CLASSIFICATION OF THE MASTIGOPHORA 



51 



pointed and are used by the animal as piercing needles for pene- 

 trating the membranes of the victims that are caught for food. 



The more than superficial resemblance of these suctoria to the 

 heliozoa gives a clue to the possible evolution of the infusoria from 

 sarcodina. We have seen that in forms like myriophrys, cilia and 

 pseudopodia are equally distributed around the body. We have also 

 seen that the central axis of such pseudopodia and flagella are of the 

 same type, and are probably homologous structures; furthermore, 

 we have seen that in actinobolus, projectile tentacles armed with 

 trichocysts can be thrown out at any point on the periphery. These 

 facts indicate the possibility of a common ancestry of the infusoria 



FIG. 19 



I 

 "< 



I 



Cilia and myonemes of infusoria: a, b and e after Johnson; c, d, f and a after Biitschli. 

 The surface view of Stentor ceruleus (c, e) shows rows of cilia inserted on the borders of 

 canal-like markings, each of which contains a myoneme (rf). These are more clearly shown 

 in the optical section (/). In Holophyra discolor (g) the canals and myonemes are inserted 

 deeper in the cortical plasm, a, the membrane of Stentor ceruleus under pressure. 



from a heliozoon-like ancestral race, represented in present-day forms 

 by types like myriophrys, hypocoma, ileonema, and mesodinium, which 

 have both tentacles and cilia. From such an ancestral group the 

 ciliata may have arisen by losing the tentacles and adapting the cilia 

 to the various needs of the cell, while the suctoria may have arisen by 

 loss of the cilia and development of the tentacles to meet all of the 

 needs of the cell, the cilia appearing in the embryos of the suctoria 

 as reminiscences of the earlier ciliated condition of the race. 



These motile organs of the protozoa, with the exception of the 

 flagella, are products of the cortical protoplasm, the flagella retaining 



