ADULT COLONY. 29 



Owing to the early disappearance of the mesenteries aborally and periph- 

 erally the organs are, at any one time, necessarily perforated for a very 

 restricted part of their course. Furthermore, as a result of the presence 

 of these synapticular bars, any mesentery must be capable of less retraction 

 than in species where the whole length of the organ is free to respond to the 

 action of the retractor muscle. 



Desmocytes, continuous with the mesoglcea of the lining of the inter- 

 septal chambers and the attached mesentery, are usually numerous around 

 the perforations of the synapticula (plate 7, fig. 41, and plate 8, fig. 45). 



MESENTERIAL FILAMENTS. 



Filaments occur on the mesenteries of all three cycles, though usually 

 they are imperfectly developed on the members of the third. On the incom- 

 plete mesenteries the organs commence a short distance above the lower end 

 of the stomodaeum, and are continued throughout the vertical extent of the 

 mesentery ; on the complete mesenteries they start from the lower termina- 

 tion of the stomodseum, the ectoderm of the latter being continuous with the 

 filament. As in all Madreporaria yet described the filaments consist of only 

 a median lobe, supported basally on an expanded mesogloeal axis (plate 7, 

 fig. 42). Trilobed mesenterial filaments having lateral lobes supported on 

 a mesogloeal axis and bearing ciliated bands (Flimmerstreifeu), like those 

 found in most Actiniaria, are not known to occur in corals. 



The fully developed filaments of S. radians are sharply separated from 

 the mesenterial endoderm by a well marked constriction on each side. The 

 endoderm is usually slightly swollen immediately behind the filament, but 

 rarely assumes the form of a definite lobe as in many other coral species. In 

 addition, the outline of the filament in section varies somewhat in the case of 

 the first-cycle mesenteries. At first it is cordate, and, histologically, is quite 

 uniform all round; but soon it becomes nearly circular, and the cellular 

 constituents of the middle part differ from those of the lateral (cf. plate 7, 

 figs. 42, 43). 



The cellular constituents of the filaments are mostly ciliated supporting 

 cells, but with these are mingled large, clear, and granular gland cells and 

 nematocyst bearing cells, especially in the middle region of the sections. 

 Towards the sides and posterior borders the cells diminish in height and 

 are nearly all supporting cells (plate 7, fig. 42). At least two kinds of 

 nematocysts occur a long, narrow, thick-walled form, similar to that in the 

 tentacular knobs, and a large, oval, thin-walled form, with the spiral thread 

 strongly marked (plate 7, fig. 44). 



