ADAPTATIONS 379 



which enable fishes to breathe air. In some cases air is 

 swallowed into the intestine, and the oxygen is absorbed by 

 the blood-vessels of the mucous membrane. This is the case 

 in one of the loaches, Misgurnus fossi/is, which lives in stag- 

 nant waters in eastern and southern Germany, and in northern 

 Asia. Intestinal respiration also occurs in species of South 

 American Cat-fishes (Siluridae and Loricariida;), e.g. CallicJithys, 

 Doras, Loricaria, and Plecostomus. 



In several other cases the adaptation for breathing air con- 

 sists in a special modification of the gill-chamber enclosed by 

 the operculum, an increase of surface being produced by com- 

 plicated foldings of the walls, usually supported by bony plates. 

 The organ so produced is known as a labyrinthiform organ. 

 These organs occur in tropical fishes. Under tropical conditions 

 both temperature and decomposition are unfavourable to the 

 supply of dissolved oxygen in water, and these fishes have the 

 power and the habit of living for considerable periods of time 

 out of water and carrying on aerial respiration. One of the best 

 known of such cases is that of the climbing perch, Anabas 

 scandens. In Anabas and other allied fishes the organ consists 

 of three or more concentrically arranged bony plates with frilled 

 margins, attached to a bony base which in turn is attached to 

 the upper end of the fourth gill-arch, and enclosed in a dorsal 

 enlargement of the gill-cavity. The vascular membrane which 

 covers the organ is supplied with venous blood by a branch of 

 the fourth afferent branchial artery, and the efferent vessel 

 from the organ joins the dorsal aorta. The possession of this 

 organ enables the fish to breathe air for a long time, and 

 although its climbing powers may have been exaggerated, 

 there is no doubt that it has been observed frequently among 

 vegetation on land. Its occurrence on the trunks of trees 

 (palms and such tropical forms), seems on the whole rare and 

 accidental. Daldorf stated in 1791 that he saw one of these 

 fish on the stem of a Palingra palm near a lake in India. It 

 was five feet from the ground when first observed, suspending 

 itself on the projections of the bark by its gill-covers which are 

 very spiny, and pushing itself upwards with its ventral spines. 

 This fish, as well as the serpent-heads, also buries itself in the 

 mud of ponds, or tanks, as they are called in India, when they 

 are dried up in the hot season. 



