FORM REGULATION IN CERIANTHUS. 2/1 



the internal pressure must be greatly increased and probably the 

 cesophageal walls are more closely appressed than before, since 

 the muscles which separate them are weak. At this stage then 

 little or no water escapes through the oesophagus. When the 

 pressure reaches a certain point the aboral pore is forced open 

 and rapid ejection of water occurs with considerable force. Thus 

 the internal pressure is relieved, and now, as further contraction 

 occurs, separation of the cesophageal walls takes place, and the 

 remaining water passes out of the oesophagus, sometimes carry- 

 ing with it mesenterial filaments in case of violent contraction. 

 The point to which I desire to call especial attention is that when 

 contraction begins the internal pressure closes the oesophagus all 

 the more tightly, and it is not until this pressure is relieved by 

 escape of water through the aboral pore that opening of the 

 oesophagus can take place. Though few observations have been 

 made on other species of actinians, I am inclined to believe that 

 distension and collapse may be accomplished in much the same 

 manner in other members of the group, the cinclides or other 

 openings taking the place of the aboral pore. It is probable, 

 however, that in many forms the muscles which separate the 

 cesophageal walls are more powerful than in Ccriatitluis and so 

 are able to bring about separation in spite of the internal pres- 

 sure. It is perhaps needless to state that the ejection of mesen- 

 terial filaments through the cinclides or mouth is purely passive, 

 the filaments being merely carried out with the water. 



One other point may be mentioned in this connection : if the 

 preceding observations and suggestions are correct it follows that 

 the form of the oesophagus usually observed in transverse sec- 

 tions, viz., an oval with grooves at one or both ends (Fig. i) is 

 not what might be called the natural form, but a form resulting 

 from contraction. Practically all fixed specimens of actinians 

 are more or less contracted. It is extremely probable that 

 when the animal is distended in the normal manner the cesopha- 

 geal walls are always closely appressed except in the region of 

 the siphonoglyphe. Dilation occurs of course in the taking of 

 food, but it probably does not extend over the whole length of 

 the oesophagus at one time and so does not cause any great loss 

 of water. That the oesophagus is widely open in contracted 



