128 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



the rotatory movements of which the name of the 

 group have been given. This organ is well developed 

 in those types which lead an independent existence, 

 but in those where movement is more restricted or 

 where the character of the organ has changed, it is 

 considerably modified and reduced in size. In 

 Philodince, (crawling Eotifers) the organ of rota- 

 tion has lost the central part, and in its place is 

 substituted a very complicated organ of prehension. 



In sessile forms such as Floscularia, the organ is 

 modified and only the primitive character of the 

 inner ring is retained, while the outer ring is 

 segmented and becomes a series of arms or lobes, 

 furnished with stiff bristles. In Apsilus, another 

 sessile form, the organ of locomotion has disap- 

 peared ; this is obviously an instance of true retro- 

 gression, for in young specimens a vibratory crown 

 still persists. 



The Nemathelminthes contain such round worms 

 as these common intestinal parasites : Ascaris, 

 Oxyuris, Strongylus, etc. These all belong to 

 the Nematode group, and possess a complete 

 digestive tube. Gfordius, however, exhibits organs 

 which are reduced in a marked degree ; in the 

 adult animal the buccal orifice of the digestive 

 tube is closed, and the posterior part of the 

 intestine has disappeared, although in the young 

 worm the alimentary canal is complete. This 

 modification, though incomplete and appearing 

 only in the adult life of Gordius, is complete 



