86 



SHELL GALLEKY. 



interior, the space around and outside of the sac being termed the 

 atrial or peri-brancbial cavity. 



The delicate walls of the branchial sac, which resemble fine 



Fig. 4. 



Ascidia mentida. 



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 

 branchial 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. 



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