SKE^iETON OF THE FISH. 35 



SKELETON OF THE FISH. 



In all fishes, the extent of ossification is less than in 

 the higher vertebrate classes. Only in the skull do we 

 find all the elements of the typical segment represented 

 by bone. In the trunk, e. ^., the htemapophyses and 

 hjBmal spines never advance beyond the fibrous stage of 

 tissue development. 



Four segments enter into the composition of the skull 

 of fishes, answering to the first four in the archetype (Fig. 

 7), and they combine to constitute the bony framework 

 of a head, larger in proportion to the trunk than in any 

 other class of animals. The skull (Fig. 9), 3, 52, hi\ forms 

 a cone, whose base is vertical, directed backwards, and 

 joined to the trunk without an intervening neck, and 

 whose sides are commonly three in number, one superior, 

 and two lateral and inferior. The cone is shorter or 

 longer, more or less compressed or squeezed from side to 

 side, more or less depressed or flattened from above down- 

 wards, with a sharper or blunter apex, in different species 

 of fishes. The base of the skull is perforated by the hole, 

 called "foramen magnum," for the exit of the spinal mar- 

 row; the apex is more or less widely and deeply cleft 

 transversely by the aperture of the mouth; the eye-sockets 

 or "orbits," or^ are lateral, large, and usually with a free 

 and wide intercommunication in the skeleton ; the two 

 vertical fissures behind are called "gill-slits," or branchial 

 or opercular apertures, and there is a mechanism, like a 

 door, 84, 35, 36, for opening and closing them. The 

 mouth receives not only the food, but also the streams of 

 water for respiration (indicated by the arrow hr\ which 

 escape by the gill-slits. The head contains not only the 



