140 THE AXAT05IY OF VERTEBRATED ANIMALS. 



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

 which respect, and in the dorsal position of the oesophageal 

 aperture of the pneumatic duct, this structure differs from a 

 lung. 



The heart consists of a single auricle, receiving its blood 

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

 single row of valves from the bulbus aortoe, which is not rhyth- 

 mically contractile. 



The cardiac aorta divides into trunks to form the branchial 

 arteries, which run upon the outer, or convex, side of the bran- 

 chial arches, and are distributed to the branchial filaments. 

 The blood is collected thence into a branchial vein, which also 

 lies on the convex side of the arch ; and, increasing toward 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 

 backward and meet in the trunk of the dorsal aorta under the 

 spinal column. 



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

 tion, a considerable carotid trunk, which passes forward under 

 the base of the skull ; and this is united with its fellow by a 

 transverse branch — so that a complete arterial circle, the cir- 

 culus cephalicus, is formed beneath the base of the skull. Be- 

 low, the anterior branchial vein gives off the hyoidean artery, 

 which ascends along the hyoidean arch, and very generally 

 terminates by one Ijranch in the cephalic circle, and b}- another 

 enters a rete mirabUe, which lies in the inner side of the hyo- 

 mandibular bone, and sometimes has the form of a gill. This 

 is the pseudohranchia. The branches of the rete mirahile 

 unite again into the ophthalmic artery, which pierces the scle- 

 rotic, and breaks up into another rete mirabile, the choroid 

 gland, before being finally distributed. 



In the Lamprey, as has been seen, the respiratory organs 

 are pouches, the anterior and posterior walls of which are 

 raised into vascular folds. The walls of adjacent pouches are 

 distinct and but loosely connected together ; and considerable 

 spaces of integument separate their rounded outer apertures. 



In the ordinary Elasmohranchii, the branchial pouches are 

 more flattened from before backward, and their outer apertures 

 are more slit-like. The integumentary spaces between the 

 slits aie correspondingly narrower, and the adjacent walls of 

 successive pouches are more closely approximated, so that they 

 art divided only by scpia ; but the vascular plaits of the sup 



