5<5 



ANIMAL BIOLOGY 



In this necessarily cursory survey of the Protozoa, at least one 

 major point must be forced upon our attention — the versatility 

 of form and function exhibited by these unicellular animals — 

 their adaptation to many and varied modes of life in the most 

 diverse environmental conditions. But their unicellular plan of 

 structure, permitting only cytological differentiation, has obvious 



Micronucleus 



Gullet 



Micronucleus 

 Gullet 



Contractile 

 vacuole 



- ' -li=£ ~ Macronucleus 



s^- Contractile 

 vacuole 



(■ O^S&Sk Contractile 

 vacuole 



fgs- Macronucleus 



Contractile 

 vacuole 



Fig. 28. — Paramecium aurelia dividing by binary fission. (From Lang.) 



inherent limitations which adaptation cannot surmount, while 

 the multicellular type of organization characteristic of all other 

 animals affords, as we shall presently see, opportunities which 

 almost transcend imagination. It makes possible both cytological 

 and histological — tissue — differentiation. However, the potential 

 of evolution of the protozoan type was expressed, it is believed, 

 when it gave rise in the geological past to the stock from which 

 the higher animals have developed. This allowed the powers of 

 adaptation pent up, so to say, in the single cell to find expression 

 in specialization and cooperation in the individual of another and 

 higher order, which multicellular animals represent. 



