314 TUNICATA.— ASCIDIAD A. 
The branchial sac occupies the upper part of the 
animal ; its interior surface is divided into cells by 
plaits or ridges, which are clothed with cilia; its 
orifice is surrounded by a cirele of tentacles, and 
its inferior extremity merges into the digestive 
tube. For as the food of these animals consists of 
microscopic organisms, which are drawn in by the 
entering currents, the same influx of water brings 
oxygen for the respiration, and food for the nutri- 
ment of the system. 
The water, in yielding its vital properties, passes 
towards the bottom of the body, and then returning 
by an upward course is discharged through an 
orifice closely resembling the other in appearance, 
and situated near to it on one side. Both orifices 
can be completely closed at will, by being drawn 
together in wrinkles, and each is commonly sur- 
ASCIDIA VIRGINEA. 
rounded with minute coloured specks, that are con- 
sidered to represent eyes. These specks are usually 
eight around the receiving orifice, and six around 
the discharging one. 
