PISCES. 143 



3, tlie superior spinous process, projecting upward 

 from the union of these pieces ; 4, the two lateral 

 processes ; 5, the inferior arch, formed as the 

 superior is, protecting great blood-vessels ; 6, the 

 inferior spinous process, pointing downward. 



The skull is considered to be but a number of 

 vertehrcB excessively altered in form, to serve a 

 special pm^pose, that of forming a box to enclose 

 the hrain, cavities to protect the organs of sense, 

 and jaws to seize and masticate the food. 



As accessory to the spinal column, we find three 

 sets of bones developed from its sides. 1, The 

 hyoid arches, which spring from the skull, and 

 form the firm framework on which the gills are 

 hung ; 2, the rihs, another numerous series of 

 arches, which protect the internal organs, and 

 serve as the support of muscles ; 3, the liiyihs, of 

 which there are two pairs, each limb consisting of 

 several pieces jointed to each other, of which the 

 first is jointed to the spine, while the last is free. 

 In Fishes, the last joint of the limb carries a 

 number of slender spines or rays, united by a thin 

 membrane in most cases, and forming a fin, to be 

 used in swimming. It may be considered as a 

 hand of many fingers. 



Both pairs of limbs are, in the present Class, 

 suspended from the vertehrce that enter into the 

 composition of the skull. The upper pair are termed 

 pectoral fins, and correspond to the human arms 

 and hands ; the lower pair, termed ventral fins, 

 represent the feet. The latter may be before, 

 beneath, or behind the former (but always nearer 

 to the line of the belly), and are sometimes 

 wanting. 



In addition to this internal skeleton^ Fishes 



