n8 



P. B. SIVICKIS. 





and in animals 12-13 mm. in length the posterior fission piece is 

 only 2-3 mm. in length, and in shorter animals it is not only abso- 



Fig. 5. P. dorotocephala, showing level at which fission usually occurs. 

 Fig. 6. P. lata, showing level at which fission usually occurs. 



lutely, but relatively, shorter. In P. dorotocephala the posterior 

 fission piece often divides again after four or five days, when it 

 begins to move about more or less normally. Apparently at this 

 stage the developing head is unable to control the whole length of 

 the posterior fission piece and fission takes place at one of the more 

 posterior zooid boundaries. Such second fission has never been 



