234 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the gill is attached at its base only (Fig. 98), and the 

 gill lamellae are free. With the disappearance of the 

 septa we have, of course, the loss of the separate gill 

 slits, and the whole of the gills of one side come to 

 lie in a common chamber, which is covered over by 

 the operculum, and has only one opening to the 

 exterior. 



The water which brings the necessary oxygen to 

 the gills enters by the mouth ; as the mouth opens 



the operculum 

 rises, and the 

 gills separate 

 from one an- 

 other, but the 

 membrane which 

 fringes the oper- 

 culum acts as a 

 valve to prevent 

 the entrance of 

 water through 

 the opercular 

 cleft (Bert). 

 When the mouth 

 closes, and the 

 pharynx con- 

 tracts, the water is forced through the pharyngeal 

 clefts into the gill clefts owing to the presence of a 

 valvular arrangement which shuts off the passage into 

 the mouth. 



In the already mentioned Amphipnous and in 

 Saccobranchus, the true gills are rudimentary, and a 

 sac with contractile walls is developed, which takes 

 in water and expels it at intervals ; the walls of these 

 sacs are richly supplied with blood-vessels, which are 

 arranged as in a gill ; that is to say, the blood that 

 passes from them goes direct to the aorta ; in the 

 climbing perch (Anabas), the internal surface of the 



Fig. 99. Suprabrancbial organ of Andbas 



scandens. 

 a, Supraforanchial organ. 



