340 ZOOLOGY. 



branchial cavities ; these are the radii branchiostegi. Behind 

 these branches there descend from the median portion of the 

 hyoid apparatus four pairs of osseous arches (a), which ulti- 

 mately are attached to the basis of the cranium by means of 

 some small bones, called superior pharyngeal bones (ph}\ 

 these arches carry the gills* and for this reason are called 

 branchial arches. Still further back, at the entrance of the 

 gullet, are the two inferior pharyngeal bones, so disposed as 

 to be applied against the superior. 



Such generally is the complicated structure of the head of 

 fishes: anomalies exist, as in the sword-fish and in some 

 species allied to the tunny ; in these the upper jaw is pro- 

 longed into a powerful weapon, with which they attack the 

 largest sea animals. Any further comparison with the osseous 

 head of mammals need not detain us here, for much uncer- 

 tainty prevails. 



479. In the vertebrate column there are but two distinct 



Fig. 310. Espadon (Xiphias Gladius, 

 Common Sword- Fish). 



portions, a dorsal and a caudal (Fig. 307). The body of 

 each vertebra is formed like an hour-glass, with the two ex- 

 tremities hollowed out into conical cavity which sometimes 

 unite by an opening : the double cavity resulting from the 

 juxtaposition of the two vertebrae, is filled with a soft elastic 

 substance. The osseous ring formed by the processes of the 

 dorsal part of the column for the protection of the spinal 

 marrow, is repeated beneath the column in its caudal por- 

 tion ; it lodges the great artery of the trunk. 



In some the ribs are wanting, in others they are very com- 

 plete, and surround the trunk ; and in some they are connected 

 anteriorly with a chain of bones representing the sternum. 

 The ribs, moreover, often carry one or two small spines, which 

 are directed outwards, and penetrate the flesh. Similar stylets 

 also sometimes proceed from the bodies of the vertebm-: and 

 these are in some very numerous, as in the herring. Finally. 

 in the median line of the body are found the interspinal 

 bones (Fig. 311, i) ; these rest on the spinous protv- 



