INTRODUCTION 



carefully observed. It is identical in its essential features with the 

 sexual reproductive phenomena of the colonial Flagellate, Volvox 

 fjlobator. Not only so, but the egg -cells and spermatozoa thus 

 developed and uniting are identical in character with the egg-cells 

 and antherozoids of a vast series of lower and higher plants, and 

 with those of the whole series of Metazoa. A very important link 

 in the genetic relationships of Plants and Animals is thus established. 

 There is no occasion to suppose that they have independently 

 developed the typical form of the male and the female reproductive 

 particles. The plants have inherited this from the Protozoa which 

 gave rise to the earliest chlorophylligerous, phytotrophic organisms. 

 It is perhaps necessary to remark that further observation is 

 necessary in these lowest forms as to the precise steps in the 

 preparation of the nucleus and its chromatin in each of the 

 conjugating gametes for the definite union of fertilisation. There 

 is abundant evidence that it is of the same nature as that which 

 occurs in the sexual cells of higher organisms, but in special details 

 we may have to recognise some differences. 



C. SEPARATION OF THE CLASSES OF PROTOZOA INTO GRADES OF 

 LOWER AND HIGHER STRUCTURE. 



The question as to whether the various classes of Protozoa are 

 to be regarded as nine separately divergent lines of descent, starting 

 from a common primitive ancestry not represented at the present 

 time by any one of them, or whether some of them possess closer 

 genetic relationship inter se than do others, is a very difficult one. 

 It has been proposed at various times to seek for evidence of such 

 closer affinity in the development of a cortical firmer layer of the 

 cell-protoplasm (as in most Sporozoa and in the Ciliata), as opposed 

 to the retention of the uniform viscid character of the protoplasm 

 (Lankester, Ency. Brit., article "Protozoa"), and again it has been 

 considered probable that all those forms which produce temporary 

 lobose or filnmentar extensions of the protoplasm, as locomotor or 

 grasping organs, may have a genetic community of origin which 

 separates them from those provided with either isolated flagella or 

 with " cilia " of vibratile protoplasm. Some or other, however, of 

 the forms which it is found necessary, on account of the affinities 

 indicated by their life-histories and other details of structure, to 

 class as Flagellata (Mastigophora) exhibit combinations of characters 

 which render both these attempts at grouping unsatisfactory. 



We find Flagellata (see the section on this group) which produce 

 extensive amoeboid processes, and yet possess a flagellum, whilst 

 the majority have a distinctly corticate protoplasm. Among the 

 Sporozoa (for which refer to the section on that group in the second 

 fascicle of Part I. of this treatise), which are with these rare excep- 

 tions strongly corticate, we find genera which produce lobe-like and 



