32 INTRODUCTION TO PROTOZOA 



outside of the cell, sometimes through a definite excretory pore. 

 Sense organs in the form of pigment spots sensitive to light, 

 and outgrowths sensitive to chemical substances, giving, perhaps, 

 a sensation comparable with taste, are present in some species, 

 especially in free-living ones. Various organelles serving the 

 function of a skeleton may be developed in the form of a tough 

 cuticle, a chitinous, calcareous or siliceous shell, a chitinous sup- 

 porting rod or " axostyle " (Fig. 30, axo), or even a complicated 

 internal skeleton of calcareous material. While no protozoan 

 possesses all of these organelles, many possess a considerable 

 number of them and exhibit a degree of complexity and or- 

 ganization almost incredible in a single-celled animal which is 

 barely, if at all, visible to the naked eye. 



Physiology and Reproduction. — In their physiology and 

 manner of life the Protozoa differ among themselves almost as 

 much as do the Metazoa. Some ingest solid food through a 

 cytostome or by wrapping themselves around it, others possess 

 chlorophyll and are nourished in a typical plant manner, and 

 still others absorb nutriment by osmosis from the fluids or 

 tissues in which they live. Acid substances corresponding to 

 the gastric juice and alkaline substances simulating the intesti- 

 nal juices may be present in the protozoan body, often localized 

 in definite regions, and acting upon the food as it circulates in 

 the food vacuoles. The waste material either is voided through 

 a cytopyge or is left behind by a simple flowing away of the 

 protoplasm. Body excretions are collected by the contractile 

 vacuoles and voided by them, or they are simply passed through 

 the body wall by osmosis. 



The multiplication or reproduction of protozoans is of two 

 quite distinct types, an asexual multiplication, more or less 

 comparable with the multiplication of cells in a metazoan body, 

 and sexual reproduction, comparable with a similar phenomenon 

 in the higher animals. Several common asexual methods of 

 multiplication occur amongst protozoans, namely, simple fission, 

 or division into two more or less equal parts; budding, or separa- 

 tion of one or more small parts from the parent cell; and multiple 

 fission or sporulation, a breaking up into a number of individuals 

 or spores. Multiplication by one of these asexual methods may go 

 on with great rapidity for a long time, but sooner or later some 

 process at least n^motoly resembling sexual reproduction usually 



