THE PERCH 215 



connected by intervertebral ligaments. They are deeply bicon- 

 cave ; the two concavities are joined by a central canal and all 

 these spaces are filled with the jellylike notochord. The noto- 

 chord thus forms a continuous structure which runs the length of 

 the vertebral column. 



The vertebral column may be divided into two regions : the 

 trunk region, in which ribs are present, and the caudal region, 

 in which they are absent. Each trunk vertebra is composed of 

 a biconcave body, or centrum, from the dorsal side of which 

 arises the neural arch, and from the ventrolateral side of which 

 projects a pair of long, curved haemal processes, which form the 

 ribs. The neural arch is composed of a pair of neural processes 

 and a long, median neural spine ; it incloses the spinal cord. 



The ribs are long, slender bones which form extensions of the 

 haemal processes. The ribs of teleosts are not homologous to 

 those of the higher vertebrates, but represent the distal ends of 

 the haemal processes. Extending from the ribs is also a series of 

 long, slender bones which lie in the muscle segments and may 

 be homologous to the ribs of the higher vertebrates; they are 

 called the intermuscular bones. 



The caudal vertebrae differ from the others in that the haemal 

 processes meet in the midventral plane and unite in a haemal 

 spine, forming thus the haemal arch, and inclosing a space in 

 which lie the caudal vein and artery ; they do not bear ribs. 



The haemal processes of the caudal vertebrae are thus homol- 

 ogous to the ribs of the trunk region. This homology is easily 

 seen by following the haemal processes forward from the caudal 

 region : at the point where the two regions meet the caudal 

 haemal processes separate from each other and become ribs. 



Count the trunk vertebrae and caudal vertebrae. Note how 

 the former grade into the latter. The spinal column ends pos- 

 teriorly with a fan-shaped bone called the urostyle. 



Exercise 21. Draw a posterior and also a side view of one of the trunk 

 vertebrae ; draw the same views of a caudal vertebra. 



Make a sagittal section of a vertebra. Examine the concavities 

 with the aid of a lens and note the lines of growth. The centrum 



