CHARACTER AND USES OF FINS. 13 



prolonged into the upper half of this caudal fin, rendering it unequally Jobed or 

 " heterocei'cal." Among the salmonidse the chai'acter of the last two or three of 

 the vertebra is somewhat intermediate between these two forms of fin, although 

 externally it is of the homocercal or equally-lobed type. The anal (a) 

 commences behind the vent (vf) and passes along the median line of the lower 

 surface.* The functions of the dorsal and anal fins arc mostly restricted to 

 increase or diminish the extent of the lateral surface, by means of their being 

 elevated or depressed, and so preventing violent lateral oscillations or the body 

 being turned upside down. 



These fishes possess two pairs of horizontal or paired fins, one on the breast 

 (p) or pectorals (also known as chowk fins), corresponding with the anterior 

 extremities of higher vertebrata ; while the second or ventral pair (v), the homo- 

 logues of the hind pair of limbs, are abdominal, so termed as being near the hind 

 end of the body. The pectoral can assist Ln a forward movement by rapid strokes 

 towards its body, and likewise in a backward movement by means of reversed or 

 forward strokes. Under certain circumstances the upper rays of this fin are useful 

 in respiration, as should a deficiency of air exist in the water, when they are 

 constantly in motion, sending a current towards the gills, thus in very young 

 fish, as salmonoids, prior to the absorption of the umbilical vesicle, they are 

 ceaselessly employed in assisting respiration. In some localities a considerable 

 development of this fin appears to be constant, but it is generally largest in 

 males. t Variations in its size are of insufficient character upon which to constitute 

 species. 



Experiments have demonstrated that should both the dorsal and anal fins be 

 removed, the fish rolls from side to side : if one pectoral is cut off, it falls over to 

 the side from which such has been taken : if both are abstracted, the head sinks : 

 should the pectoral and ventral of one side be gone, equilibrium is lost : while 

 removal of the tail fin interferes with progression. 



The form of the caudal fin amongst these fishes alters very considerably with 

 age, being more foi'ked in the young than in the adult :J while non-migratory 

 varieties of trout sometimes have it less so than do the young of the anadromous 

 races. But the variations are exceedingly numerous, and attention has to be paid 

 to the age, sexual development, and locality from whence the specimen was 

 procured, for this fin would be required to possess much greater power in rapid 

 than it would in sluggish streams, while although excessive use might, and 

 probably would augment its extent, the same influence might also curtail its size 

 by wearing off its extremities, more especially its corners, as will be more fully 

 entered upon when arriving at the description of the various forms of trout. 



Prior to examining these questions some fixed method on which to proceed 

 when investigating the conformation of this fin is desirable, for it is evident that 

 the wider it is expanded the less will be the comparative emargination at its 

 centre. To illustrate this the following figure of the caudal fin is given. Here 

 the fin is placed so that the depth of the free portion of the tail, from A to B, 

 about equals the distance from C to E, and E to D, or half the length of the 

 expanded fin (C to D), and when in this position it is seen to be but slightly 

 cleft. But should the distance from C E D be extended about one half more, or 

 from C to E and on to D', the fin then becomes merely emarginate. Or in the 

 first position we find it very slightly forked, in the second scarcely concave, and 



* These median or unpaired fins are considered by some to be appendages to the skin, but 

 believed by Balfour to be the specialized and highly-developed remnants of a once continuous 

 lateral fin along either side. 



t In the opinion of Agassiz, trout inhabiting rapid rocky streams have then- fins always much 

 developed (Richardson, Faun. Bor. Ainer. p. 169). " This opinion is not founded on observations 

 in nature. Salmonoids inhabiting such streams are distinguished by short fins, the delicate 

 extremities being worn o& by constant activity of the fish, whilst individuals inhabiting still 

 waters show the fins proportionately longer. Hence mature specimens have the fins more rounded 

 and shorter than the sterile ones, on account of their greater activity." (Giinther, Catal. vi. 

 p. 149.) 



J Widegren likewise believed that sexually mature individuals have the caudal fin less deeply 

 cleft than immature ones of the same age and size. 



