xii VASCULAR SYSTEM 34! 



to form an epibranchial artery. From the dorsal end of the fourth 

 afferent artery there arises a recurrent branch which curves round 

 the upper margin of the sixth cleft and supplies the gill-lamellae on 

 the posterior margin of that cleft, a fact which lends support to 

 the view that these lamellae are " emigrants " from the anterior 

 margin of the cleft ; the efferent vessel from the " emigrant " 

 lamellae joins the fourth epibranchial artery. The blood-supply 

 of the external or cutaneous gills is derived from the dorsal 

 extremities of the second, third, and fourth afferent arteries, 

 while the efferent vessels from these organs join the correspond- 

 ing epibranchial arteries ; in this respect there is a close resem- 

 blance between Protopterus and those larval Amphibians which 

 possess similar cutaneous gills. All four epibranchial arteries 

 unite together at about the same point to form a short common 

 trunk, the right or left dorsal aorta, which subsequently unites 

 with its fellow to form the median dorsal aorta. 



There is a so-called " hyoidean " artery, which, however, has 

 its origin, not from an anterior efferent branchial vessel as in 

 Neoceratodus, but from the first afferent branchial artery. After 

 giving off a submaxillary or lingual artery, the " hyoidean " 

 artery (a/) becomes the afferent vessel for the " opercular gill " 

 or " hyoidean pseuclobranch," l and supplies the latter with arterial 

 blood. The efferent vessel (ef) from the pseudobranch unites with 

 the four epibranchial arteries in forming the right or left dorsal 

 aorta. A " carotid " artery arises from the efferent vessel of the 

 " hyoidean pseudobranch," and a pulmonary artery has its origin 

 from the root of the dorsal aorta of its side, and . not from the 

 fourth epibranchial artery as in Neoceratodus. 



The Blood. The blood consists of a nutritive fluid plasma in 

 which float red corpuscles and leucocytes. In the Cyclostomata 

 (e.g. Petromyzori) the red corpuscles are circular, but in Myxine 

 they have the usual oval shape. In Fishes the red corpuscles 

 are almost invariably flat, oval, biconvex, and nucleated, and owe 

 their colour to the presence of the characteristic oxygen-absorb- 

 ing, iron-containing pigment, haemoglobin. They are unusually 

 large in the Dipnoi and are only exceeded in size by those 

 of certain Urodele Amphibians. The leucocytes are much less 

 numerous than the red corpuscles, although their relative propor- 

 tions are very variable, even in the same species under different 



1 This structure may prove to be a hemibranch of the first branchial arch. 



