MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 561 



MH. i:\astk.KA HYADES i Dana). 

 (I Ms. X-XI1I, figs. 74-91. ) 



External characters. — Free colonies of this species are to be found lying on the sea floor at 

 tho eastern end of Kingston Harbor, often incrusting <>r inclosing some pebble, shell, or other 

 foreign body. 1 have not met with it elsewhere around Jamaica. 



The polyps in any colony may be cither very close together or wide apart, the polygonal 

 divisions between one polyp and another being strongly marked. When the polyps are dis- 

 tended to their full extent, the column wall reaches -J or 3 mm. beyond the corallum, and is 

 cylindrical, or it may be slightly constricted at the middle and enlarge again distally. The 

 wall is adherent to the skeleton only at the polygonal line of union of adjacent polyps. 



Duriny partial expansion the column wall is somewhat infolded within the calice, and the 

 tentacles protrude from beneath, leaving the middle region of the disk and mouth exposed. On 

 complete retraction the calicular portion of the column wall is further depressed and overf olded, 

 almost completely hiding the disk and leaving hut a small central aperture (fig. 74). The extent 

 of the perithecal part of the column during retraction varies much in different regions of a 

 colony, according as the polyps are close together or widely apart. 



The column wall is smooth, thin walled, and ridged and grooved in correspondence with the 

 internal costse and mesenteries. The costal areas are practically equal, not divided into alter- 

 nately small anil large divisions, a- is more usually the case. The corallum shows very distinctly 

 through the thin, extracalicular portion of the column wall. 



The tentacles are in three cycles, arranged according to the formula •!. ti, 12, and correspond 

 in position with the septa. The total, 24, is occasionally departed from: in young polyps il is 

 less, and very rarely it is more. The tentacles of the innermost cycle are 2.5 mm. in length, and 

 differ hut little from those of the second cycle, while they are nearly double in size the members 

 of the la>t cycle On the living polyp they appear as two alternating cycles of larger and 

 smaller tentacles, tin? inner and larger communicating with the entocceles, the outer and smaller 

 with the exocoeles, Their lateral walls are provided with small urticating areas, white upon 

 a transparent background; the apex is white and rounded, or distinctly knobbed. On full 

 expansion of the polyps the tentacles are overhanging, and when all are extended they practically 

 cover the colony, giving to it a delicate, pale brown, fleecy appearance. 



The disk is smooth, circular, and radiately ridged and grooved, with rounded elevations 

 alone- the ridges, corresponding with the denticulations alone- the edges of the septa. The 

 discal diameter during full expansion is about 3.5 mm. 



The peristome is often much elevated, ending sharply in the narrow, slit like mouth. Under 

 some conditions the peristome and mouth are both rounded, and at other times the lips may 

 approximate in the middle, the two ends remaining open and serving as a means of communication 

 between the exterior and interior. The approximation may be so pronounced as to give rise 

 to the appearance of two quite distinct oral apertures. The stomodseal walls display six white 

 longitudinal ridges with alternating grooves on each side, and are capable of almost complete 

 eversion. 



Colonies as a whole are lighter and darker shades of brown, the tissues being delicate and 

 parti v transparent. The basal region of the column wall is often light brown, and the white 

 skeleton shows through; on partial retraction the intracalicular portion of the column wall and 

 tin- disk are dark brown, as well a- the tentacle-. The swollen tips of the tentacles and the 

 stinging areas are colorless, and iridescent green radiations are occasionally present on the disk 

 and on the inner face of the tentacles. In some instances the polypal tissues on the under 

 surface of a colony were perfectly colorless ami transparent, and examination revealed an 

 absence of zooxanthellse from the ejndoderm. 



The polyps do not expand during the day with the same readiness as some species, but at 

 nio-ht are -ecu opened to their full degree, irritation of one polyp is slowly responded to by 

 those immediatelv surrounding. Mesenterial filaments can be emitted through the mouth on 



