MADREPORARIA. 



193 



forked. The theca is formed by the junction (complete in the Aporosa, incom- 

 plete in the Porosa) of these forked extremities of the septa. 



From this account it is obvious that the theca must not only project into 

 the cavity of the polyp in exactly the same way as do the septa, and divide 

 it into an extra- and intra-thecal portion (Fig. 154), but also must divide 

 the mesenteries in a similar manner. This is actually found to occur in adult 

 polyps, in which the body- wall projects over the lip of the calycle and lies 

 on the outer side of the theca (Fig. 154, r). 



It appears that when the polyps of a colony are connected by soft tissues 

 (coenosark), the connection is effected by this extra-thecal portion of the polyp, 

 and that the so-called coenenchyme or hard matter filling up the valleys between 

 adjacent polyps is secreted 

 by the ectoderm on the 

 lower side of this connecting 

 coenosark. The coenosark 

 is generally broken up into 

 canals which, in the Porosa, 

 communicate with the coel- 

 enteron of the polyps by 

 apertures left in the theca, 

 and may in some cases, at 

 any rate, be embedded in 

 the superficial layer of the 

 hard coenenchyma. The 

 extra - thecal coelenteron is 

 confined to the upper part 

 of the thecae, and the 

 coenosarkal continuation of 

 it over the coenenchyme 

 (when present) may or may 

 not be broken up by con- 

 tinuations of the mesen- 

 teries. When there is no 

 coenenchyme, and the thecae 

 are isolated from one another 

 except at their base, the 

 living tissues appear to have 

 died away round the basal 

 parts of the thecae. 



As may be gathered from 

 the last statement, the 

 polyps ascend as the thecae grow and forsake the lower older parts of the cup. 

 In their ascent the ectoderm of their basal walls secretes calcareous laminae, 

 which may either completely occlude the cup as the tabulae of Serialopom 

 and PociUopora, or merely stretch as imperfect plates between the septa as the 

 so-called dissepiments. 



Synapticula are more rod-like calcareous structures passing from septum to 

 septum through the mesenteries. The columella (Fig. 155) is a central calcareous 

 projection into the theca rising up from the basal plate, and pali (Fig. 156) are 

 accessory columellae arranged in a circle round the central columella, and 

 sometimes joined to the edges of the septa : they are sometimes looked upon as 





FIG. 155. Vertical section through a polyp of Astroides 

 calycularis (after Lacaze-Duthiers). The mouth-opening, 

 oesophageal tube and mesenteries are seen ; also the 

 calcareous septa between the mesenteries, and the 

 columella Sk (the line from Sk should be produced to 

 the middle of the cup). 



O 



