38 THE SUB-KINGDOM PROTOZOA. 



Referring to the foregoing diagrammatic scheme, it will be observed that 

 the four primary sections of the Pantostomata, Discostomata, Poly- 

 STOMATA, and EUSTOMATA, inckiding their more important classes and 

 orders as embodied in the preceding table, are circumscribed by a broader 

 and double circular line. Making use of a convenient metaphor, these 

 circular sections with their varied contents may be compared to so many 

 planetary systems or constellations, all derivable from one common centre 

 and indicating at the points where their peripheries are made to intersect, 

 their mutual relationship to, and interdependence on each other. The centre 

 of the entire series and common source from whence, through the process of 

 evolution, all the various types, orders, and classes of the Protozoa have 

 probably through a more or less extensive epoch of time developed, is 

 undoubtedly to be found among the most simply organized Pantostomata, 

 finding there its typical embodiment in the amoeban order, the hypothetic 

 primeval ancestor of which may, for convenience' sake, appropriately 

 receive the generic name of Protaiucvba. Accepting this last-named 

 generic type as the common basis for departure, the dotted lines radiating 

 outwards from it exhibit, so far as it is possible to predicate, the various 

 directions apparently traversed by the several phylogenetic lines or tracks 

 of evolution before their arrival at the more complex and outlying 

 members of the series. Selecting that phylogenetic line connecting the 

 central or amceban group with the most highly differentiated Eustomata 

 typified by the class Ciliata, as a first example in illustration of this 

 proposed scheme, the following explanation may be submitted. The 

 first cycle of development in this direction, exhibiting a transition from 

 the Rhizopodal or Myxopodous towards the Flagelliform or Mastigopodous 

 structural type, is evidently embodied in the group or order of the 

 Trypanosomata. An Avuvba flapping through the water, or other in- 

 habited fluid, through the medium of a flattened, crest-like and undu- 

 lating extension of its lateral margin, constitutes to all intents and pur- 

 poses a representative Trypanosoma. Although among the few known 

 members of this series a flagellate appendage is not as yet perfectly 

 developed, such organ may be said to exist in its most pristine and 

 rudimentary condition in the tag-like prolongation of one extremity of 

 the body that constitutes so important a characteristic of the species 

 Trypanosoma sangninis, as more recently figured by Professor E. Ray 

 Lankester, and reproduced in PI. I. Figs, i and 2 of this volume. 

 The lateral crest-like extension which represents the most prominent 

 characteristic of this order, carries with it an equally, if not still more 

 important significance. A similar structure associated with the title of an 

 undulating crest or membrane, is constantly recurring among the more 

 highly organized groups of both the Flagellate and Ciliate Protozoa, and 

 undoubtedly takes its origin from this source. As illustrations in this 

 direction, reference may be made to the supplementary undulating mem- 

 branes that form a permanent characteristic of certain species of the genera 

 Trichomonas, Hcxamita, and Conchoncma, among the P'^lagellata ; and of 



