20 INTRODUCTION TO ZOOLOGY CHAP. 



Differences between Unicellular Animals and Plants. 



It is often most difficult to draw a line between unicellular 

 animal forms and unicellular plants, for amongst them we 

 find the most primitive forms of each kingdom the forms 

 which we look upon as most closely allied to the common 

 ancestor from which we suppose both animal and vegetable 

 kingdoms to have arisen, along diverging lines of development. 

 We should expect, therefore, as we approach the lower forms, 

 to find those distinctions which are well marked in the higher 

 organisms, becoming less and less well defined, until we get 

 forms of such mixed characters that we cannot say definitely 

 that one is a plant and one is an animal. In spite of this, 

 however, we do find that there are certain characteristics 

 which, as a rule, distinguish unicellular animals from plants, 

 the chief of these being the different modes of nutrition of 

 the two classes. 



Typical animals are dependent for their food 



Nutrition ll P on ^he r g an i c substances formed by other 

 animals or plants, and they take in this food usually 

 in the solid form, the albumens and fats of which it is largely 

 composed not being soluble in water, and digest it within their 

 bodies by the action of a digestive juice secreted by the 

 protoplasm ; hence animals need some special method of 

 grasping and " swallowing " such solid food. If the proto- 

 plasm is not readily penetrable at any point, the cells must 

 develop a special " mouth " and " gullet," down which the food 

 can be passed into the centre of the protoplasm, as in Vorti- 

 cella; and cilia or flagella or pseudopodia must be brought 

 into play to bring the food into the " mouth." This mode of 

 nutrition found amongst animal cells is termed holozoic. 



Typical plant -cells, on the other hand, can 



Nutrition a bsorb the greater part of their food only when 

 it is in the state of solution, but they are able 

 to make use of simple mineral salts and of gases which 

 animals cannot use for food. They absorb through their 

 surface the simpler nitrates and other mineral salts dissolved 

 in the water, but they obtain the carbon they need from 

 the carbon dioxide in the air or in the water surrounding 

 them. Within the plant -body, the carbon is separated and 

 caused to combine with the elements in the dissolved food 



