REGULATION OF HARENACTIS ATTENUATA. 13 



capable of the adhesive reaction maybe altered very considerably 

 by external conditions. It is probable that the adhesion of a given 

 region increases the physiological specification of adjoining 

 regions in the same direction, so that if these come into contact 

 with the substratum they may adhere, and in their turn affect 

 other adjoining regions and so on. Under the usual conditions 

 of life other reactions, and especially the extension of the body 

 to the surface of the sand interfere with the attachment of large 

 regions, aborally, so that the disc of attachment usually remains 

 very small. But when we eliminate the factor of extension and 

 induce instead a shortening of the body, as we do in keeping the 

 animals in water without sand, the conditions are favorable, if the 

 aboral end is in contact at all, for bringing larger and larger areas, 

 into contact and an extension of the actual adhesive region is the 

 result. How far such extension might be carried it is impos- 

 sible to say. 



On the other hand, cases like that represented in Fig. 13 seem 

 to indicate that the aboral region may lose the adhesive reaction 

 if it is forced to remain unattached or is detached as often as it 

 attaches itself. Individuals like this which had been unattached 

 for several weeks or more failed to attach themselves even when 

 the aboral region was brought into contact with the bottom of 

 the dish and kept there by propping the animal up in an erect 

 position. Such evidence is of course negative, but I believe it 

 permits at least the conclusion that long continued absence of 

 contact decreases the ability of the aboral end to react to contact, 

 by adhesion. 



4. Conclusion. 



The transformation within a relatively short time of the ex- 

 tremely elongated, burrowing type of body into the short, broad 

 form characteristic of the sessile actinians is of interest in that it 

 indicates the role which external factors play in determining the 

 actual shape, and to a considerable extent the size relations of 

 different parts. The elongated form is apparently impossible 

 without the support afforded by the sand in the burrow. The 

 chief factor in bringing about the change in form when the ani- 

 mals are removed from the sand is probably a change in disten- 

 sion of the body by water in the enteron. In animals kept with- 



