Evolution of Animals 415 



these may have originated from ciUate infusorians, through 

 repeated division of the nucleus, and succeeding separation 

 of the cell protoplasm into nucleated areas or cytoplasmic 

 masses. On the principle of stereoenergics also, if definite 

 and characteristic lines of energ>^-flow have caused the mor- 

 phological origin of definite layers, structures, or masses of 

 matter in the Infusoria, that have severally become associated 

 with definite functions, such might be continued upward from 

 the unicellular to the multicellular organism, and little altered 

 as to position or relation. 



Now in not a few infusorian genera, such as Lacrymaria 

 and Oxytricha, division of the nucleus and subsequent diffuse 

 distribution of the resulting nuclei occur. Further in the 

 liigher Ciliata, exact differentiation of the cell-substance, first 

 into a surface zone with sensory cilia and often trichocysts, 

 second into a muscular and excretory zone, third into an 

 alimentary zone with mouth, canal, and anus, is shown. Can 

 such a condition then be correlated and connected with any 

 of the simpler Metazoa? 



Of the latter, we regard the Rotifera for many reasons as 

 the foundational group. In the accompanying diagrams 

 (Fig. 14, a, b) are set forth the structural details of a composite 

 ciliate infusor and of a rotifer. Alike in the relation of ventral 

 and dorsal surface during feeding, as in the opening of mouth 

 and anus on these two surfaces ; in the possession of an anterior 

 ciliated wreath; in the differentiation of the external or surface 

 layer into a sensory tissue and the formation therefrom of cilia; 

 in the formation of myonemes or myotomes and of excretory 

 organs from the middle-body substance; in the origin of a 

 mouth, at times of a protrusible pharynx, of an alimentary 

 canal, and of an anus; in the polar actions and reactions shown 

 by both; as well as minuter details that cannot here be exam- 

 ined, both show an exact fundamental agreement. 



As the embryological studies by Zelinka and the mature 

 studies of the Rotifera by many observers show, the demarca- 

 tion of the individual cells of the Rotifera is faint, and becomes 

 increasingly so as one passes from the early to the late embryo, 



