FORM REGULATION IN CERIANTHUS. 6 1 



not due to muscular contraction, for although strong contraction 

 occurs at the time of section the muscles relax within a few 

 moments and before inrolling begins. Moreover, the muscular 

 contraction consequent upon stimulation of pieces after inrolling 

 has occurred causes in most cases a more or less complete un- 

 rolling, provided the inrolling was in the transverse direction but 

 does not affect inrolling in the longitudinal direction. In the 

 light of these facts it is difficult to escape the conclusion that the 

 inrolling is caused by some part of the body-wall axial to the 

 muscular layer, viz., either entoderm or mesogloea. The delicate 

 cellular layer of entoderm cannot be supposed to possess any 

 such elasticity ; there remains therefore only the mesoglcea. 



It now remains to consider whether the different forms of in- 

 rolling described and figured above are all explicable on the 

 basis of elasticity of the mesogloea. For this purpose we may 

 regard the tension as resolved into longitudinal and transverse 

 components. 



As regards the inrolling of cylindrical pieces with transverse 

 cut margins (cf. No. I of these studies, '03^, also Figs. I and 2 

 of the present paper), it is easy to see that it can proceed only 

 a certain distance. Since the elastic tension is present in all 

 parts of the cylindrical piece reduction in size may occur, but the 

 cut ends are the only regions where marked change of form can 

 take place. These are bent inward until the more prominent 

 folds come into contact and the size of the opening is reduced. 

 Beyond this the inrolling cannot go since contact between dif- 

 ferent parts of the margin and the radial folds into which the 

 contracting margin is thrown both oppose further change. The 

 appearance of the radial folds requires a word of explanation. 

 Their presence would seem at first glance to indicate that elastic 

 tension exists only in the longitudinal direction. A brief con- 

 sideration will show, however, that this is not the case. After 

 escape of the water from the enteron and collapse of the body, 

 reduction of the circumference occurs throughout the whole 

 piece, undoubtedly in consequence of the elasticity of the body- 

 wall. There is, however, no physical ground for greater con- 

 traction in the transverse direction at the ends than elsewhere 

 since there is no break in the transverse continuity of the body- 



