IV. VERTEBRATA: PISCES. 



•197 



The opercular apparatus does not occur in all fishes. It consists of 

 a numjjer of bony plates and processes which arise from the hyoid arch 

 and extend backwards over the gills, protecting them. It arises in 

 part (opercular bones — O, Pro, So, lo, fig. 547) from the hyomandibular, 

 in part {branchostegal rays) from the hyoid bone. The significance of this 

 apparatus will be spoken of in connection with the gills; it gives the fish, 

 skull a definite character, but hides its structure, on which account it, 

 like the infraorbilal ring, is shown in red in the figure. 



c]w pi:' p n'" V''" 



Fig. 547. — Skull of haddock. Infraorbital ring and operculum outlined in red. a, 

 angulare; ar, articulare; as, alisphenoid; de, dentar)^; ee, ectethmoid; ekt, ectopterj'goid ; 

 eng, OS entoglossum; ent, entopterygoid; epo, epiotic;/)-, frontal; li'-h', hyoid elements ; 

 Iim, hyomandibular; ih, interhyal; ma, maxilla; me, mesethmoid; mf, metapterygoid; iia, 

 nasal; ocb, ocl, ocs, basi-, ex-, and supraoccipital; 00, opisthotic;/i, parietal; /"a, palatine; 

 prm, premaxiUary; pro, prootic; ps, parasphenoid; pto, pterotic; qu, quadrate; rbr , 

 brancliiostegals; spho, sphenotic; sy, symplectic; vo, vomer; w, vertebra. Bones out- 

 lined in red; inf, infraorbital; Jo, interoperculum; O, operculum; Pro, preoperculum ; 

 So, suboperculum; i, 2, 3, axes of labial, mandibular, and hyoid arches. 



The appendages are also influenced by the aquatic hfe. In contrast 

 to the cyclostomes, there are two pairs of paired fins, the thoracic or pectoral 

 and the pelvic, ventral, or abdominal fins; in contrast with Amphibia, rep- 

 tiles and mammals, which occasionally have fin-like structures, the fishes 

 have three unpaired fins, dorsal, caudal, and anal fins. Only rarely, as in 

 the eels, the ventral fins are lacking; more rarely (Muroenida;) the pectorals 



