MORPHOLOGY 



51 



and pseudopodia occur in many Phytomastigina and Rhizomasti- 

 gina, and a flagellum and cilia are present in Ileonema (Fig. 235, 

 b, c). 



In the cytosome of Protozoa there occur various organellae, 

 each of which will be considered briefly here. 



Fibrillar structures 



One of the 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 organella, 

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



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

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



are special organellae of contraction. Many Ciliophora are able 

 to contract instantaneously when subjected to mechanical pres- 

 sure, 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 was Lieberkiihn (1857) who noted "muscle 

 fibers" in the ectoplasm of Stentor which were later named 

 myonemes (Haeckel) or Neurophanes (Neresheimer). 



The myonemes of Stentor have been studied by several in- 

 vestigators. According to Schroder (1906), there is a canal be- 

 tween each two longitudinal striae and in it occurs a long banded 

 myoneme which measures in cross-section 3-7 fx high by about 1m 

 wide and which appears cross-striated (Fig. 14). Roskin (1923) 

 considers that the myoneme is a homogeneous cytoplasm (kino- 

 plasm) and the wall of the canal is highly elastic and counteracts 



