194 ZOOLOGY SECT. 



from behind, the water is compelled to make its exit through the 

 gill-slits. 



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. 865, 

 an.) ; behind it is the urinogenital aperture, of almost equal size 

 and leading into the urinogenital sinus, into which both urinary 

 and genital products are discharged. 



The region from the snout to the posterior edge of the operculum 

 is counted as the head ; the trunk extends from the operculum to 

 the anus ; the post-anal region is the tail. 



There are two dorsal fins : the anterior dorsal (Fig. 865, d. f. 1) 

 is large and triangular, and is supported by thirteen bony fin-rays ; 

 the posterior dorsal (d. f. 2) is small and thick, and is devoid of 

 bony supports : it is distinguished as an adipose fin. The caudal 

 fin (c. f.) is the chief organ of locomotion ; it differs markedly from 

 that of most Elasmobranchs in being, as far as its external appear- 

 ance is concerned, quite symmetrical, being supported by fin-rays 

 which radiate regularly from the rounded end of the tail proper ; 

 such outwardly symmetrical tail-fins are called homocercal. There 

 is a single large ventral fin (v. f.) supported by eleven rays. The 

 pectoral fin (pct.f.) has fourteen rays and is situated, in the normal 

 position, close behind the gill-opening, but the pelvic fin (pv. f.) 

 has shifted its position and lies some distance in front of the vent : 

 it is supported by ten rays, and has a small process or adipose lobe 

 (a. I.) springing from its outer edge near the base. 



The body is covered by a soft, slimy skin through which, in the 

 trunk and tail, the outlines of the scales can be seen ; on the head 

 and fins the skin is smooth and devoid of scales. A well-marked 

 lateral line (I. L) extends along each side from head to tail, and 

 is continued into branching lines on the head. The skin is grey 



above, shading into yellowish below, and 

 is covered with minute black pigment- 

 spots which, on the sides and back, are 

 aggregated to form round spots two or 

 three millimetres in diameter. In young 

 specimens orange-coloured spots are also 

 present. 



Skin and Exoskeleton. The epi- 

 dermis contains unicellular glands, from 

 FIG 867 scale of saimo fario which the mucus covering the body is 

 JkS^f'SSSd.SrS*?! ^creted, and pigment-cells, to which the 

 b. free portion covered only by colours of the animal are due. The scales 

 (Fig. 867) are lodged in pouches of the 



dermis and have the form of flat, nearly circular plates of bone 

 marked with concentric lines, but having no Haversian canals, 

 lacunae, or canaliculi. They have an imbricating arrangement, 

 overlapping one another from before backwards, like the tiles of 



