THE CATFISH. 175 



the central nervous system, is another immensely larger 

 tube, the body cavity, which contains the organs of nutri- 

 tion and reproduction. The walls of this latter tube are 

 largely membranous, but they are in part supported by 

 the ribs and by the lower bones of the skull. A trans- 

 verse section of the body of the fish, therefore, shows two 

 tubes, one above the other. How does this arrangement 

 of nerve center and nutritive organs differ from the 

 arrangement studied in insects ? In the river mussel ? 



The Bony Skeleton. A few points respecting the 

 structure and arrangement of the parts of the bony skele- 

 ton must be studied here. Use a prepared skeleton for 

 reference, and study the parts in a disjointed skeleton, 

 which, if not furnished you, you can easily prepare for 

 yourself. 1 



I. The Spinal Column. Study the spinal column. 

 This is the most important and characteristic part of 

 the skeleton. Observe that its constituent vertebrae are 

 not all alike. Their differences are such as to make two 

 regions distinguishable in the spinal column. The an- 

 terior portion extending backward as far as the hindmost 

 pair of ribs is the body region, and the remaining posterior 

 portion without ribs is the tail region. Isolate one of the 

 middle vertebrae of the tail region, taking care not to 

 injure any of the small bony processes that arise from 

 it, and examine it. Observe a thick, solid, hourglass- 

 shaped central portion, concave on both anterior and 

 posterior faces, and perforated for the passage of the 

 spinal cord. This is the centrum (or body of the ver- 

 tebra). Observe a pair of processes arising from the 

 dorsal surface, one on either side of the median line, 

 and soon uniting to form a long, sharp neural spine. By 

 their union they form an arch above the centrum, in which 



1 For the method, see Appendix, p. 287. 



