PISCES. 



987 



In the Sharks the orifices of the pharynx 

 leading into the branchial chambers are guarded 

 by cartilaginous pyramidal processes ; but in 

 the Skates, which have the cartilaginous arches 

 much less perfectly formed, no such defences 

 are visible. 



The Cyclostomatous cartilaginous Fishes, 

 from the peculiarities of their habits, require 

 another modification in the construction of 

 the organs of respiration, seeing that, whilst 

 they rest fixed by their suctorial mouths to 

 the surfaces of stones or other foreign bodies, 

 or while they are compelled to remain with 

 their heads deeply plunged into the flesh of 

 the prey upon which they live, the admission 

 of water into the mouth and its subsequent ex- 

 pulsion through pharyngeal fissures would, in 

 their case, be impracticable. 



In the Myxine or Hag-fish the branchial 

 apparatus presents externally seven round holes 

 placed in a line along each side of the neck, 

 and situated very far back in comparison with 

 the usual situation of the gill-openings. Each 

 of the seven lateral openings leads internally 

 into a flattened circular cavity, which likewise 

 communicates by a special canal with the inte- 

 rior of the pharynx. When opened, each gill- 

 sac is found to have its lining vascular mem- 

 brane gathered into closely set folds, which in 

 turn exhibit, when accurately examined, smaller 

 plicae, and thus a great superficial extent of 

 respiratory membrane is secured, much in the 

 same manner as in the case of the Raidte ; in- 

 deed, the general disposition of the whole ap- 

 paratus is essentially similar in the two groups, 

 the great difference being that whereas in the 

 plagiostome genera the water passes in at the 

 mouth and out at the gills, in the Cyclosto- 

 mata it alternately passes in and out through 

 the same orifices by a mechanism which is best 

 exhibited in the Lampreys ( Petromyzon). 



In the last mentioned genus of Cyclosto- 

 matous Fishes, the seven holes which lead to 

 the breathing organs are seen, on removing the 

 skin, to be supported by a curiously contorted 

 framework of cartilaginous pieces, which we 

 have already noticed as being the exaggerated 

 representatives of the sternal ribs met with in 

 the Sharks, and which we shall subsequently 

 see in a much more highly developed condi- 



tive to the extent of the respiratory surface pre- 

 sented by the gills of Fishes to the action of the 

 surrounding medium. On each side of the body 

 of a Skate, says he, there are four double gills, or 

 gills with two sides each, and one single gill ; or 

 there are in all eighteen sides or surfaces on which 

 the branchial artery is spread out. On each of 

 these sides there are about fifty divisions or dou- 

 blings of the membrane of the gills. Each division 

 hasoneachside.of it 160 subdivisions, doublings, or 

 folds of its membrane, the length of each of which 

 in a very large skate is about one-eighth of an inch, 

 and its breadth about one-sixteenth of an inch, 

 so that in the whole gills there are 144,000 subdi- 

 visions or folds, the two sides of each of which are 

 equal to the 64th part of a square inch, or the sur- 

 face of the whole gills in a large skate is equal to 

 2.250 square inches, that is, to 15 square feet, an 

 expanse equal to the whole external surface of the 

 human body. Structure and Physiology of Fishes, 

 p. 15. 



tion in Branchiostoma. These form a kind of 

 elastic thorax around the region of the body 

 where the branchiae are situated, which is made 

 to perform alternate movements of contraction 

 and dilatation, whereby the water is perpetually 

 sucked in and again expelled through the exter- 

 nal openings (fig.51\, i, i). The branchiae them- 

 selves (/;,/*) present nearly the same structure as 

 in the Myxine, consisting of as many distinct 



Fig. 521. 



Respiratory apparatus of the Lamprey ( Petromyxon). 

 a, mouth with its teeth ; b, pharynx ; c, opening 

 of tube from the back of the head ; d, cavity com- 

 municating with the respiratory sacculi of both 

 sides ; e, commencement of oesophagus ; f, carti- 

 laginous process connected with hyoid tooth, like- 

 wise marked d ; g, muscles of sucking disc ; h, h, 

 respiratory cavities and branchiae contained therein ; 

 i, i, external openings of do. ; m, cartilaginous sac 

 investing the pericardium. 



sacculi as there are external openings, ranged 

 along the sides of the neck, each sacculus having 

 its vascular lining membrane finely plicated in 

 order to increase the extent of surface to be 

 exposed to the respired element. Every bran- 

 chial sacculus, in addition to its communica- 

 tion with the exterior through the lateral open- 

 ings of the neck, has a passage that leads into 

 the pharynx (d), so that by the intervention of 

 the pharyngeal cavity the respiratory sacculi of 

 the opposite sides of the body are made to 

 communicate with each other, an arrangement 

 which explains the circumstance, that when 

 the breathing holes of one side of the Lamprey 

 are kept above water, the respired fluid which 

 enters the submerged orifices, after traversing 

 the pharynx, fills the branchial chambers of the 

 opposite side, and is forcibly ejected therefrom 



