FORM REGULATION IN CERIANTHUS. 28 1 



is probably due to the combined effect of the currents about the 

 whole circumference, but the localized effect is much greater, 

 giving rise to the long marginal tentacles. In its earlier stages 

 the intermesenterial chamber formed between two regenerating 

 mesenteries is simply a small blind pocket into which the water 

 passing orally along the wall of the enteron is continually forced. 

 In the normal animal there is a current passing aborally along 

 the stomodaeum (see Fig. 2), thus carrying the water out of the 

 pocket, but in the earlier stages of regeneration the stomodaeum 

 and disc are absent ; /. c., no cilia, or few, are present to produce 

 a current in the aboral direction. On the other hand the current 

 passing orally is not necessarily diminished by section. The 

 consequence is that water is continually forced into the oral end 

 of the intermesenterial chamber and strikes the body-wall with a 

 certain pressure, but no means for its removal exists except as 

 the incoming water displaces it. 



It is clear without further discussion how the localization of 

 the regenerating tentacles and their dependence upon the pres- 

 ence of mesenteries can be accounted for as a reaction to the 

 tension produced by internal water-pressure. 



LOCAL INHIBITION OF TENTACLE REGENERATION. 



The shape of the collapsed pieces after section and before 

 closure and distension is various. Frequently some part of 

 the oral end as well as other regions becomes involved in 

 some fold or wrinkle in consequence of collapse. The new 

 tissue binds the cut surfaces together however they may happen 

 to lie, but if distension occurs within a few days the folds are 

 soon obliterated because the growth of the new tissue corre- 

 sponds to the tension. If, however, distension be prevented by 

 keeping open the aboral end while the oral end is permitted to 

 close, the new tissue uniting the cut surfaces thickens and becomes 

 more resistant to tension in the course of a few days. 



After two weeks or more, even if the piece is allowed to close 

 aborally and become distended, the fold may persist for a long 

 time, since union of the oral end has occurred in the folded con- 

 dition and the new tissue is now so resistant that only very grad- 

 ual change occurs. It is easy to see that when such a piece be- 



