THE GREGARINIDA. 



7 



Brachiopoda. 



Aseidioida. 



Lamellibra nchiata . 



Branchiogasteropoda. 

 Pulmogasteropoda. 

 Pteropoda. 

 Cephalopoda. 



MOLLUSC A. 



Pisces. 

 Amphibia. 



Aves. 

 Mammalia, j 



* 



VERTEBRATA. 



It is not necessary for my purpose that the groups which 

 are named on the preceding table should be absolutely and 

 precisely equivalent one to another ; it is sufficient that the sum 

 of them is the whole of the Animal Kingdom, and that each, of 

 them embraces one of the principal types, or plans of modifica- 

 tion, of animal form ; so that, if we have a precise knowledge of 

 that which constitutes the typical structure of each of these 

 groups, we shall have, so far, an exhaustive knowledge of the 

 Animal Kingdom. 



I shall endeavour, then, to define or, where definition is 

 not yet possible, to describe a typical example of these various 

 groups. Subsequently, I shall take up some of those further 

 classificatory questions which are open to discussion ; inquiring 

 how far we can group these classes into larger assemblages, with 

 definite and constant characters ; and, on the other hand, how 

 far the classes can be broken up into well-defined sub-classes 

 and orders. But the essential matter, in the first place, is 

 to be quite clear about the different classes, and to have a 

 distinct knowledge of all the sharply-definable modifications 

 of animal structure which are discernible in the animal 

 kingdom. 



I. THE GREGARINIDA. 



These are among the simplest animal forms of which we 

 have any knowledge. They are the inhabitants of the bodies, 

 for the most part, of invertebrate, but also of vertebrate, 

 animals ; and they are commonly to be found in abundance in 

 the alimentary canal of the common cockroaph, and in earth- 



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