552 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



protoplasm are accumulated, the first mentioned often formintr a vevy regular row. The layer 

 is thinner in the upper region of the pol^'^j, and the vacuolisation is not so complete; but even 

 here the cellular distinction is almost lost, except as regards the clear gland cells (figs. 38, 39). 



The mesoglita is very thin throughout, and the skeletogenic ectoderm is distinguishable 

 only at the growing points, as a deeplj'-staining layer without distinct cell limitations {cal, fig. 

 38). The desmoidal processes are ever3^where feebly developed. 



The individuality of the polyps is usually retained throughout their length, both under low 

 magnification of merely decalcified material and throughout microscopic sections. Each polyp 

 is, however, seen to be connected with the others, at difl'erent points along the whole of its 

 length, b}' the complicated svstem of very short canals which penetrate the skeleton; in transverse 

 sections at any level five or six canals are nearly always connected with each polyp (figs. 'J'K -12). 

 The polyps are truncated below, as in the case of the imperforate corals; the canals are thus 

 practically restricted to the lateral regions, not prolonging the polyp basally. 



The gastro-cffilomic cavity of each polyp is most distinct in the upper region, as here the 

 septal invaginations are either wanting (figs. 28, 30), or proceed but a short distance centrallj-, and 

 rarelj' with any constancy all round. Both entocoelic and exocoslic invaginations may occur, 

 and at nearly any level one or other, or both, maj' be wanting. The septa in Poritea are 

 perforate, and in the polyp this is represented b\- interruptions in the invaginations. As the 

 lower regions are approached, the septal invaginations become more pronounced, and meet the 

 spine-like, columellar invaginations in the middle, thus cutting ofl' one portion of the cavity 

 from another (fig. 39). In transverse sections the septal invaginations are nowhere as regular 

 as in nonperforate corals (cf. figs. 42, 49). 



To such a degree may the subdivision of the coelenteron proceed toward the terminal region 

 that, in sections, the individual polyps seem constituted only of a series of canals, some containing 

 mesenteries, but mostly empty. 



In addition to the basal canalicular connections, the ccelentera of the polyps communicate 

 with one another over the edge of the theca, as in the case of other corals (figs. 31, 40). 



Gonads have not been found in any of the polyps examined. In the outer polypal wall 

 appear many spheroidal bodies, which, at first sight, may be taken for spennaria. They never 

 occur, however, in the mesenteries, but usually in the endoderm of the column wall or disk 

 (fig. 36). They may be the sporogonia of the parasitic alga^ which are so prevalent within the 

 corallum, but the connection has not been traced. 



II.— CYCLOCNEMARIA. 



MADREPORAKIA IN WHICH THK MESENTERIES BEYOND THE PROTOCNEMIC STAGE ARISE IN 

 ISOCNEMIC PAIRS WITHIN THE PRIiMARY EXOCCELES. THE MESENTERIES IN THE ADULT ARE 

 USUALLY ARRANGED IN TWO OR MOKE ALTERNATING CYCLES. 



B.— SECTION APOROSA. 



MADREPORARIA IN WHICH THE BASAL DISK IS IMPERFORATE, AND THE GASTRO-CCELOMIC 

 CAVITIES OF COLONIAL POLYPS AKK IN COMMUNICATION ONLY AROUND THE PROXIMAL TERMINATION 

 OF THE CpLUMN. 



A.— GEM MANTES. 



ASTRiEID>E IN WHICH ASEXUAL REPRODUCTION TAKES PLACE BY GEMMATION, AND EACH 



POLYP REPRESENTS A DISTINCT INDIVIDUAL. THE TENTACLES, MESENTERIES, AND SEPTA ARE 



ARRANGED IN ALTEKNATtNG CYCLES, AND TWO PAIRS OF DIRECTIVE MESENTERIES ARE PRESENT 



IN EACH POLYP. 



Genus ASTRANGIA Milne Edwards and Haime." 



Polyps smooth, either isolated or connected by a thin narrow basal continuation of the column wall; 

 tissues appearing delicate; perithecal continuation of g-astro-coelomic cavity and mesenteries; form 



n "Colony incrusting. Corallites short, arising from calcareous basal expansions, close, more or less turbinate or 

 cylindriral. Calice circular, deep, and large. Columella papillary, and formed of a network of trabecuhe with 



