D4 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
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 
papille, 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 
99 
‘vessels,’ are exca- 
¢ 
which the blood lacunze, known as 
vated. These vessels are very regular in size and 
arrangement, and are so large that comparatively litle 
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). 
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