ASCIDIA. 43 
layer of the test (fig. 9, ¢.4.); in some Ascidians they 
probably form an accessory organ of respiration. In the 
adult Ascidia the vessels enter the test by a double trunk 
ventrally, near the posterior end of the left side (see PI. 
ies 10). 
~ HcCTODERM AND BoDY-WALL. 
Inside the test, and lying between its inner surface and 
the muscular body-wall (‘‘mantle’’), 1s a very delicate 
membrane, formed entirely of small cubical or more flat- 
tened cells (Pl. II., fig, 5, ec.), with delicate ‘‘ secreting 
processes” projecting into the test. This is the Kcto- 
derm, the outermost complete layer of cells in the body; 
and the test lying over it has been produced as a sort of 
gelatinous exudation upon the surface of the ectodermal 
cells, aided by the numerous mesoderm cells which have 
migrated into it, and which give it the appearance of a 
connective tissue. Besides the ordinary epithelial cells, a 
few gland cells and pigment cells may be found in the 
ectoderm. ‘The ectoderm is turned in for a short distance 
at the branchial aperture (mouth) and atrial aperture 
(cloacal), as a short stomodeum and proctodeum, lined 
in each case by a delicate prolongation of the test (Pl. IL, 
fig. 4, Br. and At.). 
Inside the ectoderm lies a thicker layer, the so-called 
“Mantle” or body-wall (parietal mesoderm), containing a 
large number of muscles, which run some along the length 
of the body, and others across it, so that they form a rude 
interlacing vet-work (Pl. IL., fig. 4, m.), which is much 
more strongly developed on the right than on the left side 
of the body. ‘The shape of the body can be changed, to a 
slight extent, by means of these muscles in the body-wall. 
When the Ascidian is killed by re-agents it is generally 
found that the muscles have contracted and drawn the 
