474 



FISH. 



and, as they do not move from a fulcrum or point of 

 rest more stable than the water, feet would be equally 

 superfluous. 



Accordingly, the motive powers of fishes are more 

 exclusively concentrated upon the spin,tl column than 

 those of any other vertebrated animal.*. Even the ophi- 

 dian reptiles form no exception to thit, though they are 

 in general destitute even of the rudiments of feet ; for, 

 as they start from a solid support in the successive por- 

 tions of their motion, they act against that support by 

 means of the scales on the under parts of their bodies 

 and their ribs, which may be considered as a sort of 

 substitute for feet ; and, though many of the ophidia 

 can swim well, even those which are habitually aquatic 

 have no proper swimming apparatus. (See the article 

 OPHIDIA.) 



Fishes have, generally speaking, rudiments of four 

 extremities, two anterior and two posterior, answer- 

 ing to the fore and hind feet of the mammalia ; but 

 these are often wholly concealed within the skin, and 

 sometimes they are altogether wanting, especially the 

 posterior ones. 



A fish which is considered of the most perfect or 

 typical form, has all these four extremities more or 

 less produced beyond the skin ; and their extremities 

 consist of membranous tissues more or less entire, and 

 supported by a framework of rays, which are in some 

 cases continuous spines, in others made up of a suc- 

 cession of articulations, and in many the two kinds of 

 rays are united in the same organ. These termina- 

 tions have a rude resemblance to the fingers and toes 

 of the human subject, or to the toes on the fore and 

 bind feet of the digitate or toed mammalia. Those 

 rayed and membranous appendages are called fins, 

 which is the general name for all the proper organs 

 of swimming or progressive motion through the water. 

 The fins, which answer to the fore feet of mammalia 

 and the wings of birds, are from their situation called 

 pectoral fins. They vary much in form and size in 

 different species, being short, or hardly, or not at all, 

 developed in some, but not so much produced in 

 others, as to answer the purpose of a sort of wings, 

 during a short flight, or rather leap, through the air. 

 They are, however, less frequently wanting than the 

 fins which answer to the hind feet. These last are 

 called ventral fins: they vary much in their situations, 

 being sometimes placed far back, at other times far 

 forwards, so as to be under, or even in advance of the 

 pectorals ; and in other cases again they are in a me- 

 dium situation. The position of ventral fins is a very 

 good character, not merely for the external distin- 

 guishing of fishes, but as an indication of the style of 

 their swimming, and this again leads to some know- 

 ledge of the nature of their haunts and habits. 



The pectoral and ventral fins of fishes are not, 

 properly speaking, organs of progressive motion in 

 swimming through the water ; but they are to keep 

 the body in such a position as that the proper swim- 

 ming organ can act with the greatest advantage, and 

 they are also useful in the keeping or the altering of 

 the course through the water. 



The really efficient fin in progressive motion is the 

 caudal fin, or that attached to the extremity of the 

 vertebral column, or tail, This fin is always spread 

 out in a kind of fan-shape, but its termination is very 

 various!}' formed in different species ; sometimes it is 

 squared over, sometimes rounded, sometimes wedged- 

 shaped, and sometimes forked ; but it is always 

 placed on the same plane with the elongated pro- 



cesses of the spinal column, or in a position exactly 

 crosswise to that of the fan-shaped tail in birds. This 

 shows the difference of use in the two organs. The 

 tail of the bird is chiefly useful in ascending and 

 descending, contributing nothing to the progressive 

 motion, and very little even to the direction of that 

 motion. 



Hrsides the pectoral and ventral fins answering to 

 the four extremities of mammalia and birds, and the 

 caudal fin, which though it occupies the position of 

 the tail in the other classes of vertobrated animals, 

 yet performs a far more important function than that 

 organ in any of them, the fishes have, in very many 

 species, other fins attached to the extremities of the me- 

 sial plane, or that plane which passes through the cen- 

 tre of the spinal column and its processes. These fins 

 are attached to additional bones, which are generally 

 articulated to the processes. Those which are on 

 the back of the fish are called dorsal, or back fins, 

 and those which are on the under part, and which 

 are always in the rear of the cavity of the body, and 

 of course of the vent, are called anal fins. The 

 dorsal fin is sometimes single, sometimes double ; and 

 there are many species in which the dorsal and anal 

 fins are joined to the caudal one, forming a sort of 

 finny margin round a considerable part of the mesial 

 plane of the body. In fact, to describe all the varie- 

 ties of those fins would be to enter very largely into 

 the detailed description of the species. 



What may be considered as the natural position 

 of a fish, that is, the position in which the greater 

 number of them are placed, both when they are at 

 rest and when they are in motion, is yvith the mesial 

 plane vertical, or in the direction of the perpendicular. 

 The dorsal, the anal, and the caudal fins, have always 

 the direction of this plane ; and therefore in a fish 

 which has what we have called the natural position, 

 those fins have their planes or flat sides laterally, and 

 their thin edges in the direction of gravity. From 

 this it is easy to see that neither of those fins can in 

 this direction produce, or assist in producing, either 

 an ascent or a descent of the fish, because their action 

 is across the direction of gravity, and therefore it can 

 neither directly assist nor directly oppose that force. 

 They strike laterally, and as we may consider that 

 when the fish gives its stroke, there is a certain por- 

 tion of their body forwards, which keeps the direction 

 in which the fish moves, without any deviation either 

 to the right or the left, we may consider some point, 

 either the centre of gravity of the fish, or some point 

 very near it, through which if a perpendicular, or 

 line in the mesial plane, be supposed to be drawn 

 at right angles to the axis of the body, then this 

 line will be the centre upon which the propelling 

 portion of the body may be supposed to turn alter- 

 nately to the right and left, just as a paddle does 

 when it. is worked right and left across the middle of 

 a stern of a boat. It will also be apparent that the 

 greater the mass and weight of the fish which is 

 before this centre of motion, the progress of the whole 

 will be the more steady. 



But though there is this analogy between the 

 action of the posterior part of a fish and a stem 

 paddle in a boat, we must not suppose that there is 

 any comparison of the two in point of efficiency. 

 The boat has no general principle of union and 

 consent with the paddle ; and it very often happens 

 that the one is the worse possibly adapted to the 

 other that we can well imagine. The fish on the 



