, and not 

 only useless, but a defect. 



The pectoral fins — those curtailed prototypes of the fore-limbs 



of other Vertebrata, with the last segment, or hand, alone jiro- 



jccting freely from the trunk, and swathed in a common undivided 



teo'umentarv sheath — present a condition analogous to that of 



& . ... 



the embryo buds of the homologous memljcrs in the higher Ver- 



' See the specimen in tiie Museum of the Roj'al College of Siir^^eons, London, 

 described in cxov. p. 5, 



