Vi PKEFACE. 



the general plan of the work is independent of 

 these artificial divisions. 



In treating of the classification of the Protozoa, 

 it will be seen that the expressions, class, order, 

 family, and genus, ai'e not here made use of. To 

 these terms a definite meaning is, or ought to be, 

 always attached, and it would, therefore, be pre- 

 mature to apply them to the subdivisions of the 

 Protozoa ; the natural groups into which this de- 

 partment is resolvable not having been yet deter- 

 mined with absolute certainty. 



Since the present work is the first of a series 

 of similar treatises, on the several departments of 

 the Animal Kingdom, it has been deemed neces- 

 sary to prefix thereto a brief introduction on the 

 general principles of zoological science. 



Queen's College, Coek, 

 Maj', 1859. 



