XV111 EXPLANATION OF THE PLATES. 



and the branchial compartment of the intestinal tube, have 

 been opened longitudinally, a little to the right side of the 

 mesial line. a a The two divisions of the hyoicl bone ; 

 b b the internal surface of the branchial chamber, through 

 the walls of which the " chorda dorsalis," the nerves, and . 

 the ventral bundles of the muscles, are seen ; c c the heart, 

 with the splanchno-ribs passing off from it on each side 

 towards the " chorda dorsalis ;" d d the abdominal muscle ; 

 c the digestive portion of the intestinal tube proceeding to 

 the anus ; fg g the abdominal folds. 



Fig. 5. The neuro-skeleton, consisting of a a the " chorda dorsalis," 

 b b the vesicular germs of the dorsal fin rays, c those of the 

 anterior, and d those of the posterior anal fins. 



Fig. 6. The nervous system, a a The spinal cord ; b the first pair 

 of nerves ; c the dorsal ; d the ventral branch of the second 

 pair. 



Fig. 7. The intestinal system, a The branchial compartment ; b the 

 digestive compartment of the intestinal tube ; c the mouth ; 

 d the anus. 



Fig. 8. The vascular system, a a The heart ; b b the primary branches 

 or branchial arteries ; c c the branchial veins uniting in 

 loops, from the angles between which trunks convey the 

 blood into d d the aorta. 



Fig. 9. a Portion of the " chorda dorsalis," to show the circular fibres 

 of the sheath ; b the superior ; c the inferior longitudinal 

 ligament. 



Fig. 10. A portion of the sheath ; shreds of the aj^oneuroses adhere 

 to it. 



Fig. 1 1 . One of the compressed vesicles which occupy the interior of 

 the sheath, and compose the mass of the " chorda dorsalis." 



Fig. 1 2. Some of the compressed vesicles removed from the sheath, to 

 show their relation one to another. 



Fig. 13. Five of the cylindrical cells, from the dorsal fin, to show their 

 relative positions, and the masses of cells which they contain 

 in their interior. 



Fig. 14. A single cell. 



Fig. 15. A portion of the spinal cord, from its anterior third, magnified, 

 to show the black matter which runs along the median line, 

 and the origins of the nerves. 



Fig. 16. A portion of the middle third, highly magnified to show the 

 nucleated cells of which it is composed, and the larger cells 

 of the dark median band. Some of the cells of the dark 

 band are filled with black pigment granules, which are repre- 

 sented escaping under the compression. 



Fig. 17. A portion of the spinal cord, magnified to show the origin of 

 the nerves by single roots, and withoul the insertion of the 

 primitive fibres of the nerves into the substance of the cord. 



Fig. 18. Primitive fibres of a nerve. 



