i6 I' HE COMING OF MAN 



muscles; the fibers of one running longitudinally, those of the 

 other transversely. These muscles can lengthen, shorten or 

 bend the body as it writhes through the water. They also give 

 complete protection to the delicate internal organs. The lin- 

 ing of the digestive tube secretes the digestive fluids and ab- 

 sorbs the digested food. As the food passes through it in a 

 more or less continual stream its different parts have begun 

 to differentiate to suit the different stages of digestion. The 

 tube is far more efficient than the mere sack. Around this 

 tube also are layers of muscular fibers. 



The space between the two tubes is the perivisceral cav- 

 ity or coelom where delicate vital organs can be packed away. 

 Here we find comparatively simple reproductive organs. 

 Many eggs are still produced but this is not the whole business 

 of life as in flat worms. We find a pair of excretory tubules 

 in each segment to remove the steadily increasing nitrogenous 

 waste. The earth worm still breathes through the outer 

 surface; the more active clam-worm has little feathery 

 or tubular gills on its fins. We find a complete system 

 of blood-vessels to insure delivery of nutriment and 

 oxygen to all p^rts of the body and removal of waste. The 

 perivisceral cavity is filled with a colorless fluid, perhaps 

 much like our lymph. All the essentials of the body of a very 

 powerful and efficient animal have here been realized to a 

 large extent. The general plan of structure and arrangement 

 will be retained in higher forms though with much improve- 

 ment in details. 



A quite complex nervous system has been already attained. 

 In every ring or segment of the body we find a nerve center 

 or ganglion controlling the muscles of that segment. These 

 are connected in a chain along the ventral surface sending 

 forward on each side of the oesophagus bundles of fibers, com- 

 missures, to an evidently superior or supreme center in the 

 first segment. The long cylindrical body is thus unified and 

 brought under one control. This chief supra-cesophageal 

 ganglion innervates the principal sense-organs. The mouth 

 has the sense of taste, and here or hard by is located the 



