IV. VERTEBRATA: PISCES. 



501 



stairway into the lumen of the intestine, increasing the digestive surface. 

 Caeca and spiral valve rarely occur in the same fish (La-mar gus, ganoids). 

 Gills of two types occur (fig. 551). In both the gill clefts, which lie 

 between successive branchial arches, begin by openings in the pharynx, 

 but differ in their external openings. In the elasmobranch type (A) the 



FIG. 551 Pharynges of (A) Elasmobranch (Zygrena) and (B) Teleost (Gadus), the 

 skull removed and on the left the gill slits cut across, a, attacnment of upper jaw to 

 cranium; as, outer gill slit; b, gill arch; bl 1 , bl' 2 , anterior and posterior gills (demibranchs) ; 

 h, dermal projection; hm, hyomandibular; is inner gill cleft; m, mouth; ma, maxillary ; 

 o, cesophagus; op, operculum; ops, opercular opening; pa, palatine; phi, inferior pharyn- 

 geal bones; pq, pterygoquadrate; pnn, premaxilla; s, shoulder girdle; uk, lower jaw; c 

 tongue. 



external openings are a series of slits separated by broad dermal bridges 

 which cover the gills (fig. 555). The gills are vascular folds of 

 mucous membrane with secondary folds distributed on anterior and 

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

 (figs. 551, 552) show, bears two rows of gill folds (demibranchs) 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 septum between the demibranchs has more 



