ORGANS OF RESPIRATION. 549 



species which frequent shores and shallow waters. Being 

 developed, like the lungs of higher animals, from the alimen- 

 tary canal, the air-sac of fishes generally communicates with 

 the oesophagus or stomach, by means of a short trachea or 

 ductus pneumaticus ; in some, however, this tracheal commu- 

 nication becomes completely obliterated, and the sac remains 

 an isolated, closed cavity, filled with its gaseous secretion, 

 as in the xiphias. It is often provided with distinct mus- 

 cular bands for its compression, and when largely developed, 

 it is often capable of producing sounds, either under water 

 as in pogonias, or in the air as in trigla, but there is yet no 

 adaptation of laryngeal cartilages for systematically vocalizing 

 these sounds. 



The air-sac of fishes presents great diversities of general 

 form, as well as of extent of development, being sometimes a 

 single simple elongated sac, or provided with tubular or ramified 

 appendices, or divided into sacculi by transverse constric- 

 tions, or symmetrically divided into two lateral cavities like 

 the lungs of higher animals, as seen in several of the plectog- 

 nathi. The highly vascular glandular organ, which secretes 

 the gaseous contents, is most developed and distinct where 

 there is no ductus pneumaticus, and is generally imper- 

 ceptible where that tracheal canal exists. The air-sac is 

 supplied with branches from the pneumogastric nerve, like 

 the lungs of higher vertebrata ; like the lungs of tritons, 

 water-snakes, turtles, and other aquatic pulmoniferous ver- 

 tebrata, this sac assists in poizing the body in that dense 

 element ; and it often communicates with the organ of hear- 

 ing, like the respiratory passages of higher animals. In the 

 herring, the long fusiform air-sac communicates by a narrow 

 ductus pneumaticus, with the fundus of the stomach ; in 

 the sturgeon it opens by a short wide duct into the cardiac 

 portion of the stomach ; in the diodons, which inhale air at 

 the surface of the water, and thus render their body tense 

 and buoyant, the large air- sac communicates with the an- 

 terior portion of the oesophagus. In rays, frog-fishes, 

 flounders, lampreys, and many others which lie at the 

 bottom of the water or reside in mud, there is na air sac 

 developed. The tracheal duct of many fishes already receives 

 atmospheric air, sometimes pure, and sometimes derived 

 from the inspired water, as that sent into the tracheae from 

 the minute openings of the branchial apparatus of some 



