40 ASCIDIANS 
CHAP. 
particles will be seen to converge to the branchial aperture and 
be sucked in by the inhalent current entering the body. After 
a short interval a certain proportion 
of the particles will be shot out from 
the atrial aperture with the exhalent 
current. 
These particles have passed through 
the pharyngeal portion of the ali- 
mentary canal and the cloacal passages, 
with the water used in respiration, 
but a considerable amount of such 
particles taken in with the water 
do not reappear, as they are retained 
by the nutritive organs and pass along 
the remainder of the alimentary 
canal with the food. The current 
of water passing in at the branchial 
and out at the atrial aperture is of 
primary importance in the life of 
the Ascidian. Besides serving for 
respiratory purposes it conveys all 
the food into the body and removes 
waste matters both intestinal and 
Fie. 15.—Ascidia mentula Linn. 
from the right side (natural 
size), Loch Fyne, N.B.; Br, 
Branchial aperture ; Af, atrial 
aperture. Arrows show the 
renal, and also expels the reproduc- 
tive products from the body. 
The Test.—The test is notable 
lirection of the wi rents. 
direction of ‘the water currents: gmongsh animal structures tor -cene 
taining “tunicine,” a substance which appears to be identical 
in composition, and in behaviour under treatment with various 
reagents, with cellulose. It is cartilaginous in appearance and 
consistency, and to some extent in structure, as it consists 
of a clear (or in some cases fibrillated) matrix in which are 
embedded many corpuscles or cells. It is the matrix that 
contains the cellulose, which may form over sixty per cent by 
weight of the entire test. As the test is morphologically a 
cuticle, being a secretion on the outer surface of the ectoderm 
(Fig. 16, ec), the cells it contains have immigrated to it from the 
body, and it has recently been shown that many of these are 
mesodermal cells (leucocytes or connective tissue wandering cells, 
amoebocytes, and in some cases embryonic “ kalymmocytes,” or 
