550 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



PORITES ASTR.EOIDES Lamarck. 

 (Pis. 1II-V, figs. 28-42. i 



External characters. — The species is met with in abundance in all the regions of coral 

 growth, from a depth of 3 or 4 feet downward; sometimes many colonics occur closely asso- 

 ciated, at other times they arc widely scattered. The colonies form large, incrusting, hemisphe- 

 l'oidal, or nearly spheroidal masses, the surface usually sub-botroidal or with gibbosities. 



The polyps are small and closely arranged, completely hiding the surface of the corallum 

 when fully expanded. In this condition each appears to arise from a pentagonal thecal margin, 

 common to the surrounding polyps, without any division into calicular and perithecal regions. 

 On retraction of the polyps, the column rests upon the septal edges, and the tentacular ring is 

 more central (tig. 34). 



During full expansion the column is smooth, erect, and cylindrical, and so thin-walled as to 

 allow the internal mesenteries to be seen through. The diameter is 2 mm., and the height above 

 the corallum 3 mm.; as a rule the proximal and distal diameters are slightly larger than the 

 middle. DistalVy the column wall passes uninterruptedly into the tentacles, and is rarely folded 

 over the disk: usually, on full retraction, it is merely drawn within the calice, the tips of the 

 tentacles and disk being still visible (fig. 35). 



The tentacles are extremely small, digitiform or acute, smooth, and arranged in a single 

 cycle. They are practically equal in size, but sometimes one of the axial members is slightly 

 larger than the others. In nearly all cases they are twelve in number, but on most colonies a few 

 larger polyps occur in which the tentacles vary from sixteen to twenty-four. Usually they arc 

 about 2 mm. in length. When the polyps are fully expanded the tentacles arc overhanging, and 

 those of adjacent polyps intermingle. The organs are freely introverted, and in some instances 

 they actually disappear, becoming part of the marginal tissues of the column wall and disk; 

 sometimes the column wall of a polyp may he extended to its full degree, but the tentacles arc 

 indicated only by twelve, lighter colored, circular or oval areas at the margin of the disk, each 

 with a minute aperture in the middle (tig. 35). 



The disk is circular, smooth, and very thin walled; the internal mesenteries can he seen 

 through, and their actual arrangement around the stomodsEum determined (tig. 32). The mouth 

 is either circular or slightly oval. 



The colors of the colonics as a whole are very variable, and often brilliant; indeed, the 

 species is one of the most gaily colored of all the West Indian corals, and. occurring in large 

 masses, often becomes an important constituent in determining the general coloration of the reel's. 

 As a rule the colonies are a bright blue, pale yellow, or yellowish green. Various colors occur 

 side by side, and sometimes one portion of a colony will lie blue and another yellowish green. 

 The pale yellow colonies frequently exhibit restricted patches more brilliant than others, some 

 even becoming brownish; other colonies may he a dull yellowish-brown, or even a blackish-brown. 



New polyps arise among the others mainly by intercalary gemmation. In all colonies many 

 young examples with less than twelve tentacles occur. Fissiparous gemmation has been observed 

 on one or two occasions (p. 513). 



Certain colonies have been found which at first sight appeared in the normal healthy 

 condition, but on careful examination no actual polyps could be discerned; tentacles were 

 indistinguishable, and the whole polypal tissues, though pigmented, seemed in a state of decay. 



Examined in the laboratory, the living polyps are seen to he constantly and quickly 

 retracting to a limited degree, and then slowly expanding again. When fully expanded, agitation 

 of the water moves them to and fro. They may retract below the edge of the calice, the tips 

 of the tentacles still showing or wholly covered (tig. 35). 



Numerous parasitic Cirripedes are usually found associated with the colonies, inextricably 

 inclosed by the overgrowth of the corallum. Around these the polyps are smaller and more 

 closely arranged. 



Anatomy mnl histology. — The column wall is thin and delicate throughout, the mesoglcea 

 appearing as a mere separating lamella. In addition to the usual supporting cells and clear 

 eland cells, the ectoderm contains numbers of cells with yellow granular contents (fig. :!tl). The 



