n6 



C. M. CHILD. 



a 



a 



structural middle : in anterior pieces the functional middle is near 

 the posterior end ; in pieces from the middle of the body it is 

 near the middle, etc. The new pharynx appears at this func- 

 tional middle or as we may more properly call it the physiolog- 

 ical pharyngeal region. With this interpretation in mind the 



difference in position of the pharynx 

 in the pieces ae, Fig. 2, is readily 

 understood. 



Now we might expect that 

 pieces from the postpharyngeal 

 region would show relations similar 

 in character but reversed, /. t\, 

 a decrease in size of the new pre- 

 pharyngeal region with increas- 

 ing distance from the pharynx. 

 As a matter of fact just the op- 

 posite occurs (Figs. 4, 5, nt, b, c, d) 

 except in very rare cases (Figs. 6 

 and 7) so that the prepharyngeal 

 and postpharyngeal regions are 

 usually equal in size in pieces from 

 the extreme posterior end. 



We must conclude from this re- 

 lation that pieces such as c and 

 d in Fig. 5 are physiologically 

 more anterior than pieces a and b. 



I believe that the occurrence of fission affords a very simple ex- 

 planation of these facts. It is highly improbable that fission 

 occurs in these forms without some physiological preparation 

 even though changes in structure may not be visible. Various 

 authors have noted that fission in certain other turbellaria is ini- 

 tiated by changes in the nervous system leading to the develop- 

 ment of new cephalic ganglia. If such changes are going on in 

 this postpharyngeal region in Platiaria there is no difficulty in 

 understanding why the new prepharyngeal region should be 

 longer in these pieces than in those farther anterior. Moreover, 



FIGS. 6, -. 



7 



