THE TELEOSTEI. 161 



and Anchovy — prolongations of the one organ being sepa- 

 rated from the other only by a membranous fenestra in the 

 wall of the skull. In the Siluroidei, Cyprinoidei, and Cha- 

 racini, and in the Gymnotini. the anterior end of the air- 

 bladder is connected with the membranoiis vestibule by the 

 intermediation of a series of bones attached to the vertebral 

 column, some of which are moveable. 



The vessels of the air-l^ladder are derived from, and empty 

 themselves into, those of the adjacent parts of the body, in 

 which respect, and in the dorsal position of the oesophageal 

 apertiu-e of the pneumatic duct, this structure differs from 

 a lung. 



The heart consists of a single aui'icle, receiving its blood 

 from a venous sinus ; and of a single ventricle, separated by 

 a single row of valves from the biilMis aortce, which is not 

 rhythmically contractile. 



The cardiac aorta divides into trunks to form the branchial 

 arteries, which ruu upon the outer, or convex, side of the 

 bramchial arches, and are distributed to the branchial fila- 

 ments. The blood is collected thence into a branchial 

 vein, which also lies on the convex side of the arch ; and, 

 increasing towards its dorsal end, opens into one of the 

 trunks of the original dorsal aorta. Of these there are two, 

 a right and a left, which pass backwards and meet in the 

 trunk of the dorsal aorta under the spinal column. 



The anterior branchial vein gives off, at its dorsal ter- 

 mination, a considerable carotid triuik, which passes for- 

 wards under the base of the skull ; and this is iinited with 

 its fellow by a transverse branch — so that a complete arterial 

 circle, the circulus cephalicus, is formed beneath the base of 

 the skull. Below, the anterior branchial vein gives off the 

 hyoidean artery, which ascends along the hyoidean arch. 

 and very generally terminates by one branch in the cephalic 

 circle, and hj another enters a rete mirabile, which lies 

 in the inner side of the hyomandibular bone, and some- 

 times has the form of a gill. This is the pseudohrancliia. 

 The branches of the rete mirabile unite again into the 



H 



