FISH, 



481 



sification of fishes, upon the form of this organ, but 

 the classification has not been followed by more 

 scientific and discerning ichthyologists ; and indeed 

 the breathing apparatus, especially in its more minute 

 variations, is but ill calculated for being the founda- 

 tion of any arrangement. In the distinguishing of 

 species it may, like the number of rays in the fins, 

 and of vertebrae in the spine, be of some use, because 

 we believe it is pretty constant in the same species ; but 

 it is so obviously a character which leads to nothing, 

 that it cannot be made the basis of anything more 

 general in an arrangement which professes to assist 

 in the study of fishes. All fishes breathe atmospheric 

 air, and they effect this through the medium of 

 water, the quantity of which is in all cases very 

 much greater than that of the air, and it is every 

 where very much the same. So that the character 

 of the breathing apparatus is to be inferred from the 

 habit of the fish, rather than the habit from the pecu- 

 liar structure of the apparatus ; and even when taken 

 in this way there is not much in the inference. Thus, 

 for instance, the eel and other apodal fishes, which 

 move about at or near the bottom of the waters, 

 generally have the gill-openings very small, and are 

 without a gill-flap, a gill-lid, or both ; while cod and 

 many other bank fishes, which feed at the bottom, 

 have the gill-openings unusually large, and the lids 

 furnished with flaps. The number of rays in the gill 

 flap, which are technically called "branchiopterygeous 

 rays," are, however, admitted into the list of charac- 

 ters in the subordinate divisions. 



In fishes which have free gills, inserted upon arches 

 of bone attached to the os hyoides, there are usually 

 eight, lour on each side, answering symmetrically to 

 each other, both in form and in magnitude ; but in 

 some there is an additional small gill on each side, 

 though it does not appear that any inference can be 

 drawn from this as to the general habits of the fish, 

 for it occurs in the herring, which is a surface fish, 

 and also in the sole, which is a bank or ground one. 

 The upper ends of the gill arches are attached 

 through the os hyoides to the general mass of bones 

 in the head, the under ends arc attached to the bones 

 of the tongue. The motion of that organ of course 

 moves the arches, and they are besides flexible through 

 their whole length. The anterior edges of the arches 

 toward the mouth are covered with the same kind of 

 integument as the rest of that cavity ; they are 

 generally more or less tuberculated ; and in many 

 species they are beset with teeth, and assist in the 

 operation of conveying the food to the stomach. 



In the CJiondropterygii, or cartilaginous fishes, the 

 gills are much less perfect. They are fixed to par- 

 titions, which answer nearly the same purpose as 

 the arches of bone in the other fishes ; but the 

 fibres are very similar to those in the others, which 

 shows that though the apparatus is different in form 

 and arrangement, the operation to be performed is 

 substantially the same. Some general notice of the 

 structure of these fishes will be found upon looking 

 back to the article CHONDROPTERYGII ; but as the 

 subject is rather a curious one, we shall add the de- 

 tails of some of the species as given by sir Everard 

 Home. " In the lamprey," says sir Everard, " the 

 organs of respiration have seven external openings 

 on each side of the animal ; these lead into the same 

 number of separate oval bags, placed horizontally, 

 the inner membrane of which is constructed like that 

 of the gills in fishes. There is an equal number of 



NAT. HIST. VOL. II. 



internal openings leading into a tube, the lower end 

 of which is closed, and the upper terminates by a 

 fringed edge in the oesophagus. These bags are con- 

 tained in separate cavities, and enclosed in a thorax 

 resembling that of land animals, only composed of 

 cartilages instead of ribs, and the pericardium, which 

 is also cartilaginous, is fitted to its lower extremity 

 like a diaphragm." In the hag, or myxine, there are 

 only two external openings, one on each side ; but 

 there are on each side six lateral bags, to each of 

 which there passes a tube from the externa. opening : 

 and close to the left external opening there is also a 

 tube which passes to the gullet, and forms a commu- 

 nication with the mouth. In the common chondro- 

 pterygii which have their mouths of similar construc- 

 tion to those of the bony fishes, and which seize and 

 swallow their prey, it may be presumed that the 

 ordinary mode of breathing resembles that of other 

 fishes, that the water is partially at least received by 

 the mouth, and discharged by the gill-openings ; and 

 the communication which there is between the gills 

 and the gullet in the lamprey' and the myxine shows 

 that they also, occasionally at least, receive water in 

 the same manner. But the lamprey occasionally, and 

 the myxine habitually, in feeding, adheres by the 

 mouth as a sucker ; and when this takes place of 

 course neither air nor water can enter by that orifice ; 

 and thus the whole operation must be performed by 

 means of the bags or cells, to the walls of which the 

 fibres of the gills are attached. Hence there must be 

 in this apparatus, a means of receiving and discharg- 

 ing the water, independently of all action of the 

 mouth ; and it is worthy of remark that in this respect 

 those fishes bear no inconsiderable resemblance to 

 insects which breathe wholly through apertures in the 

 sides, which have no communication whatever with 

 the mouth. The method in which this operation is 

 performed, in the lamprey and myxine, is thus de- 

 scribed by sir Everard Home : " In the lamprey, the 

 water is received by the lateral openings of the animal 

 into the bags which perform the office of gills, and 

 passes out by the same opening ; the form of the cavi- 

 ties being fitted to allow the water to go in at one 

 side, pass round the projecting parts, and out at the 

 other. A part of the water escapes into the middle 

 tube, and from thence either passes into the other 

 bags, or out at the upper end into the oesophagus* 

 There is a common opinion that the water is thrown 

 out of the nostril : this, however, is unfounded, as the 

 nostril has no communication with the mouth. In 

 the myxine, the elasticity of the two tubes and the 

 bags into which they open, admits of the water being 

 received ; and the pressure produced by the action 

 of the external muscles forces it into the oesophagus, 

 from whence it is thrown out by the opening at the 

 lower end of that tube. 



This operation appears to be performed by means 

 of the cartilaginous ridges which are attached to the 

 skin between the gill-openings. When the water is 

 to be received, these arches are extended outwards, 

 like a bow, by their extremities being moved by par- 

 ticular muscles ; and thus cavities' are formed, and as 

 instantly filled by the pressure of the water. When 

 this water has remained for the requisite time the 

 ridges are unbent and drawn inwards, and the water 

 is expelled ; and this operation being repeated, con- 

 stitutes the process of breathing in those peculiar 

 fishes, altogether independently of the mouth. 



In the ordinary breathing of fishes by the mouth, it 

 H H 



