Section of a spider to show the arrangement of the internal organs:
a, b, upper and under lips of the mouth; c, c, the œsophagus;
d, f, upper and under muscles of the sucking-stomach;
e, stomach; g, g, ligaments attached to diaphragm under the stomach;
J, lower nervous ganglion; k, upper ganglion;
l, l, nerves to the legs and palpi; m, branches of the stomach;
n, poison-gland; o, intestine; p, heart; R, air-sac;
S, ovary; t, air-tube; u, spinning-glands.

The intestine, o, continues backward through the abdomen to the anus, in the little knob behind the spinnerete. The brown mass which surrounds the intestine, and fills the abdomen above it, is supposed to be a secreting-organ discharging into the intestine at several points.