PHYLUM PROTOZOA 69 



tozoa muscle fibrils (myonemes) are present just beneath the 

 cuticle (Stentor, Fig. 43, A; Vorticella, Fig. 43, C); these are 

 capable of contraction and can change the shape of the animal. 

 In the stalk of Vorticella the muscle fibrils are agents for moving 

 the bell from place to place. 



Reactions to Stimuli. — Brief accounts have been given of 

 the reactions of Ameba (p. 35), Euglena (p. 43), and Paramecium 

 (p. 56) to stimuli. It has been shown that these minute organ- 

 isms are capable of spontaneous activities and respond to a num- 

 ber of different external stimuli, which are changes in their en- 

 vironment. These responses are carried on without the help 

 of a nervous system. The study of the behavior of the lower 

 organisms has become quite prominent within the past decade 

 and has led a prominent investigator in this field to the follow- 

 ing conclusion. " All together, there is no evidence of the exist- 

 ence of differences of fundamental character between the be- 

 havior of the Protozoa and that of the lower Metazoa. The 

 study of behavior lends no support to the view that the life ac- 

 tivities are of an essentially different character in the Protozoa 

 and the Metazoa. The behavior of the Protozoa appears to be 

 no more and no less machine-like than that of the Metazoa; 

 similar principles govern both." (Jennings, Behavior of the 

 Lower Organisms, p. 263.) 



Reproduction. — The usual method of reproduction in the 

 Protozoa is that of binary division. This occurs in most of the 

 types discussed in the preceding pages (Ameba, Euglena, Para- 

 mecium, etc.). During binary division the body of the Pro- 

 tozoon divides into two approximately equal parts, the 

 daughter-cells. Binary division is frequently interrupted by 

 conjugation as in Paramecium (p. 59). When the division of 

 the Protozoon is unequal, the process is spoken of as budding 

 or gemmation. The parasitic Rhizopod, Entameba histolytica 

 (p. 70), reproduces in this way. A third method of reproduc- 

 tion is by the formation of spores (Ameba, p. 33; Monocystis, 

 P- 49» Fi g- 32). 



