122 FISHES. 



few fishes the jaws themselves are modified for that pur- 

 pose. In the Sword-fishes the bones of the upper jaw form 

 a long dagger-shaped weapon, with which they not only 

 attack large animals, but also frequently kiU fishes on which 

 they feed. The Saw-fishes are armed with a similar but 

 still more complicated weapon, the saw, which is armed 

 on each side with large teeth implanted in deep sockets, 

 specially adapted for killing and tearing the prey before it is 

 seized and masticated by the small teeth within the mouth. 

 Fishes show but little choice in the selection of their food, 

 and some devour their own offspring indiscriminately with 

 other fishes. Their digestive powers are strong and rapid, 

 but subject in some degree to the temperature, which, when 

 sinking below a certain point, lowers the vital powers of 

 these cold-blooded animals. On the whole, marine fishes are 

 more voracious than those inhabiting fresh waters ; and whilst 

 the latter may survive total abstinence from food for weeks 

 or months, the marine species succumb to hunger within a 

 few days. The growth of fishes depends greatly on the nature 

 and supply of food, and different individuals of the same 

 species may exhibit a great disparity in their respective dimen- 

 sions. They grow less rapidly and to smaller dimensions in 

 small ponds or shallow streams than in large lakes and deep 

 rivers. The young of coast fishes, when driven out to sea, where 

 they find a much smaller supply of food, remain in an unde- 

 veloped condition, assuming an hydropic appearance. The 

 growth itself seems to continue in most fishes for a great length 

 of time, and we can scarcely set bounds to — certainly we know 

 not with precision — the utmost range of the specific size of 

 fishes. Even among species in no way remarkable for their 

 dimensions we sometimes meet with old individuals, favour- 

 ably situated, which more or less exceed the ordinary weight 

 and measurement of their kind. However, there are certain 

 evidently short-lived species of fishes which attain a remark- 



