II STRUCTURE—BODY-WALL, ETC. 43 
tissues—both homogeneous and fibrous—with cells, blood- 
sinuses, and many muscle- bundles large and small running 
circularly, longitudinally, and obliquely, and interlacing in all 
directions (Fig. 18, 
m). The muscles 
are all formed of 
very long fusiform 
non-striped fibres. 
The mantle in some 
Ascidians is often 
brilliantly pig- 
mented — red, yel- 
low and opaque 
white, the coloured 
cells being exactly 
like those found in 
the blood. 
The mantle 
forms two well- 
marked siphons or 
short wide tubes, 
which lead in from 
the branchial and 
atrial apertures. 
These are sur- 
rounded by strong 
sphincter muscles,’ 
and are lined by the 
invaginated ecto- Fia. 18.—Dissection of Ascidia, from right side, to show 
a anatomy. a, Anus; Af, atrial aperture ; Br, branchial 
erm and test. The aperture ; br.s, b7.s’, branchial sac ; end, endostyle ; g.d, 
one leads into the genital ducts; gon, ovary ; hyp, neural gland; hyp.d, 
. the duct leading to dorsal tubercle; m, mantle; 2.9, 
branchial sac or ganglion ; ves, oesophagus ; p.br.c, peribranchial cavity ; 
modified pharynx ren, renal vesicles ; st, stomach ; ¢, test ; tv, tentacles ; 
‘ ty, typhlosole. 
and the other into 
the atrial or peribranchial cavity (see Fig. 18, and Fig. 19, p.br). 
Figs. 18 and 19 show the relations of the branchial and 
peribranchial cavities to one another. The peribranchial cavity 
1 These sphincters close the only openings through the tough test so effectually 
that when collectors are. preserving Ascidians in alcohol it is advisable to make one 
or more slits in the test to allow the sea-water to escape and the spirit to enter. 
