CHAPTER EIGHTH. 



OF RESPIRATION. 



376. FOR the maintenance of its vital properties, the 

 blood must be submitted to the influence of the air. This is 

 true of all animals, whether they live in the atmosphere or in 

 the water. No animal can survive for any considerable period 

 of time without air ; and the higher animals almost instantly 

 die when deprived of it. It is the office of KESPIRATION to 

 bring the blood into communication with the air. 



[377. In the lowest classes of animals no special organ is 

 developed for the exposure of the nutritive fluid to the oxygen- 

 ating influence of the air contained in the water in which they 

 live. In them, the general cutaneous surface is a respiratory 

 organ such is the case in infusoria, polyps, medusae, and 

 many other invertebrata. Many parts of the cutaneous mem- 

 brane on the exterior of their bodies, or that lining the diges- 

 tive organs, are covered with vibratile cilia, by the motions of 

 which, currents of water are made to flow over these surfaces, 

 and thfveby oxygenating the nutritive fluids circulating in 

 them. 



[378- In the echinodermata special organs exist ; the up- 

 per surface of the tegumentary membrane of the Asterias is 

 covered with innumerable small transparent fleshy tubes, which 

 in the living state are seen advancing and receding through 

 openings in the integument. The interior of these tubes is 

 lined with cilia, and by their vibrations currents of water are 

 made to flow through them into the visceral cavity, into which 

 they open. The peritoneal membrane lining this cavity pre- 

 sents a considerable extent of surface continually in contact 

 with the surrounding medium, and appears to be the principal 

 seat of respiration. Its surface is covered with cilia, by which 

 currents of water *e made to flow in a determinate direction, 



