FORM-REGULATION IN CERIANTHUS. I 5 I 



functional features may determine the order in which the ten- 

 tacles are crowded out of the single row, and thus more or less 

 exactly the definitive mutual positions of the tentacles. Thus 

 far, however, I have been unable to discover any such differences. 

 The small size of the disc in open pieces as compared with 

 closed pieces has been mentioned frequently in the above discus- 

 sion. Comparison of Figs. 2 and 4 with Figs. 3 and 5 shows the 

 marked difference. In closed pieces the disk soon acquires 

 greater diameter than any other part of the body while in open 

 pieces, its diameter is usually considerably less, and never much 

 greater than that of the more or less completely collapsed body. 

 This difference, like the other noted, can scarcely be due to any- 

 thing but differences in internal pressure. In the regenerating 

 piece growth of new tissue at the oral end occurs at the cut sur- 

 faces and on the tentacular ridge, and the distance between these 

 two regions of growth being slight, the intervening tissue soon 

 becomes involved in the changes. These changes consist, as 

 was noted in my first paper (Child, '03^), of reduction in thick- 

 ness of the body-wall and the production of new tissue. The 

 result is that the whole oral end of the piece becomes covered 

 with thin new tissue, which reacts much more readily to pressure 

 than the old thick tissue of the body-wall aboral to it. The in- 

 ternal pressure, although no greater here than elsewhere in the 

 body, must cause more rapid increase in size here since the tissue 

 is much less capable of resisting tension. If we take into con- 

 sideration the circulatory currents which strike the body-wall 

 around the whole margin of the disc we have an additional factor 

 causing growth of the disc in the marginal portions. On the oral 

 side of this region is the area of thin tissue composing the more 

 central portions of the disc, on the aboral side the thick body- 

 wall. If the former responds more readily to stimuli than the 

 latter, which appears to be the case, it is possible to understand 

 why the disc spreads laterally and acquires a greater diameter 

 than the other parts of the body. At first glance it might appear 

 that if the growth of the disc were due to the pressure of water 

 it should grow only in the oral direction, but I think that the 

 presence of the mesenteries and the different conditions of the 

 new tissues on the two sides of the region of maximum pressure 



