PROTOZOA 



17 



bring it within reach. There are no special points of the body for 

 the taking in of food in the Monera, Sarcodina, Gregarina, Suctoria, and 

 those Flagellata which feed after the manner of plants. In other 

 Flagellata and in the Infusoria, there is at one special part of the body 

 (in the Flagellata at the base of the chief flagellum) a depression of 

 the ectoplasm (mouth and oesophagus), through which solid food is 

 passed into the endoplasm. In the In- 

 fusoria there are, closely connected with 

 the mouth, cilia specially arranged, in 

 circles or spirals, which whirl into it the 

 minute nutritive particles. A certain part 

 of the body may also temporarily function 

 as mouth. In this case the aperture is 

 only visible at the moment of feeding. 

 An anal aperture or anal spot can also 

 be found for the evacuation of undigested 

 remnants of food. 



I 



V. Adaptations for Excretion. 



The so-called contractile 

 may, with doubtful accuracy, 



w 



FIG. 20. Paramecium aurelia. M, 

 Medulla ; C, cortical layer ; n, macro- 

 nucleus ; n', micxonucleus ; en, cuticle ; 

 ci, cilia ; t, tricnocysts ; v, food vacu- 

 o, cell mouth or 

 1, 2, 3, 4, Form- 



vaeuoles 



be con- 

 sidered as adaptations for excretion, i.e. 

 for removing from the body the useless 

 products of metabolism. These vaeuoles 

 are found in most Ama'ba, Heliozoa, 

 Flagellata (excepting Cystoflagellata), and 



Infusoria, but are Wanting in the RhlZO- ation of vacuole (after Ray Lankester). 



poda, Eadiolaria, Gregarina, and Cysto- 



flagellata. They vary greatly in number. Where there is only one 

 it generally has a fixed position. There is no sharp distinction be- 

 tween contractile and non-contractile vaeuoles. The first are vaeuoles 

 without walls, filled with liquid, which in cases where a differentiation 

 into ecto- and endo-plasm exists generally lie in the former. They 

 gradually expand and then contract more quickly, often suddenly. 

 Before and during contraction they move towards the surface and 

 empty out their contents through an aperture only visible at the 

 moment of evacuation. They again arise as a small drop, or as several 

 drops which unite later. Perhaps they also serve the purpose of 

 keeping up an exchange of water (evacuation of water taken in by the 

 mouth), and thus of respiration. 



VI. Triehoeysts. 



Small vesicles containing threads, which can be shot out rapidly 

 like the nematocysts of the Ccelenterata, are sometimes found in the 

 ectoplasm of the Infusoria and in one of the Flagellata (Fig. 20 t). 



VOL. I C 



