FISH. 



47-1 



very brief notice of the different parts of its organisa- 

 tion ; thirdly, a short notice of the distribution and 

 habits of fish ; and fourthly, an outline of Cuvier's 

 arrangement. 



GENERAL DEFINITION. Fishes are vertebratcd 

 animals, having- a double circulation, and breathing air, 

 but only through the medium of water; their circula- 

 tion is slow, and their blood, though red, is cold. 

 The heart of a fish consists only of one auricle and 

 one ventricle, answering to the pulmonary or rigiit 

 side ones of the double heart in the mammalia and 

 birds ; and the function of this heart consists in 

 receiving the blood from the body, and sending it to 

 the breathing apparatus, just in the same manner as 

 the right side of the double heart in the mammalia 

 does. 



The breathing apparatus is very different from that 

 of animals \vhich breathe the free air. These receive 

 the air into the cellular tissue of the lungs, or into 

 the separate cells arranged along the sides of the ab- 

 domen (as in insects), and the vessels in which the 

 blood is subjected to the action of the air are ramified 

 over the walls of these cells. The breathing apparatus 

 of fishes are called gills ; and in their most perfect 

 state they consist of fringe-like appendages attached 

 to the posterior sides of certain arches of bone, which 

 are articulated to the os hyoides, situated on each side 

 of the neck, and have their ingress always by the 

 mouth of the fish, and their egress always in the neck, 

 and capable of beinsr opened and shut by means of an 

 apparatus which, taken altogether, is called the oper- 

 cu/inii, or gill-lid. The fibres or leafits of the gills 

 have their surfaces closely ramified over with a very 

 fine tissue of blood vessels, which consist in the one 

 part of arteries, and in the other of veins, as in the 

 lungs of animals which breathe free air, or not through 

 the medium of water. A current of water runs con- 

 stantly through between the arches, and bathes the 

 fringes on all sides when the fish breathes. This cur- 

 rent is alsvays from the mouth to the gill-openings; it 

 is promoted by the joint action of the two ; and if any 

 attempt is made to force a passage for the water in the 

 opposite way, or from the gill-openings to the mouth, 

 as if by holding the fish head downward with the gill- 

 lids open, then the fish is drowned, as certainly and 

 as speedily by water, as a land animal would be by 

 being kept th"e same length of time below the surface 

 of that fluid. As the current of water passes through 

 the gills from the mouth to the gill-openings, the 

 blood, which is contained in the small vessels, which 

 are ramified over the fringes of the gills, receives the 

 benefit of the air which the water contains, to the 

 same effect, if not in the same manner, as the blood 

 in the lungs of an animal which breathes the dry air 

 upon land. 



When the blood has thus passed through the gills, 

 and undergone that renovating process, it is not con- 

 veyed back to the heart as in land animals with warm 

 blood, because there is no arrangement on the left side 

 of the heart to receive it. The blood which returns from 

 the different fibres of the gills, is conveyed by veins, 

 which gradually unite with each other into an arterial 

 trunk, placed along the spine, in what may be con- 

 sidered as the safest part of the whole cavity of the 

 fish ; and this cavity answers the purpose of a left 

 auricle in receiving the blood from the breathing ap- 

 paratus, and also that of a left ventricle in distributing- 

 it all over the body, in so far as the peculiar structure 

 of that body renders the distribution, of blood neces- 



sary. The quantity of blood contained in a fish is, 

 however, much less in proportion to the size of the 

 fish than that contained in the mammalia or the birds ; 

 and so much blood is not necessary, because the 

 greater part of the muscles being articulated on the 

 spine, and inserted in the skin of the fish, appear to 

 require the application of blood at the one extremity 

 only. Thus, if a flesh wound is made in the body of 

 a fish, which does not reach so deep as to divide the 

 muscles down to the spinal column, such a wound 

 does not bleed ; and, in so far as we can judge, the 

 fish feels very trifling pain from it, in comparison to 

 that which a similar wound causes to an animal with 

 a double heart, and warm blood distributed through- 

 out all parts of the body. This exemption from pain 

 in the case of laceration of the flesh is a very beautiful 

 contrivance in the economy of fishes. Most of the 

 race are very voracious, and very indiscriminate feed- 

 ers ; and not a lew bite and tear in cases where they 

 are not capable of swallowing the object of their at- 

 tack. As the fish, when below the surface, is liable 

 to attacks of this kind in all directions, the defence of 

 insensibility to wounds which do not reach a vital 

 part, is of great service to it ; and if we attentively 

 examine the structure of this class of animals, and the 

 texture and character of their different parts, we shall 

 find that their adaptation to their element, and to the 

 circumstances of that element, is equally perfect with 

 that of any other living creatures. 



In one respect, fishes bear a nearer resemblance to 

 birds than they do to any other of the vertebratecl 

 classes, inasmuch as their progressive motion is 

 through one element, without any thing more solid, or 

 of greater specific gravity, which they can use as a 

 fulcrum or starting point to their motions, as is done 

 by walking animals. The chief difference in this 

 respect is, that fishes have to make their way through 

 a medium which is very nearly of the same specific 

 gravity as their own bodies ; and thus there is nothing, 

 independently of their own motion, to make them 

 either ascend or descend. Birds on the other hand' 

 make their way through a medium of much less spe- 

 cific gravity than even the lightest of them ; and thus 

 in their motions they have a double operation to per- 

 form, they have to keep themselves up against the 

 pressure of their own gravitation, and also to make their 

 way progressively ; the fish is absolved from the per- 

 formance of the first of these operations ; and thus it 

 can concentrate the whole of its energy upon its pro- 

 gressive motion ; and thus, though it has to make its 

 way through a much denser medium, it probably does 

 not require nearly so much exertion as the bird. Any 

 one who examines the general forms of fishes will 

 observe that, notwithstanding their almost endless dif- 

 ferences, they are all adapted for getting easily through 

 the water ; and, while the feathers of birds are in 

 many instances so constructed as to take an absolute' 

 hold on tho air, almost all, if not all, fishes have the 

 surface of the skin supplied with a mucous secretion,, 

 which prevents them from even being wetted so long 

 as they are alive, so that a fish gets through the water 

 with much less resistance of friction than a dead sub- 

 stance of the same form and weight ; and it is worthy- 

 of remark, how much mote difficult it is to pull the 

 body of a fish through the water after this mucous 

 substance has dried, than when the fish is instantly 

 dead, and this substance is fresh and abundant. As 

 fishes require no support against the pressure of tlu-ir 

 own weight, they stand in no need of wings like birds ; 



