PROTOZOA. 1 1 



and simply correspond to the anterior and posterior ends for the 

 time being. There are, however, some species which can only 

 thrust out pseudopodia from part of the body, and which there- 

 fore possess permanent progressing and following regions. 



It appears probable that a delicate film of firmer nature rapidly 

 forms on the surface of naked protoplasm. The direction of 

 pseudopodia may perhaps, in some Amoebae, be determined by 

 the rupture of such a film at definite points. The movements of 

 Amoeba are so characteristic that the special epithet, " amoeboid," 

 is given to all similar movements, wherever occurring. 



5. Irritability and Spontaneity. That Amoeba is affected by 

 external agents or stimuli may readily be noticed. Pseudopodia 

 which come in contact with food-particles flow around them, but 

 inedible substances are usually rejected. Increase of temperature 

 enhances activity, but as 35 C. is neared, this is diminished, and 

 arrested when that point is reached. Death is caused by a rise 

 of 5 or 10 more. On the other hand, activity is diminished by 

 cooling, and arrested at the freezing-point. Weak electric shocks 

 cause the animal to assume the spherical form, and become 

 quiescent. 



Amoeba is constantly moving, and it cannot be doubted that 

 some of its movements are spontaneous i.e., not directly due 

 to external stimuli, but resulting from internal causes, such, per- 

 haps, as chemical decomposition. 



2. VORTICELLA (Bell Animalcule). 



Vorticella is an animal of microscopic dimensions, which is 

 found attached to marine and fresh-water plants, &c., and may be 

 distinguished by the aid of a lens. 



MORPHOLOGY. 



1. External Characters (Fig. 2). The^ body is shaped some- 

 thing like a pear or rounded cone, and, in most individuals, is 

 attached to some object by an elongated slender stalk, into which 

 its apex is drawn out. It is convenient to speak of the attached 

 end of the body as proximal, the free end as distal. The animal 

 possesses considerable powers of movement, and may be found 

 fully expanded, fully retracted, or in an intermediate condition. 



