44. TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
mantle away from the test, so that these layers are no 
longer in continuity except on the branchial and atrial 
siphons, and at the posterior end where the vessels enter 
the test. 
The body-wall (Pl. II., fig. 5) is largely formed of 
connective tissues, both homogeneous and fibrous, with 
cells, blood sinuses or lacunse, nerves, and the many 
muscle bundles, large and small, formed of long, fusiform, 
non-striped fibres. The largest muscle bundles are found 
about the centre of the right side, where they may be 
0-5 mm. in thickness. This part in the living Ascidia is 
often brilliantly pigmented—red, yellow, and opaque 
white—the coloured cells being exactly hike those found 
in the blood. The connective tissue cells or corpuscles 
are fusiform, stellate, or amoeboid, and may become pig- 
mented or vacuolated, like. the similar cells of the test. 
At the anterior end the body-wall is prolonged outwards 
to form the two well-marked siphons, or short wide tubes, 
which lead in from the branchial and atrial apertures. 
These are surrounded by strong sphincter muscles (PI. I1., 
figs. 2,6, and 7, sph.). Inside the body-wall lies the large 
cavity called the Atrium, or the peribranchial cavity, which 
communicates with the exterior through the atrial aperture, 
and serves to convey away the water which has been used 
in respiration (see Pl. II., figs. 2 and 4, p.6r.). 
The ectodermal hning of the atrial or peribranchial 
cavity has been called by some French writers the third 
tunic—the first being the test and the second the mantle. 
The cavity of the atrium is traversed by numerous vascular 
strands of mesoderm, called connectives (Pl. I1., fig. 2, 
con.), passing from the body-wall inwards to the branchial 
Sac. 
Figure 6 on Pl. IT. shows the relations of ectoderm (with 
test over it), mesoderm, and endoderm in a section through 
