GKEGAKINJLDA, UHIZOPODA, SrONGIDA, AND INFUSORIA. I 



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 em- 

 braces one of the principal types, or plans of modification, 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 existing subdivisions of the classes are well based or 

 otherwise. 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. 



The first class of which I shall speak is the group of the 

 Gregakinida. 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 cockroach, and in earth- 

 worms. They are all microscopic, and any one of them, leaving 

 minor modifications aside, may be said to consist of a sac, com- 

 posed of a more or less structureless, not very well-defined mem- 

 brane, containing a soft semi-fluid substance, in the midst, or at 

 one end, of which lies a delicate vesicle ; in the centre of the 

 latter is* a more solid particle. (Fig. 1, A.) No doubt many 

 persons will be struck with the close resemblance of the struc- 

 ture of this body to that which is possessed by an ovum. You 

 might take the more solid particle to be the representative of 

 the germinal spot, and the vesicle to be that of the germinal 

 vesicle ; while the semi-fluid sarcodic contents might be re- 

 garded as the yelk, and the outer membrane as the vitelline 



