FORM-REGULATION IN CERIANTHUS. 147 



due to cilia continue of course wherever water is present in the 

 intermesenterial chambers. These, however, must be consider- 

 ably reduced in volume and force for several reasons : first, in the 

 contracted condition of the open pieces the surface area of the 

 intermesenterial chambers is much less than in distended speci- 

 mens and the mesenteries are often 'so closely approximated that 

 little water can enter the chambers ; second, the entodermal sur- 

 face is more or less folded and wrinkled in the contracted condi- 

 tion and so the number of cilia effective in producing currents is 

 reduced ; third, it is possible, though this is mere surmise, that 

 the cilia beat less rapidly when the body is less distended, since 

 the impeded circulation of water may reduce their supply of 

 oxygen sufficiently to bring about such an effect. There can be 

 little doubt then that the circulatory currents are at any rate re- 

 duced in volume and force in the open pieces. We find in such 

 pieces a delay in the regeneration of the tentacles in comparison 

 with pieces which are allowed to close and become distended in 

 the usual manner. The fact that tentacles do finally appear in 

 open pieces is due, according to this view, to the impossibility of 

 eliminating completely by the methods employed the circulatory 

 currents in the intermesenterial chambers. I hope in future ex- 

 periments to be able to eliminate these currents more completely 

 than has been possible with the methods employed thus far, and 

 so to determine whether tentacle-regeneration can be completely 

 inhibited by this means. 



In open pieces the tentacles do not regenerate completely, but 

 remain of small size as long as the pieces are open. As soon as 

 closure and distension occur, however, they increase in length. 

 The difference in the force of the circulatory currents in open 

 and closed pieces may account for this difference. The assump- 

 tion is justifiable that a certain degree of pressure causes a certain 

 amount of growth, not indefinite growth, since the tissues as they 

 grow apparently become less sensitive to the stimulus, or react to 

 it in some unexplained manner so that growth gradually ceases. 

 The so-called functional adaptation in bone, in the circulatory 

 system, and in various other tissues indicates that this assumption 

 is correct. Admitting this we may apply it to the case in hand 

 and conclude that the failure of regenerating tentacles to grow 



