FORM-REGULATION IN CERIANTHUS. 





The second explanation is based upon the possible effect of 

 local pressure due to circulatory currents. It is evident that with 

 approach toward the distal end of the tentacle the circulatory 

 currents must decrease in force and volume since in the confined 

 space of the tentacular cavity the friction between the currents in 

 opposite directions is relatively greater than in the enteron. The 

 force and volume of the currents must also depend upon the 

 degree of distension of the tentacle and body and therefore upon 

 the general internal pressure. If, therefore, the internal pressure 

 be reduced, the tentacular cavity decreases in size, and the circu- 

 latory currents are practically eliminated in the extreme distal 

 portion of the tentacles : local pressure at the tip of the tentacle 

 is thus reduced. It is possible that this is a factor in bringing 

 about a condition of malnutrition. According to this view the 

 atrophy of the distal portion may be the result of the reduction 

 of local pressure in the distal portion of the tentacle in conse- 

 quence of the reduction in force and volume of the circulatory 

 current. The larger, proximal portion is still of sufficient size 

 even under reduced pressure to permit the existence of these 

 currents. 



Since I have mentioned in previous papers certain facts which 

 seem to indicate the possibility that local pressure due to circula- 

 tory currents may constitute a formative factor in CcriantJins the 

 possible effect of these currents is mentioned in this connection. 

 It is possible that both reduction in local pressure and malnu- 

 trition of the distal portions of the tentacles may cooperate in 

 causing atrophy under reduced general pressure. But whatever 

 interpretation may be placed upon the facts, the occurrence of 

 atrophy under reduced internal pressure is clearly shown. 



SUMMARY. 



i. Pieces of C. membranaceus in which the enteron remains 

 widely open to the exterior may show some degree of tentacle- 

 regeneration. In such pieces the inrolling of the oral margin and 

 the pressure of the cut surface against the mesenteries may be 

 sufficient to prevent the escape of water and thus permit the 

 establishment of some degree of internal pressure within the in- 



