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 writhing.  Such a worm has the general plan of the body of the higher forms fairly well, though rudely, sketched.  Many improvements will come, and details be added.  But the rudiments of the trunk of even our own bodies are already visible.  Head, in any proper sense of the term, and skeleton are still lacking; they remain to be developed.

And yet, taking the most hopeful view possible concerning the animal kingdom, its prospects of attaining anything very lofty seem at this point poor.  Its highest representative is a headless trunk, without skeleton or legs.  It has no brain in any proper sense of the word, its sense-organs are feeble; it moves by writhing.  Its life is devoted to digestion and reproduction.  Whatever higher organs it has are subsidiary to these lower functions.  And yet it has taken ages on ages to develop this much.  If this is the highest visible result of ages on ages of development, what hope is there for the future?  Can such a thing be the ancestor of a