8UBKINGD0M PROTOZOA. 13 



CHAPTEE II. 



SUBKINGDOM PEOTOZOA. 



A SIMPLE cell, as has already been stated, possesses the 

 power of performing all the functions of life, and conse- 

 quently the existence of unicellular organisms is possible. 

 Such organisms, together with those which consist of a 

 number of cells grouped together, each cell, however, retain- 

 ing to a greater or less extent its own individuality, are 

 grouped together in a subkingdom and are collectively 

 termed Pbotozoa. In its simplest form a Protozoon may 

 show but little differentiation of its protoplasm, but in the 

 majority of cases various portions of the cell-substance take 

 upon themselves special functions, and in accordance with 

 this physiological differentiation undergo various structural 

 modifications. Locomotor and prehensile structures of vari- 

 ous forms may be developed, excretory pulsating vacuoles, a 

 permanent mouth and pharynx, special contractile bands, 

 and even pigment spots presumably connected with light 

 absorption may occur, and in addition the power of secreting 

 horny, calcareous, or siliceous skeletons, serving either as 

 protective or supportive structures, is frequently present. A 

 high degree of complexity may therefore occur in a unicel- 

 lular organism, a complexity produced by a differentiation of 

 various portions of the protoplasm composing the individual. 

 For the most part the organisms are simple, but occasionally 

 they associate together to form colonies. The individuals of 

 the colonies are as a rule all alike, each carrying on all the 

 functions of existence for itself, and there is no division of 

 labor among the various individuals. The complexity which 

 exists is individual and not colonial. A few forms, however, 

 such as Volvox, do present a certain amount of colonial differ- 

 entiation ; all the cells composing the colony are not perfectly 

 identical physiologically, some becoming, for instance, spe- 



