36 SIDERASTREA RADIANS. 



the invaginations is not readily established ; in fact, it would be almost impos- 

 sible to determine it without the assistance of the mesenteries. In addition, 

 complications are produced by the fact that the invagiuations are perforated 

 by the synapticula, and thus, in sections, their walls appear discontinuous. 



The gastro-ccelomic cavity in Siderastrea is very complicated in character 

 compared with that of an actinian polyp, or, indeed, with that of many other 

 corals. A double series of divisions, mesenterial and septal, are to be con- 

 sidered, except when the polyp is fully expanded and the column wall and 

 disc are raised wholly beyond the corallum. In this latter condition the 

 polypal cavity is merely divided into entoccelic and exoccelic chambers by 

 the mesenteries, as in an ordinary actinian polyp. Each chamber is 

 prolonged into a tentacle, but there has been found no lateral communica- 

 tion between one mesenterial chamber and another, such as frequently 

 occurs in actinians by means of stomata. 



The ccelomic cavities of the many polyps making up a colony are only 

 partly independent of one another. At the periphery of each polyp aper- 

 tures are found between the united edges of the contiguous column walls 

 and the skeletotrophic tissues, but no apertures or canals elsewhere connect 

 the different polyps. The communicating spaces are intermesenterial in 

 position, the polyps being more or less cut off from one another mesenterially 

 (plate 6, fig. 35). The spaces divide the superficial column wall from the 

 basal disc. Hence in colonial polyps the column wall and base are nowhere 

 in direct continuity with one another, except at the free part of the marginal 

 polyps ; elsewhere they are connected only indirectly by the mesenteries as 

 these pass from column wall to basal disc. 



When the level of the corallum is reached the polypal cavity is encroached 

 upon radially by the septal invaginations, and divided peripherally into inter- 

 septal chambers -or loculi, each of which contains a single mesentery. At 

 first all the chambers communicate with one another toward the middle of 

 the polyp, which is still free from any calcareous deposit (plate 7, fig. 38). 

 At the level of the columella the middle region of the polypal cavity is 

 encroached upon, the septal invaginations extend from the periphery to the 

 center, and the entire cavity is broken up into distinct loculi (plate 7, 

 n g- 39)- These are continued as far as the aboral end of the polyp, where 

 each loculus terminates blindly. 



In addition, the interseptal spaces of Siderastrea, as of other fungids, are 

 encroached upon concentrically by the synapticula which stretch across from 

 septum to septum, and in sections give the discontinuous canal-like character 

 to the polypal cavity seen towards the periphery of figs. 38 and 39, on plate 7. 



