PKOTOZOA. 



First Section. 



Protozoa. 



General Review of the Group. 



§ 60. 



I RECKON among these certain divisions of those organisms wliich, 

 owing to the simplicity of their organisation, are examples of 

 the lowest grades of living forms. The most essential character 

 appears to be the absence of organs differentiated for the most im- 

 portant functions. The imperfect limitation of this division is due 

 to this negative character ; in it^ nothing that is " typical " of all its 

 members, either in the relation of the body to its morphological 

 elements, or in its organisation, can be made out. The want of any 

 histological differentiation is a reason for considering the organisms 

 enumerated in this group, in company with others, which we are 

 wont to regard as lowly plants, as forma of life which stand between 

 the Animal and Vegetable Kingdoms. It is on this consideration that 

 the plan of uniting all the lower organisms which cannot be regarded 

 as Animals or Plants into the Kingdom of the Protista is based. If 

 we recognise this conception, it appears unnecessary to form a division 

 of the " Protozoa." But the knowledge of the stages of organisation, 

 which obtain in the Protista, are of such high value for the compre- 

 hension of animal organisms, that to pass them over completely 

 would not be in accordance with the aim of this book. I have, 

 therefore, retained in this place the division of the Protozoa, and 

 place in it a number of forms which, by the simple conditions of 

 their organisation, and by the low grade of their differentiation, are 

 well adapted for giving an idea of the group. 



First of all I exclude from it those forms which have not a 

 nucleus, that is, which do not pass beyond the cytod stage. The 

 development of a nucleus in the otherwise simple protoplasmic body 



