434 



The vertebral column is formed of vertebrae, each of which has a rounded body 

 hollowed out on both faces (amphicoelous)^ from which there projects an upper 

 arch protecting the spinal cord and terminating above in an upper or neural 

 spinous process (fig. 6). Between the neural spines are inter-spinous bones which 



Fig, 6.— Caupal Vertebra and Caudal end of Vertebral Column in the Catfish. 

 Ns, neural spine ; c, vertebral centre ; hs, haemal spine ; nsh, bony sheath of the notochord ; cr, caudal rays. 



carry the fin-rays of the unpaired fins. In the tail region there are also ventral 

 arches and spines which protect the blood-vessels running to the tail, and the ribs 

 further forward in the trunk region correspond to these, although they do not 

 meet in the middle line below. 



The caudal fin of the catfish appears quite symmetrical, and in some species 

 (plate 5), is equally forked ; a close inspection of the skeleton, however, shows that 

 it is chiefly situated on the lower surface of its axis, which is here abruptly turned 

 up. This apparent symmetry (homocercal condition) is absent in some fish like 

 the Sturgeon, (plate 1), where the vertebral column turns up, sub-dividing the 

 tail very plainly into two unequal lobes (heterocercal condition). 



In the catfish some anterior vertebrae are united very intimately to each 

 other, and to the skull and pectoral arch, in consequence of certain peculiar 

 connections between the ears and the air-bladder, but this is not usual in fish. 



I'lg. 7.— Pectoral Girdle oi' Catfish from behind. 

 Co, coiacoidal, s, scapular portion of primary shoulder-girdle; cl, clavicular, sc, supra-clavicular portions 

 of secondary shoulder-girdle ; b, basal elements, r, rays of the fin-skeleton. 



On the other hand the union of the pectoral arch to the skull is not un- 

 common, and is effected by a three-pronged bone, the supraclavicle or supra- 

 scapula (fig. 7). The arch is formed of right and left halves which fuse with 



