152 STURGEONS. 



low, grovel along the bottom, feeding in slioals on the 

 decomposing animal and vegetable substances which are hurried 

 down with the debris ol the continents drained by those rapid 

 currents. Thus they are ever busied in re-converting the 

 substances, which otherwise would tend to corrupt the ocean 

 into living organized matter. "These fishes are therefore duly 

 weighted by a ballast of dense, dermal, osseous plates, not 

 scattered at random over their surface, but regularly arranged, 

 as the seaman knows how ballast should be, in orderly series 

 along the middle and at the sides oi the body. The protection 

 against the water-logged timber and stones hurled along their 

 teeding-grounds, which the Sturgeons derive from their scale 

 armour, renders needless the ossification of the cartilaginous 

 case of the brain or other parts of the endoskeleton, and the 

 weight of the armour requires that endoskeleton to be kept 

 as light as may be compatible with its elastic property and 

 other functions. The Sturgeons are further adjusted to their 

 place in the liquid element, and endowed with the power of 

 changing their level and rising with their defensive load to 

 the surface by a large expansive air-bladder." Protection to 

 the eye is even more necessary than the other portions of 

 the body, and accordingly this is provided for, in addition to 

 the bony crust that surrounds and overtops it, by being deeply 

 sunk in its small chamber, into which probably it still more 

 deeply falls when danger threatens. 



How far the habits of these fishes will support the wide 

 interpretation applied to their rigid armature by the above-named 

 eminent philosojDher, I am not prepared to decide; but there 

 is another benefit, consistent with that already mentioned, 

 which is derived from the presence of those plates, and which 

 will be of great use to these fishes m some of the situations 

 in which they must often find themselves placed. The bones 

 of Sturgeons are remarkably soft^ even for a race of fishes in 

 which none of the bones are hard and firm; and they do not 

 possess ribs, which organs aftord so strong a fulcrum for 

 support to the action of muscles in bony fishes, but their place 

 is well supplied by those substantial plates, which are not 

 simply a covering to the surface, but dip within between the 

 layers of the organs of motion, and thus enable the muscles 

 of the body to exert such a degree of action as otherwise 

 they would not be capable of. 



