376 C. M. Child 
may call a chance factor and acts apparently without reference to 
anything in the original organization. 
The other factors undoubtedly play a part, though incidentally, 
in determining where along the line of union tentacle groups shall 
appear and where they shall persist. ‘The first of these factors 
is the distension of the enteric cavity with water. Water can enter 
these rings only through the body wall, of course, but like cesopha- 
geal pieces, they always become more or less distended after 
closure. Very frequently, however, certain regions become 
much more distended than others and in some cases distension 
appears to be confined almost wholly to certain regions. These 
local differences in distension are most marked in cases where a 
considerable portion of the mesenterial organs remains in the 
enteron and are undoubtedly due to the fact that some intermesen- 
terial chambers are filled with the parts of these organs remaining 
and so do not share in the distension of other parts. My previous 
work on actinians has shown that a certain degree of distension 1s 
a necessary condition for the appearance and persistence of ten- 
tacles in restitution and this is true also for Harenactis. It may 
be expected, therefore, that tentacles will be more likely to develop 
in the regions of greater distension than in others. But this fac- 
tor cannot determine the localization of the tentacles in radially 
symmetrical groups, and as a matter of fact such localization is 
most conspicuous in those cases where most of the mesenterial 
organs are removed (e. g., Fig. 29). It is very evidently only an 
incidental factor and one which affects rather the development 
and persistence of the groups than their localization. 
In some cases the movements and contractions of the ring after 
closure bring about a change in position of the contained mesen- 
terial organs and so a change in distension in certain regions. In 
some cases of this kind I have seen tentacle groups which had 
developed undergo reduction and atrophy because the opening 
into the enteric cavity of the ring was plugged with the mesenterial 
organs. Sometimes parts of the mesenterial filaments enter and 
fill the cavities of single tentacles and these tentacles undergo 
atrophy. ‘The effect of distension or its absence 1s evident in these 
rings as elsewhere in regulation, but it certainly does not account 
