FORM REGULATION IN CERIANTHUS. 



59 



often roll longitudinally soon after section, but by gradual inroll- 

 ing of the ends finally become rolled transversely. 



Loeb ('91) has suggested that the cut edges roll inward be- 

 cause the inner layers of the body- wall are stretched to a 

 greater degree than the outer layers ; this view assumes that 

 all layers are more or less similar in elasticity and therefore that 

 the layer that is most stretched will undergo the greatest con- 

 traction when the tension ceases. It is difficult to understand 

 why one layer of the body should be more stretched than 

 another, since all have been subjected to the same conditions, 

 viz., the tension resulting from the fluid pressure on the walls of 

 the enteron. 



FIG. 17. 



FIG. 1 8. 



FIG. 19. 



FIG. 20. 



FIG. 21. 



FIG. 22. 



FIG. 23. 



FIG. 24. 



I am inclined to believe that this remarkable capacity for roll- 

 ing which the pieces exhibit is due primarily to a difference in 

 elasticity between the different layers of the body-wall, though it 

 may be increased or modified by other factors. The succession 

 of layers in the body-wall is as follows : ectoderm', longitudinal 

 muscles, mesogloea, entoderm. The mesoglceal layer is fibrillar 

 in appearance and, while not as thick as the muscular layer, is 

 well developed. 



Judging from the fact that this layer is not folded or wrinkled, 

 even in strongly contracted animals, the inference that it possesses 



