18 THE DERMOSKELETOX. 



in the capture of the sturgeons for the sake of their air- 

 bladder, of which the most valuable isinglass consists, 

 show us how well the external defensive armor of these 

 fishes is adapted to their mode of life. The sturgeons 

 may be called the scavengers of the great rivers which 

 they frequent. They habitually swim^ low, and grovel 

 along the bottom, turning up the mud and sand with their 

 pig-like snout, testing the disturbed matter with their 

 feelers, 6, and feeding in shoals, on the decomposing 

 animal and vegetable substances which are carried down 

 with the debris of the continents drained by those rapid 

 currents; thus, they are ever busied reconverting the 

 substances, which otherwise would tend to corrupt the 

 ocean, into their own 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 every seaman 

 knows how ballast should be, in orderly series along the 

 middle and sides of the body. The protection against 

 the logs and stones hurried along their feeding-grounds, 

 which the sturgeons derive from their plate armor, ren- 

 ders needless the ossification of the immediate case of the 

 brain and spinal marrow, and, consequently, all the parts 

 of the neuroskeleton, c/i, jj/, 7?, ns, remain in the flexible, 

 elastic, gristly state; the weight of the dermoskeleton 

 requiring that the other systems of the skeleton should 

 [ be kept as light as might be compatible with its defensive 

 and sustaining functions. This view of the final purpose 

 of the dermal bony plates in the existing sturgeons affords 

 some insight into the habits and conditions of existence 

 of the similarly mailed extinct fishes which abounded in 

 the seas of the secondary periods of the geological history 

 of this planet. In most of these fishes, as in the stur- 



