74 A HISTORY OF FISHES 



resembling spines may be found at the base of the lobes (Fig. 

 8 1 a). The Sea Horse {Hippocampus), which is unique in using 

 the tail as a prehensile organ, shares with some of the Eels 

 (Apodes) and a few other fishes the distinction of being without 

 a caudal fin (Fig. 5E). The caudal fin of the Sharks varies 

 somewhat in form, but is rarely outwardly symmetrical, and 

 the supporting rays are never visible externally. The function 

 of the curious notch found in the upper lobe of the tail-fins of 

 these fishes has never been satisfactorily explained, and it is 

 possible that it may have possessed some special use in the past 

 which no longer exists to-day (Figs, ib; 32 a). The Thresher 

 or Fox Shark (Alopias) is remarkable for the great length of the 

 upper lobe, which forms half the entire length of the fish 

 (Fig. 32A). This Shark is said to swim round a shoal of fishes, 

 splashing the water with its tail, and thus driving them into a 

 compact mass, when they form an easy prey. Among the 

 bottom-living Rays (Raiidae) the caudal fin tends to be much 

 reduced in size, whilst in the more specialised Sting Rays 

 (Trygonidae) and their allies it is wanting altogether, the long 

 whip-like tail simply tapering to a fine point (Fig. 32B). 



Among Bony Fishes with outwardly symmetrical tails, the 

 shape and size of the caudal fin exhibits a good deal of varia- 

 tion (Fig. 33). Six main types of fin may be recognised, 

 described respectively as lunate or crescentic (Tunny), forked 

 (Herring, Mackerel), emarginate (Trout, Carp, Perch), trun- 

 cate (Flounder), rounded (Turbot and Lemon Sole) and 

 pointed (Goby). The shape of the tail generally provides a 

 good index of speed and agility. As a general rule, fishes with 

 lunate or deeply forked tails are capable of swimming for long 

 periods at high speed, whereas those with squarish or rounded 

 tails, although capable of sudden, short bursts of speed, are on 

 the whole comparatively slow swimmers. 



The Deal-fish [Trachypterus) and Sun-fish (Mola) may be 

 selected as examples of fishes with unusual caudal fins. In the 

 former this fin is unique in being directed upwards at right 

 angles to the axis of the body. In the young fish the rays of the 

 lower lobe are prolonged into lengthy filaments like those of 

 the dorsal and anal fins, but these become progressively shorter 

 as growth proceeds and finally the lower lobe of the fin dis- 

 appears (Fig. 12 ib). It will be recalled that in the Sun-fishes 

 the body ends abruptly behind the short, high dorsal and anal 

 fins, and this is margined by a low, rounded caudal with a 

 sHghtly wavy edge (Figs. 50; 26d). Such a tail, to which the 



