9 2 



The Organs of Respiration 



and Crossopterygians external gills are developed, but in the 

 more specialized forms these do not appear outside the gill- 

 cavity. In some of the sharks, and especially the rays, a spiracle 

 or open foramen remains behind the eye. Through this spiracle, 

 leading from the outside into the cavity of the mouth, water is 

 drawn downwards to pass outward over the gills. The presence 

 of this breathing-hole permits these animals to lie on the bottom 

 without danger of inhaling sand. 



The Gill-structures. The three main types of gills among 

 fishes are the following: (a) the purse-shaped gills found in the 

 hagfishes and lampreys, known as a class as Marsipobranchs, 

 or purse-gills. These have a number (5 to 12) of sac-like depres- 

 sions on the side of the body, lined with gill-fringes and capil- 

 laries, the whole supported by an elaborate branchial basket 



FIG. 76. Gill-basket of Lamprey. (After Dean.) 



formed of cartilage. (b) The plate-gills, found among the 

 sharks, rays, and chimseras, thence called Elasmobranchs, or 

 plate-gills. In these the gill-structures are flat laminae, attached 

 by one side to the gill-arches. (c) The fringe-gills found in 

 ordinary fishes, in which the gill-filaments containing the capil- 

 laries are attached in two rows to the outer edge of each gill- 

 arch. The so-called tuft-gills (Lophobranchs) of the sea-horse 

 and pipefish are like these in structure, but the filaments are 

 long, while the arches are very short. In most of the higher 

 fishes a small accessory gill (pseudobranchia) is developed in 

 the skin of the inner side of the opercle. 



The Air-bladder. The air-bladder, or swim-bladder, must 

 be classed among the organs of respiration, although in the 

 higher fishes its functions in this regard are rudimentary, and 

 in some cases it has taken collateral functions (as a hydrostatic 



