Spencer—TZhe Blood Vessels of Ceratodus. 5 
The four first branchial arches form membranous folds. Except at their upper 
and lower extremities, the outer edges of the arches are concealed by the projecting 
tips of the branchial lamin. The attachment of the latter, each of which is about 
1 inch in width, is shown in fig. 4. Dorsally and ventrally the lamine of each 
hemibranch pass from the arch itself on to the dorsal and ventral walls respectively 
of the successive clefts (fig. 21). The result is that of the nine hemibranchs the two 
first are connected dorsally, the third and fourth, fifth and sixth, seventh and eighth 
both dorsally and ventrally. Thus, as shown in fig. 21, the second, third, and fourth 
gill clefts are completely surrounded by branchial laminze. The ventral part of the 
second and the ventral and dorsal parts of the ninth hemibranch are not connected 
with those of other arches. 
The internal surface of each arch bears a double series of small triangular plates 
—the gill-rakers. The series of these is complete on both the hyoidean and fifth 
branchial arches, though the latter bears no laminz. The arrangement can be seen 
by reference to figs. 20 and 21. 
Each of the four afferent arteries passes up one of the branchial arches, giving 
off right and left branches to the internal attached edges of the lamine, and at the 
dorsal end gives off on either side a small branch, which follows the course of the 
corresponding hemibranch. The relative position of the afferent artery is shown in 
fig. 4, and will be dealt with later when dealing with the efferent arteries. There is 
no afferent artery carrying venous blood to the hyoidean arch such as is present in 
Lepidosiren. 
B. Efferent branchial arteries, epibranchial arteries, and dorsal aorta. 
As in Plagiostomes, there is an efferent branchial artery for each hemibranch— 
that is there are two in each branchial arch. The general arrangement of these is 
shown diagrammatically in figs. 2, 3, and 5. 
Every branchial lamina is provided with a small efferent vessel passing along its 
outer free border, and each of these opens into the efferent artery of its own side 
(fig. 4). The artery of the hyoidean hemibranch will be dealt with shortly. 
The two efferent vessels of each branchial arch run dorsally, and ¢he par in each 
arch unite together to form a single epibranchial artery (figs. 2 and 5). In each arch 
the posterior is somewhat smaller than the anterior one. 
Those of the fourth and third arches end blindly on the ventral side, but, whilst 
the same is true of the posterior vessels in the second and first arches, the anterior 
ones here give off important branches. 
