218 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



Villeneuve-Brachon (1940) reported that Stentor, Spirostomum, and 

 Blepharisma (which is non-contractile) lack any silverline system, 

 but that Condylostoma (contractile) has a well-developed linear 

 pattern with transverse connections. The implications are para- 

 doxical. It would appear that the minute vesicular structures of 

 Stentor do not bind silver, and that rigidity cannot be a consistent 

 property of all silverline systems. 



The alveoli must inevitably affect membrane activities at the 

 cell surface. Their response in living cells to variations in osmotic 

 pressure has not been investigated. They are present over most 

 of the buccal cavity of the tetrahymenids, but the actual cytostome 

 has not been studied at high resolution. They appear to be absent 

 in the buccal cavity of Stentor, where a rapid uptake of water is 

 presumed to occur ; instead, the whole cortical cytoplasm is filled 

 with vesicles. Whether alveoli may somehow be involved with 

 membrane transmission of excitation is a totally new question. 



For the intracytoplasmic membrane components of the protozoa, 

 there is nothing significant to add to the discussion in Chapter 2. 

 Closer looks are needed at both the structural and the biochemical 

 properties of many bodies identified as mitochondria in anaerobic 

 organisms. Presently unclassifiable structures — such as the 

 clouds of tiny vesicles described in Opalina (p. 167) or whorls 

 c f concentric membranes — are not uncommonly encountered in 

 protozoa, as in other cells. Some of them perhaps are related to 

 the cell's means for getting rid of used furniture and excess 

 baggage, or they may be stages in the development of other 

 organelles. Membranous structures often appear to be specifically 

 associated with fibers or filaments, as is noted below. 



Fibrous Structures 



If there is anything unique in the ultrastructural organization 

 of protozoa, it may be the extent to which they have utilized 

 fibrous materials in the construction of cytoplasmic organelles and 

 organelle systems. A bewildering profusion of fibrous structures 

 has been described, including some in each protozoan class. 



To begin with, fibrous elements can be listed according to their 

 most obvious, and least reliable, property: what they look like 

 individually. Some fibrillar structures for which available evidence 

 does not permit definite classification are included in parentheses 



