fHE COMING OF LIFE 15 



species, distributed around the front margin of the body or 

 limited to the front end. The central position of the mouth 

 and the distribution of the eyes, as well as the shape of the 

 body, in certain forms seems to point backward toward some 

 sort of radiate ancestor, though the general structure is bi- 

 lateral, like that of our own bodies. But these insignificant 

 worms seem to represent a stage where important structural 

 changes and progress were taking place. 



We have only glanced at flat worms as representing in some 

 respects a very primitive vermian stage. We must neglect all 

 the intermediate stages between them and the highest worms, 

 as well as the large number of divergent and aberrant forms. 

 We will not even attempt to condense into a few pages of 

 brief, incomplete and inadequate outline the history of the 

 most important changes in the building of the animal body, 

 whose accomplishment must have required millions of years. 

 Our sketch reduces itself to a couple of glimpses, not at all to a 

 moving picture, of animal evolution. 



We pass at a leap to the very highest worms, the annelids, 

 of which the earth worm is a fair example, though the char- 

 acteristics of the group are brought out far more clearly and 

 adequately in the " clam-worms " of the marine mud-flats. 

 Here we shall find the somewhat feeble and crude attempts of 

 the flat worms realized in a definite practical and efficient 

 form and structure. 



The body of the annelid is segmented, composed of a longi- 

 tudinal series of rings allowing great flexibility and freedom 

 of locomotion. Almost every ring has a pair of lateral, fin- 

 like projections which enable the animal to swim or creep 

 rapidly. Their possibilities will unfold in the appendages of 

 insects. 



The body is composed of two parallel concentric tubes, one 

 within the other. The smaller tube, the intestine, opens at 

 the front and hind ends of the body by becoming continuous 

 with the outer tube. The opening in front is the mouth, that 

 at the rear the anal opening. We will call the outer tube the 

 body wall. It is composed of the skin and two layers of 



