18 



secondary openings due to the breaking up or subdivision 

 of the primary Chordate gill-clefts. 



In addition to the stigmata there are generally one or 

 two pairs of much larger (up to 15 mm. in length^, 

 narrow, elongated slits placed near the posterior end of 

 the sac and close to the dorsal lamina, so as to be under- 

 neath the atrial aperture, by which water can escape 

 into the cloacal part of the peribranchial cavity. These 

 pharyngo-cloacal slits have well-marked edges which bear 

 much finer cilia than those of the stigmata. 



The lining of the branchial sac is the pharyngeal 

 epithelium (endoderm) while the outer surface is covered 

 by the lining membrane of the peribranchial cavity in free 

 communication with the outer surface of the body through 

 the atrial aperture (see PL II., fig. 2). Both of these 

 epithelial surfaces are formed of squamous cells. Round 

 the sides of the stigmata (PL IV., fig. 3) the cells on 

 the longitudinal vessels become more nearly cubical in 

 shape and bear the cilia, while at the ends of the stigmata, 

 near the transverse vessels, the cells approach a columnar 

 form (PL IV., fig. 3). The epithelium along the internal 

 edge of the longitudinal bars, and on the apices of the 

 papillse, is also cubical or almost columnar in form. 



Between the outer and the inner epithelium the wall 

 of the branchial sac is formed of connective tissue in 

 which the blood lacunae, known as "vessels," are exca- 

 vated. These vessels are very regular in size and 

 arrangement, and are so large that comparatively little 

 connective tissue is left, and so the blood is in close 

 proximity with the epithelial surface. In some branchial 

 sacs a few non-striped muscle fibres are found running 

 longitudinally in the connective tissue around the chief 

 vessels (see PL IV., fig. 1). 



