CHAPTER III 



THE WORMS AND SOME OF THEIR POSTERITY 



THE somewhat miscellaneous collection of animals that have 

 been thrown together and termed worms is of the greatest import- 

 ance for our theory of descent. Indeed, it seems probable that 

 all of the four great groups which we have yet to mention have 

 descended directly from worm ancestors. This, at all events, 

 is the view of Haeckel, although it must be admitted that many 

 other theories have been proposed. Nor can it be taken as a 

 matter for surprise that agreement concerning this part of our 

 history should be hard to reach, for the difficulties which are met 

 in it are many and perplexing. 



The worms comprise many greatly divergent groups, and the 

 difference between the lowest and the highest of these has been 

 produced by many important steps in Evolution. Of these 

 groups but few immediately concern us ; the first and lowest of 

 those which do, is that of the Turbellarians, a section of the 

 Platodes or flat-worms. The Turbellarians are small or micro- 

 scopic tongue-shaped organisms, of which the majority of species 

 live on the sea-floor, others however being found in fresh water. 

 The surface of the body is covered uniformly with cilia, which 

 serve, in the smaller forms, as organs of propulsion, while in the 

 larger they appear to have the function of maintaining a flow of 

 fresh water over the surface, and thus of assisting respiration. 

 In some respects there has been little advance from the condition 

 of the Ccelenterate. The digestive cavity is a simple or more or 

 less divided sac, communicating with the exterior only by means 

 of the mouth. Unlike the condition of affairs in the Ccelenterates 

 and Ctenophora, however, the sex glands do not discharge the 



58 



