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SHELL GALLERY. 
interior, the space around and outside of the sac being termed the 
atrial or peri-branchial cavity. 
The delicate walls of the branchial sac, which resemble fine 
Fig. 4. 
A8cidia mentula. Part of wall of branchial sac showing stigmata. 
Magnified. 
muslin, are perforated by innumerable vertical slits, termed stigmata, 
arranged in transverse rows (Fig. 4). 
The margins of the stigmata are lined with cilia which set up 
currents ; and the water which enters by the branchial orifice, passes 
through the stigmata into the atrial cavity, and thence out through 
the atrial orifice. The walls of the branchial sac are chiefly composed 
of a sieve-like meshwork of fine blood-vessels arranged in transverse 
and longitudinal rows. The currents of water passing through the 
stigmata aerate the blood in the vessels. Besides the stigmata, the 
branchial sac has two relatively large orifices, viz., the branchial 
orifice or mouth, and, at the opposite end, the opening into the 
gullet. The branchial sac is, in fact, a capacious throat or pharynx 
(Diagram Fig. 2 and Fig. 14). Inside the branchial orifice is a circle 
of fine tentacles, which guard the entrance to the branchial sac. The 
food of the animal consists of minute animal and vegetable organisms. 
It may be wondered how this food is secured, seeing that the 
currents of water are continually passing through the sieve-like 
walls of the branchial sac to the exterior again. Within the 
Dranchial orifice and above the branchial sac are two circular 
ciliated ridges with a groove between, which is full of viscid 
secretion ; the cilia on the ridges direct particles into the groove 
where they are retained by the mucus. 
