INTRODUCTION. 71 



of the animal, and protected by a bony case 

 or covering, often defended by strong spines, 

 which in the horrid array which covers some 

 species, are almost always placed on these 

 parts. This sort of respiration has been termed 

 aquatic, and, among the vertebrata, is found 

 in the larva or young state of some reptiles 

 which spend the early portion of their existence 

 in the waters, and in all the fishes. The gills are 

 placed in immediate communication with the 

 heart, and are composed of an innumerable series 

 of delicate bloodvessels arranged in a fringe-like 

 form upon the lower edges of four bony arches, 

 which form the frame work of this structure. 

 Water entering at the mouth, is forced out again 

 at the posterior opening of the covers, and thus 

 maintains almost a constant stream or rush 

 through them, entering and again expelled, at 

 intervals, similar to the respiration and expiration 

 of animals. When withdrawn from the water, the 

 delicate filamentous structure of the gills imme- 

 diately collapses, and no muscular exertion, or 

 convulsive action, can restore them to their former 

 play ; when exposed to the action of air only, a 

 kind of suffocation ensues, and death is the con- 

 sequence. This is the general principle of 

 respiration in this class of beings, but the struc- 

 ture and its application is often modified. The 

 admission and exit of the water is sometimes 



