Phylogeny of Animals 507 



once must have been a widespread set of organisms. A gen- 

 eralized illustration of such is given in Fig. 20. This probably 

 was primitively a holociliate organism whose entire surface 

 was stimulotactic. But, through repeated and continued stim- 

 ulation in relation to the earth below, to the light above, and 

 to the surrounding medium from which food and other mater- 

 ials were derived, the sensory cilia became largely or wholly 

 restricted to a lower or geotactic patch, an upper or heliotactic 

 patch, and one or two circlets above and below the lines of the 

 alimentary and excretory apparatus. Such is the example 

 shown in the figure, and which is typical of many adult Roti- 

 fera, of larval stages of Annelida, Brachiopoda, Mollusca, etc. 



Mention in last sentence of "adult Rotifera" as closely 

 resembling larval trochophores of more highly developed in- 

 vertebrates might suggest the view that the class Rotifera 

 ranks amongst the most primitive, if it is not the most primi- 

 tive, of the great classes from which the higher invertebrates 

 and the vertebrates have sprung. This view we propose now 

 to open up. 



Many zoologists, during the past quarter century, such as 

 Hatschek, Zelinka, Korschelt, and others, have remarked on 

 the striking structural resemblance of many rotifers to the 

 trochophore or the still more advanced veliger stage of mol- 

 luscs, to the trochophore of annelidan worms, or to the like 

 stage in the history of Polyzoa. But we believe that an even 

 closer and also wider phylogenetic connection exists than has 

 hitherto been suspected, between different genera of rotifers 

 and the higher classes of invertebrates. In now attempting 

 to trace this a helpful explanation may be furnished of many 

 distributional problems that have hitherto seemed inexplicable. 



As already explained (p. 415) the Rotifera are, and seem 

 to have been, of fresh-water origin, derived probably from a 

 colonial ciliate infusorian ancestry. Their eggs after fertili- 

 zation probably gave origin to a multicellular organism, whose 

 cells either by epiboly or invagination assumed different posi- 

 tions and took on different functions. Such a history is re- 

 capitulated in the development of each adult rotifer. The 

 great majority as already shown (p. 387) are fresh-water, a 

 few show considerable adaptability to fresh-water and marine 

 life, relatively few are x)urely marine. 



A generalized picture of an individual shows, as in a trocho- 

 phore larva, three areas that make up the body mass, viz.: 

 an apical or cephalic, a somatic, and a basal. But tendency 

 to constriction into minor areas, especially of the two latter 

 regions, may occur. The apical area may be uniformly cili- 



