child] form-regulation in ccelentera and turbellaria 137 



grew out, replacing the parts of the head removed. Moreover, in 

 this form regeneration of the cephahc gangha was possible. 



The difference between this form and Lcptoplana in this respect 

 may perhaps be due, as has been suggested by others in connection 

 with somewhat similar phenomena in other forms, to differences in 

 the degree of cephalization of the nervous system. 



Though Cestoplana does not give rise to a large amount of new 

 tissue after injury, it completes regeneration in another way, viz., 

 by the formation of the missing parts within the portions of the 

 body still remaining. In pieces without a pharynx a new pharynx 

 regenerates (with certain exceptions noted below) within the old 

 tissue at greater or less distance from the end. Its position in any 

 particular case depends on the region of the body from which the 

 piece in question was taken. In pieces taken from the region an- 

 terior to the old pharynx the new pharynx regenerates in the pos- 

 terior half of the piece. The further anterior the level which the 

 posterior end of the piece represents, the nearer the posterior 

 end is the new pharynx. In pieces posterior to the old pharynx, 

 the new pharynx appears near the anterior end of the piece, 

 provided this represents a level not far posterior to the old 

 pharynx : in pieces from the extreme posterior end of the body, no 

 pharynx is regenerated. I am inclined to believe that the position 

 of the pharynx is determined by the movements and pressure of 

 the internal contents : the grounds for this belief cannot be given at 

 present. A somewhat similar, though not identical explanation has 

 been offered by Bardeen for the position of the regenerating pharynx 

 in Planaria. 



The pieces of Cestoplana undergo a marked change in form, be- 

 coming relatively longer, more slender, and tapering posteriorly. 

 Here, as in Lcptoplana, the change in form can be shown to be due 

 to the longitudinal tension to which the tissue is subjected in conse- 

 quence of the antagonism between forward movement and attach- 

 ment by the posterior end to the substratum. In the posterior por- 

 tions of the body where this tension is greatest the elongation is often 

 so great that the intestine is torn apart and undergoes disintegration. 



One of the most interesting phenomena observed in this form was 

 the disintegration and disappearance of portions of the intestine in 

 specimens kept without food. In such specimens kept for a few 

 weeks in clear water the branches of the intestine in the anterior and 

 posterior regions of the body gradually lose their dark color and 

 become indistinct. Examination under a high power shows that the 

 tips of these branches are actually disintegrating ; the cells and 



