242 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



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 lamella* 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 supplied a functional 

 hyomandibular gill, are present, the vascular supply 

 gives no certain indication of the homology of the 

 organ.* 



The Pelvic Artery. — Each 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 . 



