34 RESPIRATORY MECHANISMS 



by now attained a length of 40 mm, rise to the surface to 

 breathe. At about this time also the pelvic gills of the old 

 male begin to atrophy (Agar). 



Fig. 10. Functional gills on pectoral fin of male Lepidosiren. (Agar.) 



Gills in a cavity. Gills protruding freely from the surface 

 are evidently liable to damage and may be in the way during 

 locomotion, and in highly organized forms we find the gills 

 protected in a special cavity ventilated by a current of water. 

 Owing to the low content of oxygen in water such ventilation 

 must involve a rather large volume which must be brought in 

 very close contact with the respiring surface, because the 

 diffusion rate in water is so slow. These mechanical problems 

 are solved in different ways by different animals, and in some 

 forms the efficiency of the mechanisms is very high indeed. 



The primitive mode of ventilation adopted by Ascidia and 

 most molluscs is by the beating of cilia. 



In the bivalve molluscs and in the Ascidia we find gills with 

 a very large surface, bearing cilia, and a flow of water gener- 

 ally much beyond the respiratory requirements of the animal. 

 This flow is kept up for feeding purposes, and the respiratory 

 function is incidental. 



