THE ORGANIZATION OF THE PROTOZOA 43 



Those investigators of the Protozoa who have expressed an 

 opinion on the subject have been for the most part in favour of the 

 alveolar theory of protoplasm, since it was first propounded by its 

 author, Biitschli (see especially Rhumbler). Protozoa as a rule 

 are very favourable objects in which to study the foam-like structure 

 of the protoplasm (compare Schaudinn, 130, p. 188). But what- 

 ever view be held as to the ultimate structure of protoplasm, its 

 essentially fluid nature is very apparent in these organisms, and is 

 a point upon which it is very important to be clear. The fluid 

 condition of the living substance is manifested directly by the 

 streaming movements to be observed in it, and indirectly by a 

 number of phenomena, such as the tendency, already mentioned, 

 of the body to round itself off when at rest, and the tendency of all 

 vacuoles to assume a spherical form. A vacuole is a drop of fluid 

 suspended in the protoplasmic body, and may be regarded as 

 formed by the bursting and running together of many minute 

 alveoli, just as a large bubble in a foam may arise by the union of 

 many smaller ones ; or by the gradual enlargement of a single 

 alveolus by diffusion of fluid into it from neighbouring alveoli, until 

 it attains proportions relatively gigantic. Vacuoles assume uni- 

 formly spherical contours, except when they are deformed by 

 mutual pressure from crowding together or from other causes. In 

 some cases the protoplasm rna}^ be so full of coarse vacuoles that it 

 exhibits an obvious frothy structure, which must by no means be 

 confounded with the ultimate alveolar structure of the protoplasm, 

 a structure \vhich is exceedingly delicate, only to be observed Avith 

 high powers of the microscope and with careful attention to all 

 details of microscopic technique. Examples of vacuolated bodies 

 are seen especially in Heliozoa e.g.. Actinosphcerium (Fig. 3). 



The statement, however, that protoplasm generally is of fluid 

 nature admits of its exhibiting many degrees of fluidity, and some 

 samples of protoplasm are far more viscid than others. This is 

 true both of different species of organisms, of the same species at 

 different phases of its development, and of different parts of the 

 same organism. In some cases portions of the protoplasm may be 

 stiffened to a degree that perhaps oversteps the ill-defined boundary 

 between the liquid and solid states of matter. In a great many 

 Protozoa, perhaps the majority of them, the protoplasm of the 

 body is divisible, more or less distinctly, into two regions 

 namely : 



1. An external or cortical zone, termed ectoplasm or ectosarc ; in 

 appearance and consistence typically clear, hyaline, more refringent, 

 finely granular or without visible granulations, and of more viscid 

 nature ; in function protective, kinetic, excretory, and sensory. 



2. An internal or medullary region, the endoplasm or endosarc ; 



