FORM-REGULATION IN CERIANTHUS. 265 



At first glance it may appear that the regeneration of tentacles 

 in these pieces with open enteron cannot be influenced in any way 

 by internal pressure, but a brief consideration of the matter will 

 be sufficient, I think, to show that even in these pieces some degree 

 of internal pressure is possible. It must be remembered first that 

 the circulatory currents running orally in the intermesenterial 

 chambers continue in pieces with open enteron, except where con- 

 traction may bring about the more or less complete obliteration 

 of certain chambers by approximation of the mesenteries. These 

 currents force the water orally into the inrolled region at the 

 oral end. If now the exit of the water from this region be 

 hindered in any way, some slight degree of distention may arise 

 within this rolled portion. Repeated examination of pieces of 

 this kind has led me to the conclusion that the inrolled edge 

 usually presses against, the free margins of the mesenteries with 

 some force and may thus hinder in some degree the exit of water. 

 The diagrams I and 2 will serve to illustrate this point. In these 

 figures a longitudinal section of the oral end is shown, the cross- 

 hatched portion representing the mesenteries. In Fig. I the 

 inrolled edge of the body- wall is shown. 

 Since these pieces represent only a part 

 of the whole circumference of the body 

 the inrolling at the oral end is usually 

 greater than in a complete cylindrical 

 piece, for in the latter the inrolling about 

 the whole circumference soon results FlG x FlG 2 



in mutual pressure between the various 



parts of the inrolled portion which prevents further inrolling. 

 The result of the inrolling is to press the cut surface against the 

 margins of the mesenteries. In the collapsed condition these 

 folded and thickened margins form a dense mass and the inrolling 

 of the cut edge upon this must press them still more closely 

 together. In whatever spaces remain between the mesenteries 

 the cilia force the water orally as indicated by the arrows in Figs, 

 i and 2. The margins of the mesenteries, being more or less 

 closely pressed together, do not readily permit the escape of the 

 water and consequently a certain degree of pressure may be main- 

 tained. Diffusion of water through the walls in this region may 

 also aid in maintaining pressure. 



