ASCIDIA. 41 
‘“tunicine,’ a substance which appears to be identical 
in composition, and in behaviour under various treatments, 
with ‘ cellulose’’—a characteristically vegetable substance, 
entering largely into the composition of plants. The test 
is a protective layer, usually about a quarter of an inch 
in thickness, and may be considered as an exo-skeleton. 
It is the only tissue of a skeletal nature which the adult 
Ascidian possesses. 
In shape the test is an oblong sac, pierced only by the 
branchial and atrial apertures, and forming, at its posterior 
end and left side, the place of attachment by which the 
Ascidian adheres to the rock (PI. II., figs. 1 and 4). At 
this point it may become greatly thickened and expanded 
to form a margin, or may even grow out in the form of a 
short stalk, raising the body above the surrounding surface. 
Stones, sea-weeds, dead shells, and remains of other 
animals may be overgrown by the test and imcorporated 
in its substance; many sessile animals and various kinds 
of sea-weeds may become attached to its outside; and 
some parasitic Amphipod Crustaceans (Triteta gibbosa) 
and Lamellibranch Molluscs (Modiolaria marmorata) may 
inhabit cavities excavated in its thickness. Microscopic 
parasitic Ales may also be present amongst the cells in 
the interior of the test, and help in giving the animal its 
colour. 
The test is cartilaginous in appearance and consistence, 
and to some extent in structure, as it consists of a clear 
or slightly fibrillated matrix in which are imbedded many 
celis. As the test is morphologically a cuticle, being at 
first a secretion on the outer surface of the ectoderm (Pl. 
II., fig. 5, ec.), the cells 1t contains have immigrated to it 
from the body; and it has been shown by Kowalevsky 
and others that many of these are mesodermal cells or 
wandering amcebocytes which have passed through the 
