566 



CHORD AT A. 



brane with secondary folds which extend on anterior and posterior 

 sides of the cleft. Each arch except the last, as the sections (fig. 

 594, A, and 595) show, bears two rows of gill folds (demi- 



FIG. 594. Pharynges of (.4) Elasmobranch (Zygoma) and CB) Teleost (Gadus\ the 

 skull removed and on the left the gill slits cut across, a, attachment of upper 

 jaw to cranium; as, outer gill slit; Jb, gill arch; fol 1 , foZ 2 , anterior and posterior 

 gills (demibranchs) ; ft, dermal projection ; tow, hyomandibular ; is, inner gill 

 cleft ; m, mouth ; ma, maxillare ; o, oesophagus ; op, operculum ; ops, opercular 

 opening; pa, palatine; phi, inferior pharyngeal bones; pq, pterygoquadrate ; 

 pnn , premaxilla; s, shoulder girdle ; uk, lower jaw; z, tongue. 



branchs) which belong to different clefts and are separated from 

 each other by tissue containing the cartilaginous gill rays. 



In the second type (B), which occurs in all other fishes, the 

 dermal bridges are lacking, and the tissue between the demi- 

 branchs has more or less completely disappeared, so that the 

 demibranchs of one arch become connected^ their free ends pro- 

 jecting into the water like the teeth of a double comb. Here, 

 on account of their very delicate structure, they would be ex- 

 posed to serious injury were they not protected by the operculum 

 or gill cover. This is a fold of skin arising from the hyoid arch 

 and extending back over the gill region. It is supported by two 

 groups of bones, the opercular bones proper (fig. 589, 0, /Sfc, lo, 

 Pro), attached to the hyomandibular, and the branchiostegals 



