SHAPE AND SIZE OF BODY 19 



adaptability to environment which in itself should be regarded as a sign 

 of degeneration. 



SHAPE AND SIZE OF THE BODY.— The Protozoa vary considerably 

 in size, some of them being easily detected with the naked eye. Many 

 of the ciliates and gregarines can be seen as white specks or elongate 

 filaments, while certain multinucleate amoeboid organisms may be several 

 centimetres in diameter. The majority of Protozoa, however, are so small 

 that they cannot be seen without magnification. The adult individuals 

 of any species may vary considerably in size amongst themselves, and 

 there may be marked differences in size between the mature and immature 

 stages of development. 



Protozoa may be of almost any conceivable shape, and the exact form 

 of the body may be regarded as a direct adaptation to their mode of life 

 and environment. When living in fluid media, unless the shape is deter- 

 mined by a relatively tough outer membrane or a skeletal support, there 

 is a tendency for the organism to assume the spherical form. Amoebae, 

 in which an outer membrane is entirely absent or represented by an 

 exceedingly fine pellicle, are spherical unless temporary contractions of 

 the cytoplasm or pressure of any body against which they come in contact 

 or over which they are moving overcomes the physical forces to which 

 they are subject (Fig. 5). So soon as relaxation occurs the spherical 

 form is resumed. In the majority of Protozoa the body is definitely 

 elongated even in a condition of repose, and it is evident that this form 

 is retained without any effort on the part of the organism itself. This 

 is due in most cases to the development of an elastic outer layer of cyto- 

 plasm, which retains its shape unless this is temporarily altered by pressure 

 or the contractions of the cytoplasm (Fig. 6). This outer layer of the 

 cytoplasm or periplast may attain a high degree of complexity. It may 

 be so tough, as in many of the Mastigophora and Ciliata, that the shape 

 of the body is practically constant. 



In the case of certain Mastigophora, like Cercomonos, which are adapted 

 to a creeping mode of existence as well as a swimming one, the body is 

 constantly changing its shape when it is moving over a surface, with a 

 tendency to the assumption of an elongate form during progression 

 through a fluid (Fig. 7). In the majority of Mastigophora and Ciliata 

 which swim through liquids the body is elongated, and may even have 

 a spiral form, when movement is associated with revolution about the 

 longitudinal axis (Figs. 143 and 509). Certain Mastigophora and Ciliata 

 which lead a swimming existence as well as a creeping one upon the 

 surface of various objects are frequently flattened dorso-ventrally. In the 

 swimming forms there is a tendency for one end of the organism to be 

 more pointed than the other. Certain Protozoa become permanently 



