MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 549 



invaginations; the contorted mesenteries may tlien be pressed into the various canals, and the 

 individuality of the polypal cavity is largely lost. 



In its simplest condition, as in the free portion of individual polyps, the canal system consists 

 of the following: (1) A series of longitudinal and transverse peripheral canals within the grooves 

 between the contiguous ridges of the skeleton, and more proximally in the depressions .between 

 the echinulations. Their uninterrupted vertical extent is not great (tig. 2), and they become 

 very irregular in the echinulate region; even in the more distal areas the canals are connected 

 laterally, and by this means are placed in communication with one another. Below the costate 

 region of a corallite, both the vertical and lateral canals are so interrelated as to produce a more 

 or less regular peripheral reticulum. (2) As shown in figs. 3 to 6, the peripheral canal system 

 communicates l)y short radial canals with the ca?lenteric cavity of the polyps, several such canals 

 usually- occurring in each transverse section. The broader skeletal region of tig. i reveals, how- 

 ever, that internal enlargements and lateral communications between one radial canal and another 

 make their appearance within the wall of the corallite, so that the thicker regions of the corallum 

 arc penetrated throughout by a canal reticulum. 



At the apex of the corallite, the peripheral system of canals also communicates with the 

 central polypal cavity over the edge of the theca (fig. 2). 



The peripheral canals are somewhat flattened on their outer aspect, and often triangular in 

 transverse sections, but the internal are more circular; so close, however, is the reticulum in the 

 more apical region of the polyps that the canals are rarely seen in circular sections, but as 

 elongated, irregular communicating spaces. In the older parts of the colony, whei'e the space 

 occupied by the skeleton and by the canals is more equal, the latter are in more regular 

 concentric series. 



The lining of all the canals is a very thin unilaminar layer of ciliated endodermal cells, the 

 deeply-staining nuclei of which are arranged with considerable regulai'ity; zooxanthellw are 

 usually absent, but are sometimes found in the skeletal layer of the superficial canals, and 

 even in the older and deeper regions of the corallum. The niesogkea is everywhere a thin 

 supporting lamella. The skeletogenic layer is represented in the growing regions by a broad 

 columnar layer (fig. S), but is scarcely determinable in the more terminal regions. Desmoidal 

 processes are numerous in places, and in the actual growing regions the skeletal matrix may 

 remain after decalcification (figs. 7. IS). 



Ripe polyps seem very rare; ova only have oeen met with on one occasion. 



Genus PORITES Milne Edwards and Haime. " 



Polyps small, distinct, forming' incrusting:, massive, foliaceous, or dendroid colonies, fixed or free; 

 united closely with one another along- a common thecal edge, without perithecal continuation of the 

 mesenteries. Column wall smooth and cylindrical, may partly fold over the disk on retraction, 

 no sphincter. Tentacles small, twelve in number, rarely more, unicyclic, sometimes in bilateral 

 larval stage, digitiform, smooth or tuberculated, introvertible. Stomodseal walls smooth. 



Mesenteries unicyclic, rarely more than six pairs, when increase takes place by the addition of 

 bilateral pairs within the sulcar or sulcular entocoele, only three or four pairs complete and filamentifer- 

 ous. Septal invaginations usually twelve, entoccelic and exocoelic, interrupted, unite centrally 

 (columella) below, forming six distinct loculi, which are abruptly truncated. Gastro-ccelomic cavity of 

 polyps in communication throughout colony by a basal canal system, and also by marginal apertures. 



Asexual reproduction by intercalary columnar gemmation, rarely by fissiparous gemmation. 

 Viviparous. 



Examples. — Poritts at!tneoide.sLia.m., I', di-caricata Les., P. f areata Lam., P. dararid Lam. 



«" Colony ramifying, or in tufts, or foliaceous, often massive, and lobed or low, incrusting or not. A basal 

 'epitheca invariable in the last instance, frequent in all. Corallites with trabeculate and perforate walls, not distinct 

 from those of their neighbors, and therefore without intermediate coenenchyma. Calices small, pentagonal. Septa 

 twelve or less, feebly developed, trabecular or spinulose. A small columella, forming at its free edge a knob or a 

 trabecular point or a style. Pali five or six, and not very distinct from the septal ends, in a circle arouii<l the 

 columella. Endotheca exists sparingly, and may be dissepimental or tabulate, or may be mere stereoplasm." 

 (Duncan, 188.5, p. 187.) 



