GILLS OE FISHES. 



475 



312 



communicates with it witliout any intervening valve ; the auricle 

 receives the vein frt)m the air-bladder by a distinct aperture, close 

 to the o]3ening into the ventricle ; regurgitation into the vein 

 being prevented by a hard valvular 

 tubercle, which also projects into the 

 ventricle. The ventricle (fig. h) is 

 single, like the auricle; its inner 

 parietes are very irregular : a ' tra- 

 becula ' projects from the lower 

 part of the cavity, like a rudimental 

 septum : a smaller transverse ' tra- 

 becula ' arches over and acts as a 

 valve to the single auriculo- ventri- 

 cular opening, but there are no 

 proper membranous semilunar 

 valves. 



The muscular parietes of the 

 ' l:)ull)us arteriosus ' are distinct in 

 all fishes from those of the ventricle ; 

 they may be overlapped by these, 

 but an aponeurotic septum inter- 

 venes between the origin of the bulb 

 and the overlapping ventricular 

 fibres.' 



§ 84. Gills of Fishes. — The primary division of the branchial 

 artery in the Myxinoids has been already described. Each gill- 

 sac receives, either from tlie trunk or its bifurcations, its proper 

 artery. The leading condition of the gills in other fishes may ha 

 understood by supposing each compressed sac of a Myxine, fig. 



CirculatiDg and respiratory organs, 

 LepiUusin;li 



313 



314 



Two gill-sacs, SdcUo- 

 stoma 



Two gill-sacs, Lamprey 



313, m, to be split through its plane, and each half to be glued by 

 its outer smooth side to an intermediate septum, which would then 

 support the opposite halves of two distinct sacs, and expose their 

 vascular mucous surface to view. If the septum be attached by 



XX. vol. ii. p. 39, prep. no. 910. 



