566 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



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

 tocysts and gland cells occur (fig. 72a), and the mesenterial epithelium immediately behind is not 

 always swollen to the degree usual in the Madreporaria. In the lower regions large oval nema- 

 tocysts predominate. 



The filaments on certain of the mesenteries undergo a glandular differentiation 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. 47o). Where best developed the filamental cells, with the exception of the 

 supporting cells, are all modified, and are tilled with a finely granular substance (figs. 69-71). 



Septal invaginations occur within all the entoeu>les and exocceles, but the entocoelic extend 

 by far the most centrally: the exocoelic never appear as more than shallow internal depressions 

 of the skeletotrophic tissues. The entocoelic invaginations are usually twelve in number, and 

 are approximately 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 entocoelic septa will therefore represent the first and second orders, corresponding with 

 the primary and secondary orders of mesenteries, wdiile the twelve septa, corresponding with 

 the exocoeles, 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 commu- 

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

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

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

 the first and second 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. 68), 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 (tig. 73). 



The skeletotrophic mesogleea is everywhere very thin, except at the insertion of the 

 mesenteries on the corallum, where it is broadened and bears numerous desmocytes. 

 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 where echinulations are present. 



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

 the edges of the costaa and septa. At these the layer retains a considerable thickness (fig. (Hi): 

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

 basal area, and the mesogloea 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. 



Zooxanthelhe occur everywhere in the endoderm, and algal filaments penetrate the skeleton 

 throughout. 



Genus SOLENASTRiEA Milne Edwards and Haime." 



Polyps smooth, very close or more distant, united along- a polygonal base, 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. 

 Stomodteal wall ridged. 



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

 invaginations entoccelic 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. Solmastrsea hyades (Dana). 



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



short, united by a well-developed exotheca, which extends beyond the small costse. ( 'a lice- with tree margins, which 



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



■ lit. imperforate, dentate. Kndotheca fairly developed. Gemmation extracalicinal." (Duncan, 1885, p. 107.) 



