310 THE ZOOLOGIST. 



need protection from enemies in just the same measure as 

 Halteria, should otherwise have become extinct, or nearly so ?* 

 But that this has not occurred our microscopes tell us. How 

 then can selection have played any part in the evolution of 

 Halteria' s spines ? 



6. Trophic Cilia of Bursaria. 



This genus is characterised, among other points, by the 

 possession in the vestibule of a series of short, very stout cilia 

 arranged in regular rows, which work to and fro in unison and 

 act as a very efficient trophic apparatus. 



Now in essence these trophic cilia are simply an instance of 

 the extreme development of the larger oral cilia found in all 

 Heterotricha. In this genus they have reached a very high 

 stage of development, forming an extremely efficient apparatus. 

 The differences between the two kinds are, indeed, not over- 

 whelmingly great, yet the trophic cilia of Bursaria have all the- 

 appearance of being due to real adaptive modification. But 

 there can be no question that natural selection has played no 

 part in their development, since the other species of similar 

 habits are without them. 



7. Setose Processes in Euplotes and Belated Genera. 



These are well marked, stiff, limb-like processes of a different 

 nature to the trophic cilia of Bursaria. They serve as loco- 

 motor organs, and have a powerful system of muscular fibrils 

 developed around their roots. By the energetic contractions of 

 these the organism is propelled with a running motion with 

 great velocity. 



Considered as locomotor organs, they are far more efficient 

 than the ciliary ring or coat. Indeed, in the most highly- 

 developed forms like Euplotes the cilia have vanished, super- 

 seded by the styles and setae. A progressive differentiation, too, 

 can be detected in the Hypotricha, from forms without setse to 

 forms in which they are the only tropho-motor organs. W& 

 appear, therefore, to have here again an instance of progression 

 towards greater efficiencj^ and this entirely independent of 

 natural selection, since there is no sign of extinction of the less 



