animals, but plants. They are sold in the fish markets of many of the
Italian coast-towns with other lower marine animals under the name of
"sea-fruit" (frutti di mare). There is nothing about them to show that
they are animals. When they are taken out of the water with the net
the most one can perceive is a slight contraction of the body that
causes water to spout out in two places. The bulk of the Ascidiae are
very small, at the most a few inches long. A few species are a foot or
more in length. There are many species of them, and they are found in
every sea. As in the case of the Acrania, we have no fossilised
remains of the class, because they have no hard and fossilisable
parts. However, they must be of great antiquity, and must go back to
the primordial epoch.

The name of "Tunicates" is given to the whole class to which the
Ascidiae belong, because the body is enclosed in a thick and stiff
covering like a mantle (tunica). This mantle--sometimes soft like
jelly, sometimes as tough as leather, and sometimes as stiff as
cartilage--has a number of peculiarities. The most remarkable of them
is that it consists of a woody matter, cellulose--the same vegetal
substance that forms the stiff envelopes of the plant-cells, the
substance of the wood. The tunicates are the only class of animals
that have a real cellulose or woody coat. Sometimes the cellulose
mantle is brightly coloured, at other times colourless. Not
infrequently it is set with needles or hairs, like a cactus. Often we
find a mass of foreign bodies--stone, sand, fragments of
mussel-shells, etc.--worked into the mantle. This has earned for the
Ascidia the name of "the microcosm."

(FIGURE 2.220. Organisation of an Ascidia (left view); the dorsal side
is turned to the right and the ventral side to the left, the mouth (o)