﻿DUERDEN] morphology of coral polyps 97 



generally folds over the tentacles and greater part of the disk, <Uk- 

 to the action of the circular musculature; but in some few cases the 

 disk and tentacles remain wholly exposed, even when the greater 

 part of the polyps is withdrawn within the skeleton. The term edge- 

 zone, or Randplatte, so frequently employed in coral literature, refers 

 to the lower, pericalicular portion of the column wall, the thecal wall 

 having divided the polyp vertically into intra-calicular and extra-calic- 

 ular parts, each containing portions of the mesenteries and gastro- 

 coelomic cavity. 



( oral polyps in general remain retracted or partly retracted during 

 the day, and expand to their full degree only at night time or when 

 placed in the shade (negatively phototropic I. 



TENTACLES 



The tentacles of coral polyps are arranged mostly in alternating, 

 hexameral cycles, the inner larger than the outer, as in the majority 

 of hexactinians. Usually the cycles are close together around the 

 margin of the disk, hut in Fungia, Siderastrea, Agaricia, and one or 

 two other genera the individual tentacles are widely separated and 

 spread over nearly the whole of the disk. One tentacle arises from 

 each mesenterial chamber, and thus corresponds in position with 

 the septa below; in Agaricia and some other genera the outermost 

 cycle of exotentacles is wanting, and no instance of the stichodactyli- 

 nous arrangement (/'. c, in radial rows, several tentacles from each 

 chamber) has been found. Perhaps the majority hear a knob of 

 stinging cells at the apex, and smaller batteries along the stem. All 

 are simple in form, except in Siderastrea, where the entoccelic mem- 

 bers are bifurcated about midway along their length. Frequently 

 the tentacles undergo invagination within the polypal cavity, even 

 when the polyps are fully expanded : and occasionally on fullest 

 expansion they temporarily disappear as outgrowths, becoming part 

 of the discal expansion. 



In fissiparous genera the hexameral plan of the tentacles is alto- 

 gether lost, and in extreme cases (Mccaiidrina, Pectinia) the organs 

 are merelv dievclic, having an inner entoccelic and an outer exoccelic 

 series. 



Frequently in adult gemmiferous polyps the last entoccelic and the 

 exoccelic cycles are hexamerously incomplete, that is, growth ceases 

 before the completion of the last entoccelic cycle commenced : other- 

 wise the cvcles follow the normal hexameral plan with the formula, 

 (\ 6, 12, 24, etc. 



