STRUCTURE OF FISHES. 417 



minds of naturalists whether that animal ought 

 not properly to be ranked among this latter 

 class. Its pretensions to be included among the 

 vertebrata are, indeed, but slender and equi- 

 vocal ; for, in place of a series of bones com- 

 posing the vertebral column, it has merely a soft 

 and flexible tube of a homogeneous and cartila- 

 ginous substance, exhibiting scarcely any trace 

 of division into separate rings, but appearing as 

 if it were formed of a continuous hollow cylinder 

 of intervertebral substance, usurping the place of 

 the vertebrae, which it is the usual office of that 

 substance to connect together, and having in its 

 axis a continuous canal filled with gelatinous 

 fluid. This, however, is not the channel intended 

 for containing the spinal marrow, for that ner- 

 vous cord is on the outside of this column. The 

 cartilage, indeed, sends out no processes to bend 

 round the spinal marrow, and forms no canal for 

 its passage and protection. The nervous matter 

 here consists merely 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 requires very minute atten- 

 tion to detect. The partial protection thus af- 

 forded to so important an organ is not greater 

 than that given by the cartilaginous lamina of 

 the cuttle-fish, which in form, texture, and 

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