Morpliolojy of the }[adrej>oniria. 47.5 



IcuiHiig to its extrusion a::jaiii * ; there seems to be no special 

 musculature involved. It is but a step to consider that some 

 of the tentacles may be permanently introverted or even 

 develop in this position, as seems to be the case in Sti/lophont. 

 Thoui^h from Krempf's description the ingrowths in this 

 genus differ in no respect from ordinary introverted tentacles, 

 yet in Pocil/opora an 1 So'iatopnni the structural modification 

 has gone much further than could possibly result from more 

 introversion. Certain of the tentacles in individual polyps 

 of these two genera were probably never formed as out- 

 growths, but in their place we have a solid downgrowth of 

 endoderm and mesogloea, not accompanied by the ectoderm, 

 unless, indeed, the latter has disappeared as a result of its 

 unusual position. 



In a long series of valuable papers appearing in the 

 'Biological Bulletin,' from Oct. 11)03 onwards, Dr. C. M. 

 Child has shown the great influence which internal pressure 

 exerts upon the growth of Actinian polyps {Ceri(inthus),eindl 

 am inclined to think that some such considerations may assist 

 ill the solution of the present problem. 



The dorsal directive entoccele and the exocoele on each side 

 of it being smaller than the other mesenterial chambers, their 

 walls may be subject to less pressure than the others; hence 

 during growth there may not be the same tension on the disk 

 which closes the chambers above and from which the tentacles 

 normally arise. Given a diminution of pressure in any 

 chamber, or a negative pressure such as probably occurs 

 during introversion, we may then have a hollow or solid 

 downgrowth of the tissue which otherwise would form the 

 hollow jirotruding tentacles. In Seriatopora and Stylophora 

 Krempf finds the modified |)olyps dis|)os-id with much regu- 

 larity in the coral stock, and it may be that the regions where 

 they occur are more subject to diminished pressures. What- 

 ever may be the ultimate explanation, it must be confessed 

 that the haphazard character of the downgrowths, both as 

 regards their number in any one polyp and the number of 

 polyps concerned, suggests that their formation is not a 



* It may be tliat the variations in the pressure of the internal polvpal 

 fiuid influencing the intioveision of i.clividual tentacles are brou"-lit 

 abdut by changes in the doiniiiant beat of the internal cilia. However, 

 we have as yet no experimental proof that the internal cilia can change 

 the direction ot their beat, thongli reversal luvs been fully demonstrated 

 for the stomod;f al cilia (see Parker, " The Reversal of Ciliary Movements 

 in Metazoans,'' Am;'r. Joiu-n. Physiol, vol. xiii. 1!>0.5). In colonial cornl.s 

 tlie internal tliiid-prtssure may vary in ditfiirent regions, and thus parts 

 of individual polyj)* may vary in pressure under the inllucuco of 

 neiglibouiing polyps. 



