xiir 



PHYLUM CHORDATA 



193 



The large eyes have no eyelids, but the flat cornea is covered by 

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

 is the double nostril (nal, na%), each olfactory sac having two 

 external apertures, the anterior one (nal) 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. 865, 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 outlines of which can be made 

 out through the skin ; they are the opercular (Fig. 866, op.), 

 pre-opercular (p. op.), 

 sub-opercular (s. op.), 

 and inter - opercular 

 (i.op.) ; the last is 

 attached to the angle 

 of the mandible. The 

 ventral portion of the 

 operculum is produced 

 into a thin mem- 

 branous extension, the 

 branchiostegal membrane 

 (br. m.), supported by 

 twelve flat, overlapping FIQ 

 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 



866. Head of female Salmo fario. br. m. 

 branchiostegal membrane i. op. interopercular ; mud. 

 mandible ; mx. maxilla ; nal, anterior, and na 2, posterior 

 external nostril ; op. opercular ; pet. /. pectoral tin ; 

 pmx. premaxilla ; p. op. preopercular ; s. op. suboper- 

 cular ; t. tongue. 



