52 THE WHENCE AND THE WHITHER OF MAN 



of these flat worms reproduction was the dominant 

 function, which had most nearly attained its possible 

 height of development. From this time on the mus- 

 cular and nervous systems were to claim an ever-in- 

 creasing 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 ; per- 

 haps 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., moUusks, arthropods, and vertebrates. 



It was probably rather long and cylindrical, resem- 

 bling 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 ceUs, and the work 

 is better done. Three portions can be readily dis- 

 tinguished : fore-intestine with the mouth, mid-intes- 

 tine, 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. 



