48 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
which encircle the anterior end of the branchial sac. The 
anterior band forms a complete ciliated ring, but the 
posterior 1s interrupted in the ventral and dorsal median 
lines: its ends becoming continuous respectively with 
the marginal folds of the endostyle (ventrally), and with 
the front of the. dorsal lamina, where before joining, they © 
bound a narrow triangular cavity lined by ciliated epi- 
thelium, the epibranchial groove (see Pl. I1., fig. 7, above 
d.l.). Behind the peribranchial bands the proper wall of 
the branchial sac commences. 
BRANCHIAL SAC AND ATRIUM. 
The wall of the branchial sac is penetrated by a large 
number of channels, through which blood flows. Some of 
these run in one direction and some in another, so as to 
form complicated but perfectly definite networks, which 
differ in their arrangement in different kinds of Ascidians. ~ 
Between these blood channels there are clefts (the second- 
ary gill-slits or “‘ stigmata’’) in the wall of the branchial 
sac, by means of which the water from the interior passes 
into the large external or peribranchial cavity—the atrium. 
The transverse section (Pl. I1., fig. 2) shows how this 
atrium surrounds the branchial sac on all sides except the 
ventral, where the wall of the branchial sac becomes 
continuous with the body-wall. The right and left halves 
of the atrium may be called the right and left peribranchial 
cavities (v.br.). They unite along the dorsal edge to form 
the cloaca, and there open to the exterior. The cavity 
of the branchial sac communicates with the surrounding 
atrium by means of the stigmata, as shown on the upper 
half (left side) of fig. 2 (Pl. II.). The section on the 
right side is shown passing along a transverse vessel 
between two of the rows of stigmata, 
