550 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



PORITES ASTRyEOIDES Lamarck. 

 (Pis. III-V, fig.s. 28-42.) 



External characters. — The species is met with in ul)undance in all the regions of eoral 

 trrowth. fi-oin a depth of ;3 or \ feet downward; sonietiiiies many colonies occur closely asso- 

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

 roidal, or nearly s])hin'i)idal masses, the surface usually sul)-botroidal or with gibbosities. 



The polvps are small and closely arranged, completely hiding the surface of the coralhmi 

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

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

 On retrai'tion of the polyps, thi^ 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. Distally the column wall passes uninterruptedly into the tentacles, and is rarely folded 

 o\er the disk; usually, on fidl retraction, it is merely di'awn within the calice, the tips of the 

 tentacles and disk being still visible (lig. 3.5). 



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

 cycle. They are practically e(|ual 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 polj^ps oicur in which the tentacles vary from sixteen to twenty-four. Usually they are 

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

 those of adjacent jwlyps intermingle. The organs art> freely introverted, and in som(> instances 

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

 sometimes the colunui wall of a polyp may l)c extended to its full degree, but the tentacles are 

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

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



The disk is ciri'ular. smooth, and very thin walled; the internal mesenteries can be seen 

 through, and theii- ai'tual arrangement around the stomoda'um determined (tig. 32). The mouth 

 is either circular or slightly oval. 



The colors of the colonies 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 reefs. 

 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 be blue and another yellowish green. 

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

 even becoming brownish; other colonies may be 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 health3^ 

 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 be constantly and quickly 

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

 of the water mo\es them to and fro. They may retract l)elow 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 liy the overgrowth of the corallum. Around these the polyps are smaller and more 

 closely arranged. 



Aiiiifdiinj and Idstology. — The column wall is thin and delicate throughout, the mesoghea 

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

 gland cells, the ectoderm contains numbers of cells with yellow granular contents (tig. 3t>). The 



