PISCES. 



985 



which is interposed between the skull and the 

 surface of the brain. 



Organs of respiration. The respiration of 

 Fishes is purely aquatic, the oxigenization of 

 the blood being accomplished throughout the 

 entire class by its exposure to the oxygen 

 dissolved in the surrounding medium as it 

 passes through the network of extremely mi- 

 nute vessels that is spread out over the exten- 

 sive surfaces furnished by the gills or branchiae. 

 These organs consist of vascular fringes or 

 laminae placed on each side of the neck, over 

 winch, in the great majority of species, the 

 water taken in at the mouth is made to pass 

 as it issues through the opercular cavities ; 

 and in this way the branchial surfaces, being 

 perpetually bathed with aerated water, perform 

 the same office as the lungs of an air-breathing 

 animal. 



But while respiration is thus accomplished 

 throuehout the whole class by means that are 

 essentially similar, there are several modifica- 

 tions in the mechanical arrangement of the re- 

 spiratory apparatus, each of which will demand 

 our especial notice. 



Throughout all the extensive division of os- 

 seous Fishes (with the exception of the Lopfio- 

 brancliii) the construction of the breathing 

 organs will be found to accord with the fol- 

 lowing general description. To the external 

 convex surface of each of the four branchial 

 arches (Jig. 522) is attached a double series of 

 flat, elongated, cartilaginous laminae, tapering 

 gradually towards their extremities, the whole 

 forming a crescent-shaped pectinifonn frame- 

 work, over which is spread the highly vascular 

 membrane that constitutes the respiratory sur- 

 face. On making a transverse section of the 

 gill it is found that towards their base, whereby 

 they become attached to the branchial arch 

 (Jig- 520, />), the two series of branchial laminae 

 are united to each other, and, moreover, the 

 structure of each leaflet of the branchia becomes 

 apparent. The branchial artery (c), whereby 

 the blood is brought to the gills for the purpose 

 of respiration, is seen running along the con- 

 vexity of the supporting arch in the middle of 

 the base of the branchial lamina 3 opposite each 

 pair, of which it gives off two branches, which 

 pass outwards to the end of the substance 

 which unites the two layers of gills at their 

 bases, and then severally subdivide, one of the 

 ramuli extending along the internal margin of 

 each branchial lamina to its extremity, the other 

 retrograding to its base. From these two ra- 

 muli minute transverse vessels are given off, 

 which distribute the blood over the general 

 surface of the laminae, and ultimately form the 

 branchial veins, from which the systemic artery 

 is continued. Besides the respiratory laminae 

 the branchial arches support a series of un- 

 vascular processes, which project from their 

 concave margins, and serve to prevent sub- 

 stances taken into the mouth from escaping 

 through the branchial fissures and thus getting 

 among the gills; these processes in the Mullet 

 (Mugil chelo) are extremely beautiful, forming 

 long and delicate fringes along the concavity 



Fig. 520. 



of each branchial arch, adapted to bar the 

 passage of minute or finely 

 comminuted food through 

 the branchial interspaces. 

 These internal unvascular 

 appendages to the branchial 

 arches act. therefore, the 

 same part as the epiglottis of 

 mammiferous animals. Fre- 

 quently there are likewise 

 tubercular projections from 

 the contiguous margins of 

 the concave surfaces of the 

 branchial arches, for the pur- 

 pose of preventing the gills 

 from becoming too closely 

 approximated to each other, 

 and thus interfering with the 

 free circulation of the blood 

 over their surfaces. 



The mechanism of the 

 respiratory process is, there- 

 fore, in these osseous Fishes 

 exceedingly simple. The 

 water which is constantly 

 taken into the mouth passes 



,,.,, . through the branchial fis- 

 Dtattramofthecir- , . ., ... . . 



culation of the sures > and ls forcibly driven 

 Wood through the by the simultaneous action 

 of the branchial arches of the 

 os hyoides, of the palato- 

 temporal flaps, and of the 

 branchial arch ; a, O p ercu l a) through the inter- 



branchial 

 lets. 



b, 



leaf- 



section 



d, d, e, e, the arte- passes out through the wide 

 rial and venous fissure upon the side of the 

 trunks derived neck beneath the branchios- 

 from them. tegous membrane. 



In such genera as have this external opening 

 very large and patulous, as it is, for example, 

 in the Herrings and numerous other races, the 

 death of the fish ensues almost immediately 

 on its removal from its native element, not so 

 much on account of a deficiency of oxygen 

 wherewith to aerate the blood in the branchiae, 

 seeing that that might be derived from the 

 atmosphere, but because the gills, being no 

 longer floated out, collapse, and thus, by pre- 

 venting the passage of blood through the deli- 

 cate vessels which ramify over the branchial 

 laminae, put a stop to the circulation as com- 

 pletely as strangulation could do; but in some 

 genera a provision is made to permit of a more 

 lengthened existence out of the water where 

 the habits of the fish render such an arrange- 

 ment necessary. In the whole tribe of Eels, 

 for example, the external fissure is removed 

 very far back and reduced to a very small ver- 

 tical slit, converting the cavity wherein the 

 branchiae are lodged into an elongated cham- 

 ber, wherein a considerable quantity of water 

 can be retained : in such Fishes, therefore, the 

 circulation of the blood is by no means put a 

 stop to by taking the fish out of the fluid it 

 usually inhabits, but, on the contrary, many 

 species can exist for a considerable length of 

 time in the air, and even make their way to 

 a distance from their native ponds, the water 



