INTRODUCTION 29 



frequently measures less than IbiJ. (Fig. 7); and has a different 

 appearance from the more youthful stages. 



A. Form-relations of Protozoa.— The more important factors 

 which determine form in Protozoa are: (1) The density or con- 

 sistency of the protoplasm; (2) the presence of lifeless secretions 

 and deposits in the form of membranes, shells and skeletons; (3) the 

 mode of life; (4) the mode of reproduction; (5) inheritance. 



(1) Protoplasmic Consistency.— All protoplasm contains the same 

 fundamental chemical elements, C, H, X, O, and P which are 

 necessary for the performance of vital activities. With these 

 fundamental elements are associated mineral elements of one kind 

 or another, Xa, K, Ca, Mg, Fe, etc., usually as salts of different 

 kinds and water. The ph\'sical properties vary with the composi- 

 tion in different cases and some types are more fluid, some more 

 dense, than others. As a jelly-fish or medusa is obviously more 

 fluid than the closely related hydroids or sea anemones, so it is with 

 Protozoa. Some types are remarkably watery in their make-up 

 while others are dense and stiff; a Nuclearia delicatula is much more 

 fluid than Amoeba proteiis, and the latter more fluid than a Pelomyxa 

 palustris. 



These dift'erences in consistency of the protoplasm have much to 

 do with the form assumed by Protozoa, and more fluid forms, if not 

 confined by resistant cell membranes, readily change in form 

 according to en\ironmental conditions, or b\' virtue of forces coming 

 from metabolic activities within. Amoeba proteus and other species 

 of Amoeba are amorphous and are constantly changing in shape, a 

 characteristic phenomenon to which the term amoeboid movement 

 is applied, and the same protoplasm ma\' be spherical in form, or 

 flattened on the substratum, or extended in ^'arious ways. Many 

 forms, under certain pressure conditions in the surrounding medium 

 due to evaporation or reduced volume of water, will suddenly burst 

 and disappear lea^•ing no trace whatsoever of their previous presence. 

 This phenomenon has been repeatedlv' mentioned by earlier observ- 

 ers in connection with t\pes of Protozoa belonging to all classes, 

 and the term diffluence was applied to it by Dujardin. In such cases 

 the fluid protoplasm is usually confined by a resisting membrane 

 or cortex which remains intact during the ordinar\' phases of 

 acti\it\' but when the pressure from within becomes too great for 

 the resistance of the membrane the latter collapses, the cell disap- 

 pearing with all the characteristics of a miniature explosion. 



Another evidence of the difference in density between different 

 species of Protozoa is the reaction after cutting with a scalpel. 

 Some species, for example Paramecium caudatum, are extremely 

 difficult to cut successfully owing to the fluid character of the inner 

 protoplasm which, as soon as the cortex is cut, flows out and disin- 

 tegrates; in my experience not more than 20 per cent out of more 



