414 



BORER. 



from the simplest and almost rudimental condition in which it 

 exists in the Myxine and the Lamprey, to that of the most 

 perfect of the osseous tribes. Its condition in the former of 

 these animals presents a close analogy with some structures that 

 are met with in the molluscous, and even in annulose animals. 

 So near is the resemblance of the spinal column of the Myxine, 

 more especially, to the annular condition of the framework of 

 the Vermes (worms,) that doubts have often arisen whether 

 that animal ought not to be ranked among this latter class; for 

 in place of a series of bones composing the vertebral column, 

 it has merely a soft and flexible tube of a homogeneous and 

 cartilaginous 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 vertebra?, which it is the usual office of that 

 substance to connect together, and having in its axis a continuous 

 canal filled with gelatinous fluid. The nervous cord is on the 

 outside of this column, and the cartilage forms no canal for 

 its passage and protection," as is formed by bony processes in 

 the higher orders of fishes. "The nervous matter here consists 

 merely of two slender cords, which run parallel to each other 

 in a groove on the upper part of the spinal column; and 

 these cords are only covered by a thin membrane, the presence 

 of Avhich it requires very minute attention to detect. As we 

 ascend from this rudimental condition, which resembles that of 

 the cuttle (sepia,) we find it in the Lamprey more distinctly 

 divided into rounded portions, which appear like beads strung 

 together, or like rings with a canal throughout the whole. 

 There is also a skull to hold the brain, which, as in other 

 fishes, does not fill the cavity, which on either side contains 

 a considerable space that is occupied with an organ of hearing; 

 but while there are nerves from the brain, as in other fishes, 

 that important j)ortion of the brain, the cerebellum, is absent." 

 Ekstrom says that the colour of this fish is greyish flesh- 

 colour when alive. 



