Spencer—Zhe Blood Vessels of Ceratodus. 1g 
A.— The lung (figs. 17 and 18). 
The lung, as deseribed by Dr. Giinther, arises from the ventral side of the 
cesophagus to the right of the median line. It then passes up the right side being 
closely apposed to the cesophageal wall and for the rest of its course lies on the 
dorsal surface of the alimentary canal beneath the vertebral column being only 
separated from this by the dorsal aorta the ventral face of which is firmly fused with 
the median dorsal line of the lung. Save for a median ventral and dorsal stripe the 
whole external surface shows small rounded protuberances corresponding to the 
“air-cells” within. The whole surface again, save in these two parts, is marked by 
a reticulation of small blood vessels which pass off from the five main ones to be 
shortly described. By the broad dorsal stripe it is firmly attached, as above said, to 
the ventral wall of the aorta and also to the peritoneal membrane lining the body 
cavity immediately on either side of this. A strong median mesentery connects, for 
the greater part of its length, the mid ventral wall of the lung to the mid dorsal wall 
of the intestine. A mid ventral mesentery, again, passes from the ventral intestinal 
to the ventral body wall, though this is not continuous but broken up into a median 
part lying half way along the mtestine after which comes a short gap followed by a 
more posterior portion after which again is a further gap across which pass thin 
strong slips of mesentery (fig. 19, mes.). The extent of the development of the 
ventral mesentery appears to vary somewhat in different examples. 
The internal structure of the lung is beautifully shown in Dr. Giinther’s 
memoir.* To quote his description :—“ The septa separating the compartments 
are imperforate membranes, so that each compartment can be separately filled with 
fluid, only the side towards the median line of the lung being open. The bottom of 
the compartments is again divided into a number of larger and smaller cells by 
reticulated septa, which are very irregularly disposed.” According to this author, 
the compartments are arranged almost symmetrically: in the four specimens of 
Ceratodus whose lungs I cut open this only obtained in the anterior third ; backward 
from this the septa begin gradually to change their relative position until about 
half way down and for the remainder of its length each septum lies opposite to the 
centre of a compartment on the other side of the lung. 
B.—The pulmonary arteries (figs. 17 and 18). 
The fourth epibranchial artery, as described by Boas, gives off on each side a 
large vessel prior to its junction with the third, and the course of the pulmonary 
vessels thus formed varies on each side. 
gil) Be c.0-qgur hp ites 
