8 THE INVERTEBRATA 



intact organism processes which in an intact metazoon we observe as the 

 activities of a complex in which protoplasm is masked and conditioned 

 by other components of the body : in short, in the Protozoa we observe 

 the normal activities of protoplasm more directly than in the Metazoa. 

 Again, a life cycle comprising more than one generation, which is com- 

 paratively rare among metazoa, is universal among protozoa, and its 

 varieties are extraordinarily interesting. Finally, while every meta- 

 zoon is thoroughly an animal, the Protozoa present an unbroken series 

 from wholly plant-like organisms, through various intermediates, to 

 members whose nutrition and behaviour are those of animals — or 

 rather, as we shall see, there are several such series. 



The Protozoa are all of small size. Most of them are minute, ranging 

 from a few thousandths of a millimetre to a little over one millimetre 

 in length: a few reach dimensions of several, or even of many centi- 

 metres, but these for the most part consist of a relatively thin layer of 

 protoplasm (certain mycetozoa). With the small size of protozoa is 

 probably to be connected, not only, as we have seen, the relative un- 

 importance to them of dead skeletons, but also their characteristic type 

 of organization. In larger organisms, the regions differentiated for 

 special purposes must usually be correspondingly larger, and therefore 

 require the services of nuclei of their own, the absence of which is the 

 hall-mark of a protozoon. The actual size varies very much in each 

 group. It is, on the average, least in the Mastigophora. The order of 

 magnitude of certain representative species may be gathered from the 

 approximate magnifications stated for figures below. 



The bodies of the Protozoa vary greatly in shape. Whereas in each of 

 the metazoan phyla there is a fundamental type of body form to which 

 the members of the phylum conform in essentials, however aberrant 

 from it they be, the Protozoa have no such type. When the surface of 

 the protoplasm is virtually fluid and is not retained by a shell, it takes, 

 while it is at rest, a spherical form. When there is a firm surface layer, 

 the individual tends, if it be a flagellate, to have an egg or spindle shape, 

 if it be a ciliate to be bilateral with a spiral twist at one end, in the 

 Suctoria to be cup-shaped. Concerning the body form of the Sporozoa, 

 which are parasitic, no generalization can be made. Bodies of any of 

 these shapes may be anchored, and have then usually a stalk, which 

 may be of dead material as, for instance, in Acineta and Codosiga 

 (Figs. I, 49), or a part of the living protoplasm. In the latter case it 

 has generally a cuticular covering, as in Vorticella (Fig. 2), but may 

 be naked (various flagellates). 



Stalked forms, and occasionally others, may be colonial; that is to 

 say, a number of zooids, each having a nucleus and the shape and 

 complete organization of an individual of related solitary species, are 

 united by protoplasmic connections to form a single living being. The 



