BIOLOGY. 313 



VARIATIONS OF THE FINS OF FISHES. 



Form. — A sharp-pointed fin, especially if sickle-shaped with 

 posterior edge concave, indicates a very rapid swimmer; as seen 

 in the pectorals and ventrals of many scomberoids and in all the 

 lins of some selachians. This corresponds to what occurs in birds 

 of rapid flight, as in the sharp-pointed wings of the falcon, swal- 

 low, frigate-bird, etc. A deeply-forked tail denotes a good swim- 

 mer, especially if the caudal is supported by a pedicle, as in the 

 mackerel family. In like manner birds of high flight have a very 

 forked tail. On the contrary, the fins of fishes of moderate swift- 

 ness, other things being equal, have rounded outlines ; so the ob- 

 tuse wings and tail characterize the birds of moderate flight. 



Size and Number. — Every one is familiar with the fact that 

 large fins are especially observed in rapid swimmers ; for exam- 

 25]e, the tail of the mackerel, the pectorals in the tunny and flying- 

 fish, etc. The same is true of the number of fins ; as in the se- 

 lachians and cod family, whose dorsals and auals are numerous. 

 In the latter the number of these appendages compensates for the 

 rounded and obtuse shape of the fins. It seems to be a general 

 law that the vertical extent, or the length of the rays, of the dor- 

 sal and anal fins is in proportion to the distance of the line of 

 insertion above or below the antero-posterior median line ; for 

 instance, in the squammipennes, many scomberoids, and sparoids 

 with arched back. That which is true of the entire fin is often ap- 

 plicable to a relatively small portion of it, as in pomotis, myripris- 

 tiSf etc., where portions of the dorsal and anal tins are developed 

 in height in relation to the fleshy prominence on which they are 

 implanted. Sometimes, however, the dorsal is very high without 

 a coiTesponding elevation of the region of implantation above the 

 median axis, as when the great extent of the dorsal is designed to 

 make up for the absence of the ventrals (Xiphias), or their small 

 development or reduction to simple rays (Istiophoi^us, Tetrap- 

 turusy FferacUs, etc). In the last the anal is elevated as well as 

 the dorsal. These are examples of organic balancing, showing 

 that rapidity of swimming is a product of many factors, whose 

 principal variations are in their form, size, and number. A very 

 great antero-posterior extent, in number of rays, of the dorsal and 

 anal fins, becoming even continuous with the tail, is seen in many 

 flat fishes (flounder family) and in very elongated ones (eel fam- 

 ily, etc.). 



Situation. — Ichthyologists have made use of this character in 

 their classification of fishes, as Linna3us, Lacepede, Cuvier, Du- 

 meril, etc. The ventrals are either jugular or thoracic, in fishes 

 with large heads {Trigla, Coitus, etc.) ; in those wiiich, without 

 having the head very large, have the anterior part of the body 

 thick (percoids, scioenoids, etc.) ; and in almost all fishes which 

 have the centre of gravity within the anterior two-fifths of the 

 body. On the contrary, abdominal fishes have neither a large 

 head nor thick shoulders, but a fusiform or even elongated body 

 (cj'prinoids, herrings, etc.). Hence we may establish the law, 

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