200 



ZOOLOGY 



SECT. 



On the ventral suiface of tlie body, at about two-thirds of the 

 distance from the snout to the end of the tail, is the anus (Fig. 854, 

 (in.); behind it is the urhuKjrnital apcrhtrc, of almost equal 

 size and leading into the urinogcnital 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 t<(il. 



There are two dorsal fins: the anterior dorsal (Fig. 854, d.f. 1) 

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

 the 2^osterior 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 (pet. /.) has fourteen rays and is situated, in the 

 normal position, close behind the gill-opening, but the p7ric fin 

 (^w. J.) has shilted its position and lies some distance in front 

 of the vent : it is supported by ten rays, and has a small process 

 or adipose lohe (((. I.) springing from its outer edge near the base. 



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

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

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

 Icdercd line (I. I.) 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 milli- 

 metres in diameter. In young specimens 

 orange-coloured spots are also present. 



Skin and Exoskeleton. — The epi- 

 dermis contains unicelkilar glands, from 

 which the mucus covering the body is 

 secreted, and pigment-cells, to which the 

 colours of the animal are due. The 

 scales (Fig. 856) 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, lacuna^, or canaliculi. 

 They have an imbricating arrangement, 

 overlapping one another from before backwards, like the tiles of 

 a house, in such a way that a small three-sided portion (l) of 

 each scale comes to lie immediately beneath the epidermis, while 



Fig. S.Ot;.— Scale c.f Salmo fario. 

 a. ciiiterioi' portion ci.vci'eil liy 

 overlap of )irccediiig stales ; 

 //. free poition oovered only by 

 pigmented epidermis. 



