60 



PROTOZOOLOGY 



are either prehensile or usually suctorial. The prehensile tentacle 

 appears to be essentially similar in structure to the axopodium 

 (Roskin, 1925). The suctorial tentacles are tubular and this type is 

 interpreted by Collin as possibly derived from cytostome and cyto- 

 pharynx of the ciliate (Fig. 13). 



Although the vast majority of Protozoa possess only one of the 

 three organelles of locomotion mentioned above, a few may possess 



jjjgjt 



Fig. 13. Diagrams showing the possible development of a suctorian 

 tentacle from a cytostome and cytopharynx of a ciliate (Collin). 



pseudopodia in one stage and flagella in another during their de- 

 velopment. Among several examples may be mentioned Naegleri- 

 idae (Fig. 183), Tetramitus rostratus (Fig. 155), etc. Furthermore, 

 there are some Protozoa which possess two types of organellae at the 

 same time. Flagellum or flagella and pseudopodia occur in many 

 Phytomastigina and Rhizomastigina, and a flagellum and cilia are 

 present in Ileonema (Fig. 306, b, c). 



In the cytoplasm of Protozoa there occur various organellae, each 

 of which will be considered here briefly. 



Fibrillar structures 



One of the fundamental characteristics of the protoplasm is its 

 contractility. If a fully expanded Amoeba proteus is subjected to a 

 mechanical pressure, it retracts its pseudopodia and contracts into a 

 more or less spherical form. In this response there is no special or- 

 ganella, and the whole body reacts. But in certain other Protozoa, 

 there are special organellae of contraction. Many Ciliophora are able 

 to contract instantaneously when subjected to mechanical pressure, 

 as will easily be noticed by following the movement of Stentor, 

 Spirostomum, Trachelocerca, Vorticella, etc., under a dissecting 

 microscope. The earliest observer of the contractile elements of 

 Protozoa appears to be Lieberkiihn (1857) who noted the "muscle 



