CHORDATA 437 
A constant current of water, maintained by the action of 
the cilia lining the pharynx, enters the mouth and passes out 
through the slits in the branchial sac into the atrial cavity, 
and out at the atrial pore, carrying with it the waste matter 
Fic. 253.—Diagrammatic dissection 
of Ascidia mentula to show the 
anatomy of a simple Ascidian. 
After Herdman. 
at. Atrial aperture. 
br. Branchial aperture. 
a. Anus. 
brs. Branchial sac. 
dl. Dorsal lamina. 
dt. Dorsal tubercle. 
end. Endostyle. 
h. Heart. 
z. Intestine. 
m. Mantle. 
ng. Nerve ganglion. 
c. Oesophagus. 
@.a. Oesophageal aperture. 
ov. Ovary. 
por. Peribranchial cavity. 
7. Rectum. 
st. Stomach. 
(i UNG 
tn. Tentacles. 
vd. Vas deferens. 
ngl. Subneural gland. 
from the alimentary canal and also the generative products 
when ripe. 
A little distance within the oral opening is situated a ring 
of tentacles, which project across the mouth and _ possibly 
function as a filter; the area between this ring of tentacles 
and the branchial sac is termed the prebranchial zone. The 
branchial sac is a large space with perforated walls; it is 
attached to the mantle or body-wall along the median ventral 
line, and the large ventral blood-vessel, which communicates 
behind directly with the heart, is situated along this line of 
attachment. The rest of the branchial sac is separated from 
the mantle by the peribranchial or atrial chamber, which 
opens to the exterior by the atrial pore. The rectum and 
