560 



CHORD ATA. 



unpaired, while in all other vertebrates it is paired. Most con- 

 stant of the cartilage bones are the ethmoids (the paired ecteth- 

 moids, ee, and the sometimes paired mesethmoid), and the four 

 occipitals. On the other hand the otic and optic regions vary 

 considerably; the otic region, from its great size, has several bones, 

 usually (fig. 589) five in number: pterotic, pto, often called 



ol ic. ns. 



< \ . 



v g/i.Jf. jio 



CO. 



S. 7. 



FIG. 588. Cranium, visceral arches, and part of vertebral column of Mustelus vulgaris. ao 

 antqrbital process; co, copula; gp, foramen for glossopbaryngeal; H, otic capsule and 

 hyoid; Hm, hyomandibular; ic, intercalare; Md, mandible (Meckel's cartilage); 

 JV, nasal capsule; o, optic foramen; 06, neural arcb ; po, postorbital process; Pq, 

 ptery goquadrate ; ps, spinous process ; J?, rostrum ; r, ribs ; tr, trigeminus foramen ; 

 v, vagus foramen; 1-8, visceral arches: 1, labial; 2, mandibular; 3, byoid; U-8, gill 

 arches. 



squamosal; sphenotic, spho, frequently called postfrontal; epiotic, 

 epo; prootic, pro; and opisthotic, 00, the last sometimes lacking. 

 In the region of the eye the cartilaginous sphenoids are rarely 

 well developed, the large parasphenoid taking their place. The 

 same is true of the ali- and orbitosphenoids, tliese sometimes form- 

 ing an interorbital septum (fig. 560) or a more or less wide in- 

 terorbital fenestra (fig. 589). 



The character of the visceral skeleton is related to the aquatic 

 life. All fishes have numerous gill arches (five to seven, mostly 

 five), which, since their function gill supporting is similar, are 

 similar in structure. So far as they are not degenerate they con- 

 sist each of four parts and are connected by unpaired copulas, these 

 often being fused. The upper ends are frequently toothed and, 

 in chewing, are opposed by the rudimentary last arch, on which 

 account these are spoken of as the superior and inferior pharyngeal 

 bones. The anterior visceral arches are greatly different in car- 

 tilaginous and bony fishes. In the former (fig. 588) the pterygo- 

 quadrate (pq) and the Meckelian cartilage bear teeth and oppose 

 each other in biting. In the bony fishes (fig. 589) the teeth of 



