This latter organ is lined with a smooth thick chitinous
membrane, and is surrounded by weak longitudinal, but powerful
transverse muscles. Perrier saw these muscles in energetic action;
and, as he remarks, the trituration of the food must be chiefly
effected by this organ, for worms possess no jaws or teeth of any
kind. Grains of sand and small stones, from the 1/20 to a little
more than the 1/10 inch in diameter, may generally be found in
their gizzards and intestines. As it is certain that worms swallow
many little stones, independently of those swallowed while
excavating their burrows, it is probable that they serve, like
mill-stones, to triturate their food. The gizzard opens into the
intestine, which runs in a straight course to the vent at the
posterior end of the body. The intestine presents a remarkable
structure, the typhlosolis, or, as the old anatomists called it, an
intestine within an intestine; and Claparede {13} has shown that
this consists of a deep longitudinal involution of the walls of the
intestine, by which means an extensive absorbent surface is gained.
The circulatory system is well developed. Worms breathe by their
skin, as they do not possess any special respiratory organs. The
two sexes are united in the same individual, but two individuals
pair together.
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