32 BULLETIN 18 8, UNITED STATES NATIONAL MUSEUM 



continued beyond the muddy pockets in which the fishes have taken 

 final refuge. 



One of the striking phenomena connected with fresh-water fishes 

 in Thailand, as in other Oriental countries, is the sudden appearance, 

 on a field or swamp bed that has been dry for months and cut off 

 from any watercourse, of good-sized fishes after a torrential downpour 

 at the beginning of the rainy season. Such a manifestation, so mysti- 

 fying to the uninitiated, is easily explained when one recognizes the 

 presence of buried aestivating fishes, which eagerly make their way 

 to the surface when a copious rain floods and softens the dry soil. 



The ability to breathe atmospheric air and to exist out of water 

 for a considerable time has led to another extraordinary habit in 

 Clarias, Anabas, and other fishes, namely, the voluntary leaving of 

 the water and traveling on land, sometimes across dusty roads, some- 

 times on dry lawns. The impelling influence may be a desire to find 

 improved physical conditions, to seek a better food supply, or to 

 escape enemies. Progress out of water is by lateral movements of 

 the tail as in swimming, while the body is maintained in an upright 

 position by the paired fins. Detailed accounts of actually observed 

 performances of Clarias hatrachus and Andbas testudineus will be 

 found in the appropriate systematic discussions. 



A large number of cyprinoid fishes, representing many genera, which 

 are constantly or during a part of the year subjected to a swift cur- 

 rent, exhibit flanges on the fin rays. The radial flanges are most 

 prominent on the dorsal and anal fins but are to be seen also on the 

 ventrals and pectorals. They extend outward and backward along the 

 whole length of each ray and decrease in width from the base out- 

 ward. They form a complete overlapping buffer for the interradial 

 membranes. No description of these accessory parts of the fin rays 

 of swift-water fishes has been met with in the literature, and no ex- 

 planation of their function seems to have been given. 



In the writer's opinion the radial flanges serve to protect the delicate 

 interradial membranes from the injury they would receive from the 

 constant impact of particles of sand and silt in rapid downstream 

 motion. With a fish in its normal position, with head upstream, 

 sharp-edged particles are effectively deflected from the membranes. 

 It is conceivable that, in the absence of such protection, the mem- 

 branes either might become completely worn through, and thus the 

 usefulness of the fins be destroyed, or, responding to the constant ir- 

 ritation, might become thickened and thus impair the flexibility of 

 the fins. 



These appendages reach their highest development in fishes that 

 live in mountain streams, and are absent or only feebly developed 

 in lake, pond, and sluggish-stream fishes. 



