184 ZOOLOGY SECT. 



each ramus of the mandible is bounded mesially by a deep groove. 

 The floor of the mouth is produced into a prominent tongue (t.) 

 bearing a double row of teeth. In old males the apex of the lower 

 jaw becomes curved upwards like a hook. 



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 (na 1 , no?), each olfactory sac having two- 

 apertures, the anterior one (na 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. 806, 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 



it 



FIG. 806. Salmo fario. a. 1. adipose lobe of pelvic fin ; an. anus ; c. /. caudal fin ; d. /. 1, first 

 dorsal ; d.f. 3, second dorsal or adipose fin ; 1. L lateral line ; op. operculum ; pet. /'. pectoral 

 fin ; pv. f. pelvic fin ; r. /. ventral fin. (After Jardine.) 



out through the skin; they are the opercular (Fig. 807, op.), pre- 

 opercular (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 membranous 

 extension, the branchiostegal membrane (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. 



On the ventral surface of the body, at about two-thirds of the 

 distance from the snout to the end of the tail, is the anus (Fig. 806, 

 an.) ; behind it is the urino-genitcd aperture, of almost equal 

 size and leading into the urino-genital sinus, into which both 

 urinary and genital products are discharged. 



