Spencern—Zhe Blood Vessels of Ceratodus. 27 
On the other hand, the lateral cutaneous veins are very well developed, and their 
course easy to follow after injection. Each runs superficially to the muscles at the 
level of the lateral line from the hinder border of the scapula to the posterior end of 
the body, its calibre becoming very small in the tail region. It receives small 
branches from the muscles along its course and at the shoulder girdle passes inwards 
behind the lower end of the scapula where this joins the coracoid. At this point it 
unites with the subscapular vern (figs. 8 and 15, s.-sc. v.). At the point of union is a 
sinus-like structure into which enter a number of branches from the muscles lying 
immediately behind the scapula. The subscapular vein passes downwards (fig. 15) 
and slightly forwards, thus running roughly parallel to the coracoid and beneath the 
latter. It then joins the main trunk, into which the brachial and superior jugular 
veins fall close to the entrance of the latter. 
At about one-third of the distance between the hind limb and the end of the tail 
the cutaneous vessel of each side sends inward a well-marked branch which passes 
between two hzmal arches and opens into the caudal vein (fig. 16). 
There is a striking resemblance between Mustelus and Ceratodus in this vein, 
which in both cases runs at the same level, opens anteriorly into the subscapular 
sinus and posteriorly communicates with the caudal vein. 
V.— GENERAL CONSIDERATIONS. 
A. The arterial systent. 
In certain respects this shows a resemblance to that of Amphibia and in others 
to that of Elasmobranch fishes. 
The especial amphibian features are (1) the origin of the branchial afferent 
vessels almost simultaneously from the anterior end of the conus, (2) the manner of 
union of the epibranchial arteries to form the dorsal aorta and (3) the origin on 
either side of a pulmonary artery from the fourth epibranchial artery. In these 
respects it is very closely similar to an amphibian in which both lungs and gills are 
present. 
Marshall and Bles have pointed out* that in the case of Rana temporaria the 
pulmonary artery is developed at a very early stage and further that the lungs 
appear very early and are used for a considerable time simultaneously with the gills. 
* Loc. cit. pp. 222 and 263. 
