PHYLUM CHORDATA 



199 



pop 



pmac 



a transparent layer of skin. A short distance in front of the eye 

 is the double nostril (na l , no 2 ), each olfactory sac having two 

 external apertures, the anterior one (via 1 ) provided with a flap- 

 like valve. There is no external indication of the ear. 



On each side of the posterior region of the head is the operculum 

 (Fig. 854, op.) or gill-cover, a large flap which, when raised, displays 

 the gills; between it and the flank is the large crescentic gill- 

 opening, from which the respiratory current makes its exit. 

 The operculum is not a mere fold of skin, as in Holocephali, but 

 is supported by four 

 thin bones the out- 

 lines of which can be 

 made out through the 

 skin ; they are the oper- 

 cular (Fig. 855, op.), 

 preopercular (p. op.), 

 sub-opercular (s. op), 

 and inter - opercular 

 (i. op) ; the latter is 

 attached to the angle 

 of the mandible. The 

 ventral portion of the 

 operculum is produced 

 into a thin membran- 

 ous extension, the 

 branchiostegal mem- 

 brane (br. m), supported by twelve flat, overlapping bones, the 

 branchiostegal rays. The narrow area on the ventral surface of the 

 throat which separates the two gill-openings from one another is 

 called the isthmus. The gills, seen by lifting up the operculum, 

 are four red, comb-like organs, each having a double row of free 

 gill-filaments ; alternating with the gills are the five vertically 

 elongated gill-slits, opening into the mouth. 



The Trout breathes by the drawing in of water through the 

 mouth and its passage outwards through the gill-slits. The 

 inspiration or inward movement of the water is effected by the 

 opercula being moved outwards, the space internal to them thus 

 being widened, and water flowing in through the open mouth 

 to fill the vacuum, the branchiostegal membrane at the same 

 time closing the gill-opening and thus preventing the water 

 from flowing in from behind. Expiration is brought about by 

 the opercula moving inwards and forcing the water out. Owing 

 to the action of a pair of transversely directed membranous folds, 

 the respiratory valves, one attached to the roof, the other to the 

 floor of the mouth, which are so directed as to become expanded 

 and block the passage when water presses on them from behind, 

 the water is compelled to make its exit through the gill-slits. 



FIG. 855. Head of female Salmo fario. Ir. m. branchio- 

 stegal membrane ; i. op. interopercular : mnd. mandible : 

 mx. maxilla ; nal, anterior, aud no?, posterior external 

 nostril ; op. opercular; pet. /. pectoral fin; pmx. pre- 

 maxilla ; p. op. preopercular ; s. op. subopercular ; t. 

 tongue. 



