ventral side of the branchial sac runs a ciliated groove--the
hypobranchial groove which we have previously found at the same spot
in the Amphioxus. The food of the Ascidia also consists of tiny
organisms, infusoria, diatoms, parts of decomposed marine plants and
animals; etc. These pass with the water into the gill-crate and the
digestive part of the gut at the end of it, at first into an
enlargement of it that represents the stomach. The adjoining small
intestine usually forms a loop, bends forward, and opens by an anus
(Figure 2.220 a), not directly outwards, but first into the mantle
cavity; from this the excrements are ejected by a common outlet (a
apostrophe) together with the used-up water and the sexual products.
The outlet is sometimes called the branchial pore, and sometimes the
cloaca or ejection-aperture. In many of the Ascidiae a glandular mass
opens into the gut, and this represents the liver. In some there is
another gland besides the liver, and this is taken to represent the
kidneys. The body-cavity proper, or coeloma, which is filled with
blood and encloses the hepatic gut, is very narrow in the Ascidia, as
in the Amphioxus, and is here also usually confounded with the wide
atrium, or peribranchial cavity, full of water.

There is no trace in the fully-developed Ascidia of a chorda dorsalis,
or internal axial skeleton. It is the more interesting that the young
animal that emerges from the ovum HAS a chorda, and that there is a
rudimentary medullary tube above it. The latter is wholly atrophied in
the developed Ascidia, and looks like a small nerve-ganglion in front
above the gill-crate. It corresponds to the upper "gullet-ganglion" or
"primitive brain" in other vermalia. Special sense-organs are either
wanting altogether or are only found in a very rudimentary form, as
simple optic spots and touch-corpuscles or tentacles that surround the
mouth. The muscular system is very slightly and irregularly developed.