234 LIFE • OUTLINES OF GENERAL BIOLOGY 



the multitudinous cilia that cover the surface of their body. Nemer- 

 tean worms are also covered with cilia, but they have a highly 

 developed musculature. A starfish is richly provided with external 

 cilia, but they are used for wafting food particles, not for locomotion. 

 In short, after the level of the Turbellarians, the locomotor func- 

 tion of external cilia wanes, and its place is taken by muscles. But 

 there are some exceptional cases of much zoological interest. The 

 larv.e (trochospheres) of the marine Annelids swim actively by means 

 of cilia. The same is true of the larvae (trochospheres and veligers) of 

 aquatic molluscs; yet when a young mollusc has no larval stage, 

 but hatches out as a miniature of the adult, as is well illustrated by 

 cuttlefishes, there are no external cilia. There is no doubt that ciliary 



Fig. 43. 



The Luminescent Night-light Infusorian, Noctiluca. which is about the size 

 of a small pin-head. From a specimen. N, the nucleus; P, the much 

 vacuolated protoplasm; FL, the large cross-striped locomotor flagellum; 

 M, the mouth, raised too prominently, containing in its internal pro- 

 longation a small food-wafting flagellum (F). 



locomotion is an old-fashioned feature, and it is interesting to 

 find it at such a relatively high level as the larval lancelet and the 

 larval irog. The newly hatched tadpole moves about by the cilia 

 of its ectoderm. The peculiar little invertebrate animals known as 

 Wheel animalcules or Rotifers, swim actively by means of cilia in 

 front of the mouth, and in this connection it is interesting to notice 

 the view of some zoologists, that Rotifers may be interpreted as 

 relatives of Annelid worms which have remained permanently 

 arrested at a larval level, though at the same time specialised there. 

 Hut the big fact is that above the level of Turbcllarian worms, 

 with rare exceptions, cilia cease to be locomotor in adult life. They 

 remain, however, of great internal importance in many types, as 

 from covering the food-wafting gills of molluscs to lining the 

 windpipe of man and other lung-breathing Vertebrates. They are 



