.XIII 



PHYLUM CHORDATA 



185 



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. 806, 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 cainl"! 

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

 that of Elasmobranchs in being, as far as its external appearance 

 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 h.omocercaL There 

 is a single large central fin (v. /.) supported by eleven rays. The 

 pectoral fin (pet. f.) has fourteen rays and is situated", in the 

 normal position, close 

 behind the gill-open- 

 ing, but the pelvic fin 

 (pv.f.) has shifted its 

 position and lies some 

 distance in front of 

 the vent : it is sup- 

 ported by ten rays 

 and has a small pro- 

 cess 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, I.) extends along each side from head to tail. 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 epidermis has no stratum 

 corneum ; it contains unicellular giands, from which the mucus 

 covering the body is secreted, and pigment cells, to which the 

 colours of the animal are due. The scales (Fig. 808) 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, 



m/ici 



FIG 807. -Head of female Salmo fario. L>,: ,, ( . branchio- 

 stegal membrane ; i. op. inter-opercular ; ,nnd. mandible ; 

 mx. maxilla; ,uii, anterior, and /i-, posterior nostril; 

 "/>. opercular ; pet. /. pectoral fin ; pnix. premaxilla ; 

 ji. oji. pre-opercular ; s. op. sub-opercular ; t. tongue. 



