6 Macieay Memoriat Vouume. 
On the dorsal surface of the gills the efferent artery of each hemibranch 
bifureates, the larger of the two branches thus formed passing upwards to unite with 
its fellow of the same holobranch, the other following the course of the hemibranch 
at the base of the lamine and uniting with that of the hemibranch immediately 
caudad or cephalad of it as the case may be. The relationship of the various 
branches will be seen by reference to figs. 2, 3, and 5. 
The relation of the various efferent and afferent arteries to each other and to the 
other parts of the gill is seen in fig. 4, which represents semi-diagrammatically a 
transverse section across one of the branchial arches. 
The cartilaginous arch itself lies on the cesophageal side and has its inner surface 
coneave in outline ; on either side of the central membranous fold which forms the 
arch external to the cartilage lies a branchial lamina. In the middle line and 
immediately external to the cartilage lies the single afferent vessel which is of 
considerable size ; from this a small branch arises on either side, running along the 
inner attached end of a lamina. It breaks up into a plexus of small vessels which 
pass outwards towards the free border of the lamina and there communicate with an 
efferent vessel which runs towards the base of the lamina and opens into the efferent 
vessel of its own side. The afferent branches on each side are united across the 
median membranous fold of the arch by small cross-vessels. 
The two efferent vessels run one on either side of the cartilaginous arch and 
between the anterior efferent and the afferent artery lies a single nerve. There do 
not appear to be two present, as there are in such forms as Mustelus. 
Though (as described already by Boas) two efferent vessels are present in each 
arch, it may be noted that these differ markedly from those of Mustelus and other 
Plagiostomes in that the pair in each arch unite together to form an epibranchial 
artery ; whereas in the latter the posterior efferent vessel of one arch unites with the 
anterior of the one lying posteriorly to it. In this respect Ceratodus retains in the 
adult what is an embryonic feature, according to Dohrn (as quoted by Parker), in 
Pristiurus, where, prior to the connection between the efferent branches of adjacent 
arteries, the pair of each one falls into the dorsal portion leading to the dorsal aorta. 
In the development of the amphibian branchial blood vessels there is present, as 
described recently by Marshall and Bles* in Rana, an afferent and single efferent 
vessel ; there appears to be no trace of a second efferent and as the one present lies 
to the anterior side of the afferent, it must be regarded as the homologue of the 
anterior of the two present in Plagiostomes and Ceratodus, in both of which again it 
* Studies from the Biological Laboratories of the Owens College (1890). The Development of the Blood Vessels in 
the Frog. Marshall and Bles. p. 186. 
