170 VERTEBRATES. 



separate bones of the spinal column are the vertebrce. 

 The slender, curved bones extended laterally, a pair 

 from each vertebra, are the ribs. Between the ribs 

 observe the white spinal nerves, coming out from the 

 spinal cord, which is a part of the central nervous system 

 located in a hollow that extends longitudinally through 

 the center of the spinal column. 



Gills. Study the gills. Continue the ventral incision 

 forward exactly on the median line to the mouth. The 

 narrowed, anterior portion of the chest between the gill 

 openings is the isthmus. Observe the branches of the 

 branchial artery going to each gill. These will be better 

 seen in an injected specimen. 1 Deepen the cut until it 

 divides the upper wall of the pharynx, the floor of the 

 mouth, and the mandible. Then draw the edges of the 

 cut apart, and expose the white gill arches. Observe 

 the vertical series of teeth on the concave side of each 

 arch. These are the gill rakers. Note their position, and 

 the size of the teeth on the different arches. What is 

 their probable use ? 



Raise and depress the floor of the mouth of one side, 

 and study the action of the gill arches and the rakers. 

 Observe the joints in the arches. 



Cut through the isthmus of one side between the first 

 gill and the branchiostegal membrane, and expose the red 

 gills on the back or convex side ^ of the gill arches. Ob- 

 serve that they are in a double row on the back of each 

 arch. 



Find on the branchiostegal membrane an elongated 

 reddish patch, which is a rudiment of an anterior gill. 

 With sharp scissors cut out of a gill arch a thin trans- 

 verse section bearing one or two pairs of gill filaments, 

 and examine the section under lowest power of a micro- 



1 See Appendix, pp. 284, 285. 



