IlS C. M. CHILD. 



The data afforded by the two pieces do not permit a decision 

 between these alternatives. It is probable that extensive modifi- 

 cations of the greatly thickened and differentiated mesenterial 

 margins aboral to the oesophagus occurred before relations be- 

 tween the intermesenterial chambers and the aboral end of the 

 piece approached those at the oral end. If this is the reason for 

 the late appearance of the aboral tentacles it is unnecessary to 

 assume that the tissues of the aboral end are less capable of 

 forming tentacles under proper conditions than those of the oral 

 end or in other words the difference in the time of appearance of 

 oral and aboral tentacles does not indicate the existence of a 

 physiological polarity except so far as the structure at the aboral 

 end was originally different from that at the oral end. 



On the other hand it may be that the aboral ends of such 

 pieces are inherently less susceptible to tentacle-forming stimuli 

 and that, therefore, action of the stimuli during a much longer 

 time than at the oral end is necessary. If it shall be possible in 

 the future to obtain sufficient material of this kind for a thorough 

 histological examination of the alterations in the mesenteries at 

 the aboral ends of pieces we shall be better able, perhaps, to de- 

 cide the question. 



The cesophageal pieces described in a previous paper (Child, 

 '04^) present conditions somewhat similar to those in the two 

 pieces with heteromorphic tentacles, in that the oesophagus unites 

 orally and aborally with the body-wall forming a series of closed 

 intermesenterial chambers within which the circulatory currents 

 pass orally on the body-wall and aborally on the oesophagus. If 

 these currents are factors in tentacle-formation and if they are 

 equal in force in both directions it would seem that tentacles 

 must appear on both oral and aboral ends of such pieces at the 

 same time. Yet in no case has such a result been attained. 

 The cesophageal pieces regenerate short tentacles on the oral 

 end, but the internal pressure soon disappears and regeneration 

 is replaced by reduction. In the two pieces described in the 

 present section the internal pressure continues for a much longer 

 time than in the cesophageal pieces. It appears from this that 

 cesophageal pieces might produce aboral tentacles if the internal 

 pressure continued for a sufficiently long time. But even if this 



