development.
From this time on the muscular and nervous systems were to claim an
ever-increasing share of the nutriment, and the balance for
reproduction is to grow smaller.

At the close of this lecture I wish to describe very briefly a
hypothetical form. It no longer exists; perhaps it never did. But
many facts of embryology and comparative anatomy point to such a
form as a very possible ancestor of all forms higher than flat
worms, viz., mollusks, arthropods, and vertebrates.

It was probably rather long and cylindrical, resembling a small
and short earthworm in shape. The skin may have been much like
that of turbellaria. Within this the muscles run in only
two-directions--longitudinally and transversely. Between these and
the intestine is a cavity--the perivisceral cavity--like that of our
own bodies, but filled with a nutritive fluid like our lymph. This
cavity seems to have developed by the expansion and cutting off of
the paired lateral outgrowths of the digestive system of some old
flat worm. But other modes of development are quite possible. The
intestine has now an anal opening at or near the rear end of the
body. The food moves only from front to rear, and reaches each part
always in a certain condition. Digestion proper and absorption have
been distributed to different cells, and the work is better done.
Three portions can be readily distinguished: fore-intestine with the
mouth, mid-intestine, as the seat of digestion and absorption, and
hind-intestine, or rectum, with the anal opening. The front and
hind-intestine are lined with infolded outer skin.

The nervous system consists of a supra-oesophageal ganglion with
four posterior nerve-cords--one dorsal, two lateral, and one (or
perhaps two) ventral. There were probably also remains of the old
plexus, but this is fast disappearing. The excretory system consists
of a pair of tubes discharging through the sides of the body-wall,
and having each a ciliated, funnel-shaped opening in the
perivisceral cavity. These have received the name of nephridia.
Through these also the eggs and spermatozoa are discharged. The
reproductive organs are modified patches of the peritoneum, or
lining of the perivisceral cavity.

The number of muscles or muscular layers has been reduced in this
animal. But such a reduction in the number of like parts in any
animal is a sign of progress. And the longitudinal muscles have
increased in size and strength, and the animal moves by w