98 
up of nearly cubical cells and contains numbers of goblet 
cells, in this respect resembling the general wall of the 
pharyngeal cavity or the epithelium covering the bran- 
chial arches. 
Probably the pseudobranch is not a functional 
respiratory organ, though its structure is very similar tc 
that of any demibranch of the posterior series of true gills. 
The walls of the vascular lamellz resemble mucous 
epithelia rather than membranes through which gaseous 
interchange may take place. And the blood reaching the 
organ has already passed through respiratory plexuses in 
the first holobranch. Undoubtedly it is part of the holo- 
branch which was formerly situated on the hyomandibular 
arch, and its situation suggests that it is the posterior 
demibranch of that gill. There are no traces of the pre- 
sence of a vestige of the anterior demibranch of this gill, 
and the structure of the organ is exactly that of a demi- 
branch, the respiratory surfaces of which are greatly modi- 
fied. Since no traces of the afferent vessel originating in 
the ventral aorta, which would have suppled a functional 
hyomandibular gill, are present, the vascular supply 
gives no certain indication of the homology of the 
organ.* 
The Pelvic Artery.—EHach of the 2nd efferent bran- 
chial vessels gives origin to an artery which almost imme- 
diately unites with its fellow of the opposite side, and the 
azygos trunk so formed runs backwards in the floor of the 
pharynx in the middle line of the body. Various small 
vessels are given off to the ventral portions of the branchial 
arches. This pelvic artery (A. pe.) then gives origin to a 
small vessel supplying the pericardium, the pericardial 
artery (A. per.), and continues backwards between the 
* Cp. the cranial nerves for a discussion of the nerve supply of the 
pseudobranch. 
