7. Anthozoa (incl. Hydrocorallia). A. Zoantharia. 27 



relaxed and undergo partial atrophy, and the aboral region of the body becomes 

 greatly deformed and often ruptures, being unable to sustain the existing pres- 

 sure. In some specimens a functional adaptation to the altered conditions 

 occurs and the body wall gradually acquires the strength necessary to support 

 the pressure. In such cases the partially atrophied tentacles may increase in 

 length but in no case did they attain the length of tentacles of specimens 

 living in burrows. Regulatory tentacles also fail to attain full length when 

 specimens are kept without sand but they do attain full length when the speci- 

 mens are permitted to burrow. 



Child (-) describes observations on Harenactis attenuata, a slender elongate 

 actinian found imbedded in fine sand in False Bay and San Diego Bay, Cali- 

 fornia. Outside its burrow the body wall is unable to support the normal 

 internal pressure and the animal never extends as completely as when in its 

 burrow, a condition which is correlated with the decreased internal pressure. 

 When extended in water the animal cannot maintain itself erect and it gradu- 

 ally decreases in length from day to day and only after a week or two it 

 may stand erect. At first these shortened anemones have transversely folded 

 and wrinkled body wall but extensive regulatory changes occur, especially in 

 the folds where resorption of tissue probably takes place. The tips of the 

 tentacles atrophy, so that the tentacles are reduced to about half their original 

 length, in consequence of the decreased internal pressure. Contraction and 

 invagiuatiou of the oral end are usually rapid and considerable in specimens 

 recently removed from their burrows, but as the length of the body decreases 

 this reaction becomes less and less marked due to two factors: (1) the retractor 

 muscles undergo atrophy from disuse, (2) the coelenteron, being smaller than 

 it was originally, is almost wholly occupied by mesenteries, mesenterial filaments 

 and gonads, and little room is left for water, the expulsion of which therefore 

 brings about very little decrease in size. At the end of 4y 2 months the 

 specimens which had been often disturbed had apparently lost all power to 

 attach themselves, the aboral end showed no trace of distinct foot and the body 

 was a rounded sac with little power of elongation or contraction or of invagin- 

 ation of the oral end. Specimens which had been less frequently disturbed 

 from their attachment still retained power of fixing themselves. Mere contact, 

 without attachment, is sufficient to bring about flattening of the aboral end, 

 and then specimens which were originally long and slender acquire a form 

 similar to that of those actinians which are normally sessile on rocks, etc. 

 External factors therefore determine the shape and, to a considerable extent, 

 the size relations of different parts. Specimens removed from their burrows 

 and placed upon the sand usually succeeded in working their way back again 

 into it by means of the attenuated posterior end but, after the regulation 

 described above has taken place, they appear to be incapable of extending 

 sufficiently to insert the aboral end into the sand or else they have lost their 

 earlier method of reaction. The foot region is not markedly different ana- 

 tomically from other parts of the body wall; the area capable of the adhesive 

 reaction, small in animals recently removed from sand, becomes larger in some 

 of the flattened fixed forms while in the unattached rounded ones this reaction 

 appears to be quite lost. 



Rand states that when a piece of a tentacle of Condylactis or Aiptasia is 

 removed by a transverse cut the open cut end of the stump is closed by a 

 concentric in-bending of the wall of the tentacle. The newly closed end is 

 hemispherical except for the presence of a small nipple-like projection at its 

 centre. The closure is effected by the contraction of the circular muscle fibres, 



