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_ The vertebral column is formed of vertebrze, each of which has a rounded body 
hollowed out on both faces (amphiccelous), 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 
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Fig. 6.—CAUDAL VERTEBRA AND CAUDAL END OF VERTEBRAL COLUMN IN THE CATFISH. 
Ns, neural spine ; c, vertebral centre ; hs, hemal 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 vertebre 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, 
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big. 7.—PectoraL GIRDLE OF CATFISH FROM BEHIND. 
Co, coracoidal, s, scapular portion of primary shoulder-girdle ; cl, clavicular, sc, supra-clavicular portions 
of secondary sboulder-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 
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