FLYING FISHES AND THEIR HABITS. 



509 



ai-v 



As almost ovory cxlornal feature is charaeteristic, so arc many 

 internal |)ai-ts. In connection with the lonj>:itu(linal arch or convex- 

 ity of the back, so different from the straightness of that of the Gur- 

 nards, a very remarkable deviation of the air bladder from normal 

 relations is noteworthy; the dorsal curvature, indeed, is a coordinate 

 of an otherwise unexampled j)osition of the bladder. 



The air, or swimming, bladder is unique in character, as Calder- 

 wood states, in that " it is not situated below, but [mostly] above, 

 the vertebral column, not forming part of the abdominal contents, 

 but situated dorsally in a special cavity [recess] of its own. When 

 the abdominal cavity is opened ventrally, and the viscera removed, 

 only the ventral surface of the bladder is seen, forming part of the 

 dorsal boundary of the cavity. Seen from this point of view, it is 

 iormed of a broad central portion, white and tendinous, and of two 

 lateral portions strongly muscular." The physiological significance 

 of this conies into view when 

 we consider one of the habits 

 or aptitudes of the fish. 



The structure and position 

 of the air-bladder are adapt- 

 ed for keeping the Dactylo]i- 

 terid with back upward in 

 the air in spite of the form 

 of the body and its relation 

 to the vertebral axis. " The 

 bladder, being prevented 

 from expanding when the 

 pressure from the surround- 

 ing w^ater is suddenly re- 

 moved, the high dorsal position of the secondary portion becomes of 

 the greatest possible advantage." It helps the fish to emerge from 

 the water and maintain its equilibrium in the air. 



Such are the most characteristic features of the Dactylopteruls com- 

 mon to all the members of the family. The species are few — about 

 half a dozen — and closely related to each other, all being strictly 

 j congeneric. 



It might naturally be supposed that fishes so specialized in form, 



so peculiar in ajipearance, and so remarkable for the development of 



their pectoral fins and their unusual functions, would have received 



, distinctive English names, but such is not the case. Flying gurnards 



I or flying gurnets are the general names given in most works, but 



they are mere book names and not of the common people; further, 



, the names are deceptive and have often misled, for the fishes are not 



j very closely related to the true gurnards or gurnets, although more 



j so than to any other fishes. Bat-fish is a designation for the coimnon 



Fiu. 2. 



Ductylopterus volituns: Shoulder girdle. 

 (After Bouleuger.) 



