SKELETON OF FISHES. 373 



of two slender cords, which run parallel to one 

 another in a groove on the upper part of the 

 spinal column ; and these cords are covered only 

 by a thin membrane, the presence of which it re- 

 quires minute attention to detect. The partial 

 protection thus afforded to so important an organ 

 is not greater than that given by the cartilaginous 

 lamina of the cuttle-fish, which in form, texture, 

 and situation is somewhat analogous to the spine 

 of the myxine. 



As we ascend from this rudimental condition of 

 the spine, we find it, in the lamprey, more distinctly 

 divided into rounded portions, appearing like beads 

 strung together. These rudimental bodies of ver- 

 tebrae have not yet completed the cup-like hollows 

 on their two ends, but are shaped like rings, being- 

 perforated in the centre, so as still to form a con- 

 tinuous canal throughout the whole column. 



Proceeding to more advanced developements, we 

 find, in the Sturgeon and other cartilaginous fishes, 

 a greater condensation of substance produced by 

 the deposition of granules of osseous matter ; the 

 central canal becomes divided into lozenge-shaped 

 compartments by the closing in of the sides of the 

 body of each vertebra.* Frequently the sides do 

 not quite meet, and the leaves, which are developed 

 from the upper surfaces of the vertebrse, now form 

 arches over the spinal cord, and are united above 

 by spinous processes. Yet the whole skeleton in 



* A small aperture still remains, establishing a communication 

 between tbe cavities the whole length of the spine. This is supposed 

 to be designed to obviate the compression of the fluid in the different 

 cells or cavities during the motions of the spine. The vertical sec- 

 tions, Fig. 189 and 190, of two contiguous vertebrse in different 

 fishes, will convey an idea of this gradation of developement. 



