ASCIDIA. 49 
Tf an Ascidian expanded in sea-water, in a healthy 
condition, be closely watched, it will soon be noticed that 
there is a constant stream of water pouring in through 
the branchial aperture, and another flowing out from the 
atrial (as represented in fig. 1, Pl. II.); and if some fine 
and insoluble coloured powder be dropped into the water 
near the branchial aperture, it will rapidly be drawn in 
with the current, and after a short time some of the 
powder will make its appearance in the water ejected 
from the atrial aperture. 
Hence, it is obvious that there is a current of sea-water 
flowing through the body of the Ascidian. This current 
has four distinct uses or functions: (1) it enables the animal 
to breathe, by bringing in fresh supplies of oxygen; (2) in 
the first part of its course it carries the microscopic food 
particles into the Ascidian’s body; (8) in the last part of 
its course it carries out of the body various waste materials, 
which must be got rid of; and (4) it ejects the mature ova 
and spermatozoa from the body. The course which this 
water-current takes is :—in through the branchial aperture 
to the branchial sac, then through the clefts in the wall of 
that organ into the surrounding atrium, and lastly out 
through the atrial aperture to the exterior. The direction 
of this current may occasionally be temporarily reversed ; 
and, when the muscular body-wall contracts, a sudden 
current may be ejected through both apertures simul- 
taneously. 
All the clefts or stigmata in the wall of the branchial 
sac (shown in fig. 8, Pl. II.) are bounded by cells which 
bear a number of cilia (Pl. IV ., fig. 3) projecting across the 
cleft. These cilia, so long as the animal is alive, are in 
constant motion, lashing rapidly from the branchial sac 
towards the atrium, so as to drive the water in the cleft 
outwards; and it is this constant action of these very 
