ADAPTATIONS 381 



which has a special air-breathing organ. (Plate XXXII., B.) 

 In this case the organ is a long sac extending along the side of 

 the body from the first gill-cleft to the tail. The sac is 

 situated in a deep internal position close to the back-bone. 

 The right sac is supplied by a branch of the first afferent 

 branchial artery, the left by a branch of the fourth, and the 

 wall of the sac is highly vascular. These sacs therefore re- 

 semble lungs and have a similar function, but open into the 

 gill-cavity not into the gullet. The fish lives for days out of 

 water. 



Amphipnous is a fish allied to the eels and of similar 

 elongated shape, without either pectoral or pelvic fins, and even 

 the vertical fins are rudimentary. The air-breathing organs 

 consist of a pair of bladder-like sacs opening from the branchial 

 cavity above the first gill-cleft. The true gills are in this fish 

 much reduced, the first arch having no gill- filaments, all its 

 blood going to the air-sacs. There are only three branchial 

 arches altogether, and the second alone bears gill-filaments. 

 The fish, known familiarly as the cuchia, is, like most of these 

 already mentioned, an inhabitant of the fresh and brackish waters 

 of India and Burmah. It grows to two feet in length. It rises 

 to the surface of the water to inspire air, and spends much 

 of its time out of water in the grass on the banks of ponds, like 

 a snake. 



It was formerly a very general idea among zoologists that 

 the lungs of higher vertebrates were evolved from the air- 

 bladder of fishes. There is very good reason for regarding 

 the two structures from the point of view of comparative 

 anatomy as homologous, which means that they correspond to 

 each other when we compare the general plan of structure of 

 a fish with that of an animal breathing air by means of lungs. 

 The explanation given by the doctrine of evolution of homology 

 in such a case is that one of the two structures compared has been 

 evolved from the other, or that both have been evolved from 

 the same structure in some common ancestral form which lived 

 at a former period. The progress of knowledge, however, has 

 led to the conclusion that the air-bladder was evolved from 

 lungs rather than vice versa. This may at first seem a paradox, 

 but it does not mean that fishes are descended from terres- 

 trial animals, and when we examine the evidence we shall see 



