GILLS OF FISHES. 



475 



312 



communicates with it without any intervening valve ; the auricle 



receives the vein from the air-bladder by a distinct aperture, close 



to the opening into the ventricle ; regurgitation into the vein 



being prevented by a hard valvular 



tubercle, which also projects into the 



ventricle. The ventricle (fig. b) 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 e tra- 



becula' arches over and acts as a 



valve to the single auriculo-ventri- 



cular oi^ening, but there are no 



proper membranous semilunar 



valves. 



The muscular parietes of the 

 e bullous 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. 1 



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

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

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

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

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



Circulating and respiratory organs, 

 Lepidosireu 



313 



314 



Two gil]-sacs, Bdello- 

 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 



1 xx. vol. ii. p. 39, prep. no. 910. 



