566 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The mesenterial filaments exhibit the usual histological details. Two or three kinds of nema- 

 tocysts and g-jand cells occur (fig. T^a), and the mesenterial epithelium innuediatoly l)ehind is not 

 alwaj's swollen to the degree usual in the Madreporaria. In the lower regions lai-ge oval nema- 

 tocysts'predominate. 



The filaments on certain of the mesenteries undergo a glandular difl'erentiation within a 

 limited portion of their extent, and all stages in the process, from the normal filament to the 

 wholly glandular, can be followed. The general characteristics of these organs have already 

 been given (p. 473). Where best developed the filamental cells, with the exception of the 

 supporting cells, are all modified, and are filled with a finely granular sul)stance (figs. G9-71). 



Septal invaginations occur within all the entoc«les and exocoeles, l)ut the entocoelic extend 

 by far the most centrally; the exocadic never appear as more than shallow internal depressions 

 of the skeletotrophic tissues. The entoc(jelic invaginations are usually twelve in lumiber, and 

 arc ajjproximately equal in their radial extent; six correspond with the mesenteries of the first 

 order, and six with the mesenteries of the second order. Although practically equal in size, the 

 twelve entocadic septa will therefore represent the first and second orders, corresponding with 

 the primai'y and secondary orders of mesenteries, while the twelve septa, cori'esponding with 

 the exocfjeles, will constitute a third order. An examination of the skeleton reveals that a slight 

 distinction in thickness and in radial extent can often be made between the septa of the first and 

 second order, while the members of the third never project far from the thecal wall. The 

 interseptal loculi are not completely separated from one another, that is, they remain in coumni- 

 nication centrally, indicating that the columella does not wholly occupj' the center of the calice. 

 Adjacent septa do not as a rule fuse within the central I'egion, and the center of the calice is 

 occupied for the most part by the large vertical teeth, one of which arises from (>ach septum of 

 the first and secontl orders. Outwardly, the costal evaginations are strongly developed, and are 

 on the same radii as the .septal invaginations. 



The endoderm of the skeletotrophic tissues is a narrow layer in the upper region of the 

 polyps (fig. OS), and contains zooxanthelhe and conspicuous granular gland cells. As the more 

 internal regions are approached, the layer becomes much broader, and its cells undergo the usual 

 vacuolization, most of the contents being aggregated toward the free surface. Zooxanthelhe 

 and gland cells occur, however, as far as the basal extremity (fig. 73). 



The skeletotrophic mesoglcea is everywhere ver}' thin, except at the insertion of the 

 mesentei'ies on the corallum, where it is broadened and bears numerous desmoc3'tes. 

 Corresponding with the smoothness of the theca, and of the faces of the septa, there is an 

 absence of indentations on the skeletal tissues, such as occur whei-e echinulations are present. 



The skeletogenic ectoderm has almost wholly disappeared, except at certain places, such as 

 the edges of the costw and septa. At these the layer retains a considerable thickness (fig. 6(3); 

 longitudinal sections also reveal that the layer may he in an active condition along the extreme 

 l)asal area, and the mesoglcea here becomes a little broader (fig. 73). It is manifest that the 

 activity of the skeletotrophic layer in this region is associated with the formation of dissepiments. 



Zooxanthelhv occur everywhere in the endod(>i'm. and algal filaments penetrate the skeleton 

 throughout. 



Genus SOLENASTR/EA Milne Edwards and Haime." 



Polyps smooth, very close or more distant, united along- a polygonal bas-e, perithecal continuation 

 of the gastro-coelomic cavity and mesenteries very restricted, sometimes none at all; form light, massive, 

 incrusting or free colonies. Column cylindrical, polygonal at the base, on retraction upper part folds over 

 the disk; no sphincter muscle. Tentacles tricyclic and hexamerous, tuberculated, swollen at apex. 

 Stomodaeal wall ridged. 



Mesenteries dicyclic, hexamerous, both cycles filamentiferous, two pairs of directives. Septal 

 invaginations entocoelic and exoccelic, tricyclic, united centrally (columella), thus forming a separate 

 chamber below for each mesentery, transversely truncated below. 



Asexual reproduction by intercalary, marginal, and fissiparous gemmation. 



Example. — ,So/c/ia><tr;tii hyadc.s (Dana). 



""Colony massive but light, convex above, incrusting or tall, often gibbous, rarely plane. Corallitea long or 

 short, united by a well-developed exotheca, which extends beyond the small costiP. Calices with free margins, which 

 are usually circular, but sometimes unsymmetrical in outline. Columella spongy or feebly developed. Septa thin 

 or stout, imperforate, dentate. J^ndotheca fairly developed. Oemmation extracalicinal." (Duncan, 1885, p. 107.) 



