76 REST III A TORY SYSTEM OF COLD-BLOODED ANIMALS. 



is carried on by the whole surface of the body which is bathed by tlie water. In 

 animals still more highly developed, Ave find canals carrying water into all parts 

 of the system. In many individuals, bloodvessels accompany these canals, and 

 ramify around their walls. An incessant motion through this aquiferous respira- 

 tory system is maintained by cilia lining their interior. These canals open upon 

 the exterior of the body and into the visceral cavity. In many animals of this 

 class, the digestive cavity, which is bathed continually by fresh portions of water, 

 performs the function of respiration. 



In the higher orders of the Invertebrata, the respiratory system is confined to a 

 definite portion of the exterior or internal membrane, which is developed within a 

 small space into a great extent of surface, so as to render the contact with the air 

 or water as extensive as possible without any loss of room or power. 



According as the fluids are elaborated, and the solids correspondingly developed, 

 the respiratory system becomes more condensed and perfected. 



In the Ampldoxus, or the pulmonary apparatus corresponds with the degraded 

 type of the cerebro-spinal system and all the organs, and, like that of many inver- 

 tebrate animals, is lodged in the same cavity with the liver, generative apparatus, 

 kidney's, and the greater portion of the alimentary canal. In the Invertebrate 

 animals and the Amphloxus amongst the Vertebrate, the circulation of the water 

 through the branchia3 is maintained principally by ciliary action. 



In Fishes, however, of higher organization, whose blood is more highly elaborated 

 and circulates Avith greater rapidity, mere filamentous tufts hanging to the side of 

 the neck will not suffice for the aeration of the blood. It is necessary that large 

 streams of Avater be constantly and forcibly propelled through the branchial 

 apparatus, in order that the blood may be exposed as much as possible to the 

 action of the air so scantily contained in the Avater. This is accomplished by the 

 connection of the gills Avith the cavity of the mouth, the muscles of which send 

 rapid currents of water through the branchial passages. The structure and posi- 

 tion of the heart, also, is such that it propels all the venous blood through the 

 branchiae before its distribution to the body generally. 



At first sight, the circulation and respiration of Fishes appear to be more perfect 

 than that of Reptiles. This, lioAvever, is not the case. By a reference to the 

 table of the comparative Aveights of the heart in different animals, it will be seen 

 that the heart of Fishes is about y oVo^^^j whilst that of Reptiles is about 4I oth of 

 the Aveight of the entire body. 



The heart of Reptiles is relatively more than tAvice as large as that of Fishes, 



The table of the comparative rapidity of the heart's action in diflferent animals, 

 shoAved that the circulation of Fishes is much slower than that of Reptiles. 



The aeration of the blood, also, is much sloAver and less perfect in Fishes, from 

 the fact that the amount of air contained in the Avater is infinitely less than that 

 of the atmosphere. 



In several remarkable Fishes having strongly marked reptilian characters, as the 

 Garfish [Lepisosteus osseiis) and the common Mudfish [Amia calva) of our southern 

 swamps and ricefields, we find both gills and a pulmonary organ. The lung of these 

 Fishes has been considered by many physiologists and anatomists as analogous to 



