PISCES 



107 



cleft opens separately and is not covered by any flap or operculum; 

 though in Chlameidoselachus the primitive "frilled shark" (Fig. 67, A) 

 each cleft has a backwardly directed flap or gill-cover. In the Holo- 

 cephali the flrst three clefts are covered by an operculum and only 

 the fourth, or last functional cleft, opens freely to the outside. In the 

 great majority of Teleostomi and in the Dipneusti the five clefts are 

 covered with a flap-like operculum, capable of opening and closing 

 and effectively protecting the branchial filaments from injury. In 

 some of the eels and in other specialised types of teleosts the gills are 



Fig. 58. — Diagram of gills of fishes. A, Horizontal section through the head of 

 an Elasmobranoh; B, Similar section of a Teleost. 6c, branchial cavity; bl, 

 branchial lamellse; c, coelom; eba, external branchial aperture; hy. a, hyoid arch; 

 hy. c, hyo-branchial cleft; Is, interbranchial septum; n, nasal organ; oes, oesoph- 

 agus; op, operculum; pq, palatoquadrate cartilage; Ph, pharynx; sp, spiracle; 

 s. ps, spiracular pseudobranch; 1-5, 1st to 5th branchial arches. (From Bridge, 

 after Boas.) 



completely covered with a fold of skin and the only exit is through 

 one or a pair of small water-pores. 



Two quite different and distinct kinds of gills are found among 

 fishes: external and internal gills. 



External gills are purely larval or embryonic organs and are not 

 functional in any adult fish; though their homologues are found in 

 the perennibranchiate Amphibia, believed to be psedogenetic or per- 

 manent larval types. External gills are finely branched processes of the 



