THE TUNIC ATE8. 
137 
of eel-grass. On placing a good-sized As- 
cidian in a vessel of fresh sea-water it will 
be found to consist of a semi-transparent or 
quite opaque test with two openings, one 
lower than the other, as seen in MoJgitla, 
which looks, when the two orifices are pro- 
truded, like a double-necked bottle (Fig. 177). 
The anterior or higher orifice or mouth is 
for the passage of currents of water into the 
respiratory sac; and the posterior, usually 
lower, excurrent orifice for the passage out- 
wards of fecal matter. The test or outer 
skin is either delicate and semi-transparent, 
or it may be quite tough and opaque. 
The Tunicates may in general terms be 
characterized as having a usually rounded or 
sac-like body, which is sometimes barrel- 
shaped, bilateral, with a dorsal and ventral 
symmetry, protected by a transparent or 
dense test, containing cellulose, lined within 
by a tunic surrounding the body-cavity. 
There are two openings in the test, one oral, 
the other ''^ atrial the mouth leads into a 
capacious pharyngeal respiratory sac, open- 
ing posteriorly by an oesophagus into the 
stomach, which is provided with a liver; 
the intestine is flexed, and ends near the 
oesophagus. The nervous system is bilat- 
eral, forming a double ganglionated chain 
in Appendicularia, but is reduced in the 
typical Ascidians to a single ganglion, sit- 
uated within the tunic between the two open- 
ings. There is a tubular heart, opening at 
each end, and its beatings are often reversed, 
the blood flowing in and out at either end. 
A singular group of Tunicates is repre- 
sented by Salpa, which is a pelagic form. 
fl 
Fig. 178— Structure 
of a compound 
Ascidian, Ama- 
rcEcium. A, bran- 
c h i a 1 sac ; m, 
stomach; fc, 
intestine; c, 
mouth; o\ testis; 
rr', efferent duct 
of the testis; C, 
ovary; p\ egg in 
the body-cavity; 
p'\ eggs in the 
atrium; n, anus; 
o shows the site 
of the heart; I, 
liver; e, openings 
in walls of bran- 
chial chamber. 
