FRESH-WATER FISHES OF SIAM, OR THAILAND 31 



the Clariidae the accessory organ, occupying a large chamber above 

 each gill cavity, is an outgrowth of the fourth gill arches and consists 

 of an arborescent structure that provides a large surface for the 

 absorption of the atmospheric air reaching it through the mouth. In 

 the Anabantidae a similar organ, arising from the fourth gill arch, 

 is composed of a set of superimposed leaflike plates, which afford a 

 large absorptive surface. In the Heteropneustidae an entirely differ- 

 ent accessory breathing organ is found. Extending from the pharynx 

 among the muscles of the back on each side of the vertebral column 

 is a long cylindrical tube, richly supplied with blood vessels and 

 serving as a primitive lung. These tubes, into which both water 

 and air are taken and from which water and air are forced by muscu- 

 lar action, enable the fish to obtain the requisite quantity of oxygen 

 while living in hot, shallow, stagnant ditches and in other places 

 where the water does not contain enough air to support life. 



Although the Ophicephalidae lack the elaborate air-breathing 

 organs met with in the aforementioned families, they have a large 

 suprabranchial cavity lined with puckered vascular epithelium, which 

 serves the same purpose. 



It has been shown by repeated observation and experimentation on 

 the clariids and anabantids in Thailand that even in well-aerated 

 water the gills may not provide enough oxygen to satisfy the re- 

 quirements of the system, and death may ensue in a comparatively 

 short time (20 to 30 minutes in some cases) if the fish are prevented 

 from going to the surface to expel vitiated air and take in gulps of 

 fresh air. We may recognize here an evolutionary process that in 

 time may eliminate the gills and make these fishes entirely air- 

 breathing. 



Associated with possession of air-breathing apparatus is the ability 

 of some kinds of fishes to live out of water for protracted periods, 

 if the respiratory chambers remain moist. Under the stress of 

 drought, when the waters gradually evaporate and ponds and small 

 streams disappear until the return of the rainy season, some of these 

 fishes habitually go deeper and deeper into the mud and ultimately 

 occupy damp pockets, at depths up to a meter, where they aestivate. 

 Aestivation in the case of Thailand air-breathing fishes consists of en- 

 forced inactivity and greatly reduced metabolism while they are 

 buried under parched earth in a stratum of damp mud during the 

 dry season, the limited nutritional needs of the system being sup- 

 plied by the absorption of stored fat and other food material and 

 the vital processes being maintained largely by the utilization of at- 

 mospheric air, which reaches the respiratory chambers through the 

 cracked or porous overlying earth. One can imagine the subter- 

 ranean tragedies that may occur when the desiccation of the soil is 



