The body is
well provided with muscles, but their arrangement is still far from
economical or effective.
Within the body-wall is the parenchym. This is a spongy mass of
connectile tissue in which the other organs are embedded. The mouth
lies in the middle, or near the front of the ventral surface. The
intestine varies in form, but is provided with its own layers of
longitudinal and transverse muscles, and usually has paired pouches
extending out from it into the body parenchym. These seem to
distribute the dissolved nutriment; hence the whole cavity is still
often called a gastro-vascular cavity as serving both digestion and
circulation. There is no anal opening, but indigestible material is
still cast out through the mouth.
The animal can gain sufficient oxygen to supply its muscles and
nerves, which are the principal seats of combustion, through the
external surface. It has, therefore, no special respiratory organs.
But the waste matter of the muscles cannot escape so easily, for
these are becoming deeper seated. Hence we find an excretory system
consisting of two tubes with many branches in the parenchym, and
discharging at the rear end of the body. This again is a sign that
the muscles are becoming more important, for the excretory system is
needed mainly to remove their waste.
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