540 



NATURE 



[Oct. 2, 1879 



As these animals are all aquatic, so they all have more 

 or less long and powerful tails. Almost always they swim 

 by striking the water right and left with the tail, and as 

 they breathe by gills, without coming to the surface, so 

 the tail and its hinder end are flattened from side to side, 

 and not from above downwards, as wc have seen to be 

 the case with the air-breathing whales and porpoises. 



Some fishes, however, progress largely by means of 

 great lateral fins, as is the case with the rays, or skates, 

 and in them the tail is comparatively small. 



There arc certain fishes which go by the name of Sea 

 Horses, though they are but small creatures. These 

 fishes swim through the water in a remarkable way. They 

 appear as if they glided at will without effort. But there 

 is on the back a small fin, which by its constant undula- 

 tions acts like the screw of a screw-steamer. It is by this 

 the creature moves, and the long tail takes no part in such 

 progression, and is relatively thin and small, except as to 

 its length. 



In this tail, however, we find once more that prehensile 

 character, such as we saw in the chameleon, the kinkajou, 

 and others. The sea-horse anchors itself by clinging with 

 its tail round sea-weed, or some similar object, much as 

 do the animals mentioned. 



The tails of fishes are like those of beasts and reptiles, 

 supported by an extension of the back-bone, and, as a 

 rule, contain no body-carity. But the mode in which the 

 vertebral column ends varies in different fishes in a way 

 worthy a passing mention. 



In such a fish as the sturgeon the end of the tail is 

 furnished with a fin, divided into two unequal lobes. 

 The end of the vertebral column runs along in the upper 

 lobe, but there is nothing similar in the lower lobe, so 

 that the lobes of the tail are very unequal, not only in 

 size, but also in structure. The tail-fins of sharks are 

 similarly conditioned. In such a fish as the cod-fish, on 

 the other hand, the vertebral column seems to stop short 

 in the middle between the two lobes of the tail, which 

 lobes are equally developed. 



This difference may seem trivial, but, in fact, it is 

 characteristic, not only of different groups of fishes, but of 

 fishes of different geological periods. The unequal tail 

 end is the more ancient, and has gradually given place to 

 the other apparently quite symmetrical form. I say 

 apparently, because, in fact, when the symmetrically 

 formed tail is minutely examined, it turns out that even 

 here, the end of the vertebral column is turned up, 

 extending dorsally as in the sturgeon and sharks. But it 

 is only a minute portion which is thus turned up. How- 

 ever, it is turned up, and very strongly so, and thus the 

 curious fact of the upward inclination of the tail is seen 

 to be a general character of fishes, whether their tail-fins 

 are apparently symmetrical or not. 



Fig. 10. — Post-axial termination of the vertebral column in a salmon. 



Such are the main varieties of tail which are to be found 

 in back-boned animals, that is to say, in beasts, birds, 

 reptiles, frog-like creatures, and fishes ; and thus we see 

 that one general structure underlies the various varieties 

 of external appearance which the tails of such creatures 

 may present. 



But there are many creatures of quite different nature 



and build, which are said to have "tails." Thus, for 

 example, we speak of the "tail" of a lobster, or the 

 " tail " of a scorpion. 



Now, of course, I do not mean to assert here, any more 

 than I asserted about the feathery so-called "tails" of 

 birds, that you should depart from ordinary usage. Still, 

 the so-called "tail" of such animals is really utterly 

 unlike the tail of back-boned creatures, and is, in fact, 

 but the backward prolongation of the body. 



The lobster's body is made up of a series of more or 

 less similar segments, in great part more or less aggluti- 

 nated together. It contains a body-cavity which is 

 traversed by the alimentary canal. The so-called "tail'' 

 does not differ from the lobster's body as the tail of a cat 

 differs from the cat's body. For the tail of the lobster is 

 also composed of a series of similar segments, also 

 contains a body-cavity, and is also traversed for its 

 whole length by the alimentary cavity— that is, by the 

 intestine. The same is to be said with respect to the 

 so-called "tail " of the scorpion, which, although so mucli 

 more slender than the so-called tail of the lobster, is no 

 more really a " tail" than is the latter. 



We see, then, that the word tail in its proper significa- 

 tion means the prolongation backwards of a backbone 

 (with the soft structures which surround it) beyond the 

 body-cavity and behind the posterior end of the alimen- 

 tary canal. This is the strictest meaning of the word 

 " tail." 



But other structures which, by their position, posterior 

 extension, slendemess, or some other analogical resem- 

 blance, more or less resemble what is most properly and 

 strictly called a tail, have the same term also applied to 

 them. 



While freely adopting popular usage in this matter, 

 and calling, without scruple, by this term whatever is 

 commonly and generally so termed, it is none the less 

 well to bear in mind the differences which have been here 

 pointed out as existing between the various more or less 

 different structures which are thus spoken of by one 

 common term. 



The survey we have made has also another result. 

 Different organs have not only their proper forms ani 

 structures, but also their proper uses. 



The uses to which we have seen that tails are applie 1 

 are more or less varied. Sometimes, indeed, the tail may 

 serve as a fifth hand, as in the spider-monkey; but tails 

 are generally related to locomotion, or at least to the 

 balancing of the body, and prehensile tails are important 

 aids to safe locomotion, especially in climbing. 



But tails are most generally and largely developed in 

 the class of fishes, and altogether the most effective aid 

 to locomotion which tails offer is the aid they give in 

 swimming. As, then, the essential structure of a tail is 

 a backward prolongation of the vertebral column withoiit 

 any body-carity, so the essential and fundamental use of 

 a tail seems to be to act as a swimming organ. As the 

 class of fishes seems to have been the first class of back- 

 boned animals to come into existence, so we may deem it 

 probable that a tail first appeared as a swimming organ 

 added to a body in front of it, somewhat, perhaps, as we 

 find the tail of our existing tadpoles. 



This matter, however, is but one of speculation. But the 

 world around us, as it exists now, affords us many examples 

 of beautiful adaptation and utility in the structures in the 

 examination of which we have been concerned to-day. 

 The perfection of the hand, the varied adjustments of 

 limbs, the wonderful complexity of the head, are matters 

 for which eveiy one would of course be fully prepared. 

 But our survey may perhaps have sufficed to show that 

 utility, beauty, and adaptation are exhibited to no small 

 extent by organs the structures and functions of which are 

 so rarely treated of and so slightly noticed as those with 

 which we have been occupied, namely, the organs called 

 "tails." 



