si VASCULAR SYSTEM 341 
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 (af) becomes the afferent vessel for the “opercular gill ” 
or “hyoidean pseudobranch,”' and supplies the latter with arterial 
blood. The efferent vessel (¢/) 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. Petromyzon) the red corpuscles are circular, but in J/yxine 
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. 
