18 



HIGH SCHOOL ZOOLOGY. 



connection with the internal skeleton in the catfish : we know this by 

 the development of their two constituent parts, the dentine — a member 

 of the connective-tissue group allied to bone — and tlie enamel) which is 

 formed by the modification of epidermal cells. (Fig. 3.) 



12. In the skeleton we distinguish axial and appendicular 

 parts, § 3 ; to the former belong the skidl with the liard parts 

 of the gills, and the vertebral column with the ribs. 



We shall study the latter first ; it is formed of a series of 

 separate bones, the vertebrae, which vary considerably in 

 I'orm in difierent regions of the column, but are all characterized 

 by a central part (the body or centrum), which is hollowed out 

 like a cup, on both anterior and posterior faces, (amphiccelous.) 

 (Fig. 5.) 



^^^^^« 



Fig. 5.— Caudal Vertebra and Caudal end of Vertebral Column in the Catfish. 



Ns. neural spine ; c, vertebral centre ; hs, hnemal spine ; nsh, bony sheath of the 



notochord ; or, caudal rays. 



Within the space so formed is contained the gelatinous remains of the 

 notochord, a rod present in the youngest stages of all Vertebrates, 

 around which the vertebral column is built, but rarely continuous in the 

 adult, except in the lowest Vertebrates. 



Each centrum bears on its dorsal surface an arch, the 

 neural arch, which terminates in a neural spine. The series of 

 centra constitutes a flexible rod of great importance in loco- 

 motion, the series of neural arches forms a canal serving to pro- 

 tect that i)art of the nervous system known as the sjnnal cord, 



