18 



the whole caudal fin is stretched out on rays only. The end of 

 the notochord turns up at an angle at the root of the fin, so as to 

 point upwards. This kind of tail is found in most of the recent 

 fishes, sometimes simply fan-shaped, as in the Sticklebacks ; 

 sometimes more or less forked, as in Perch and Herrings. See 

 Figs. 7 and 8. 



Now a very notable thing is this ; that iu the development of a 

 fish from the egg, it has first a simple membranous fin, very like 

 that of a tadpole ; afterwards it becomes (in some cases at least) 

 distinctly heterocercal, like the old-fashioned types of fish, before it 

 finally assumes the homocercal form. For example, the common 

 Stickleback, shortly after hatching, has a tail like Fig. 6. The 

 notochord occupies the upper part, and the fin-rays are all on the 

 lower side. As the fish grows, the notochord turns up more and 

 more, and the end part dwindles away ; so that in the adult 

 Stickleback the upper half of the tail seems to match the lower, 

 although nearly all the rays really spring from the lower side of 

 the spinal axis. 



Besides this caudal fin, most fi^^hes have an anal fin below, on 

 the foremost part of the tail ; and the dorsal fin of the back often 

 extends along the tail, sometimes joining the caudal. In the 

 Flat-fishes, such as soles and plaice, the vital organs occupy a very 

 small part of the animal ; the rest is all tail, with its continuation 

 in the high ridge of the back. In these flat-fishes the roe extends 

 backward far into the tail. 



The bony skeleton of a fish's tail consists of a series of vertebrae 

 along the notochord ; each vertebra carrying a neural arch above, 

 to protect the nervous cord, and a hcemal arch below, covering the 

 great blood-vessels. Both these arches are often prolonged into 

 spines at top and bottom ; as is well seen in the Flat-fishes. 

 Among the curiosities of fish-tails are the prehensile tails of Pipe- 

 fishes and Sea-horses, by which they anchor themselves among 

 seaweed. Also the strange whip-like tails of Skates, rather 

 suggestive of mammalian tails than any other class. In early life, 

 however, the Skate's tail is of the ordinary tapering form. 



Above the Fishes come the Amphibia, including the Efts, Frogs, 

 and Toads. All these in their early stages have tails such as 

 already described in the tadpole. The efts keep this primitive 

 tapering tail all their life, with neural and hsemal arches as in 

 Fishes. The fin reaches round the back as far as the neck ; but 

 never acquires any fin-rays, or specialized tip. When the animal 

 takes to the land, the fin disappears, being no longer needed. 



In the frogs and toads the whole tail is gradually absorbed. 

 The white corpuscles of the blood carry it back into the body 

 particle by particle, thus nourishing the tadpole during its great 



