REGULATION OF HARENACTIS ATTENUATA. / 



The body-wall usually shows numerous transverse wrinkles 

 and folds at this stage of regulation, but these gradually dis- 

 appear with time. 



2. The Later Stages of Regulation. 



Any distinction between the earlier and later stages of this 

 regulation must be more or less arbitrary. The important point, 

 however, is that what is at first merely a condition of partial 

 contraction and capable of rapid alteration under changed con- 

 ditions, becomes, in consequence of partial atrophy of certain 

 parts and other changes, a relatively permanent form. The 

 transition from the one condition to the other is of course gradual 

 and proceeds at different rates in different parts. 



The first directly visible indication of atrophy occurs at the 

 tips of the tentacles. In Cerianthus (Child '04^, '05) such 

 atrophy of the tips of the tentacles occurs when the internal 

 pressure due to water in the enteron decreases, and as will 

 appear elsewhere, the same process occurs in Harenactis under 

 similar conditions. In the present case the tentacle-atrophy is 

 undoubtedly a consequence of the decreased internal water 

 pressure which follows removal from the burrow. Usually the 

 tentacles underwent reduction to about half their original length 

 during the four and a half months of the experiments, though 

 individual differences were of course considerable. 



As noted above, the transverse folds and wrinkles of the body- 

 wall gradually disappear. Extensive regulatory changes occur 

 throughout the body-wall. In Cerianthus regions of the body- 

 wall which are sharply folded or contracted for any considerable 

 time undergo more or less atrophy (Child, '05, '08) while on the 

 other hand stretched regions grow. The present case is similar : 

 undoubtedly more or less atrophy occurs throughout the body- 

 wall in consequence of the decreased distension, but the atrophy 

 is greatest in those regions where the conditions depart most 

 widely from the original conditions of distension, viz., in the 

 folds. The tissue of these parts is not functioning or is function- 

 ing only in slight degree, and its substance probably undergoes 

 resorption in consequence of the demand of other more actively 

 functioning parts for nutrition. 



