ANATOMY OP THE MADREPORARIAN CORAL FUNGIA. 307 



chambers which are lined with endoderm, and if a cast be 

 made of all those chambers it will represent the space occupied 

 by the coelenteron. Such a cast I have attempted to represent 

 in fig. 16. The peripheral chambers of the ccelenteron are 

 divided by the mesenteries into exocoeles and entocoeles ; in 

 those corals in which, as in Fungia, all the septa are entocoelic, 

 the entocoeles are almost obliterated by the septa which rise 

 up within them, but morphologically lie wholly outside them, 

 since every part of the corallum is invested with its proper 

 layers [viz. a layer of cells lying next to the calcareous sub- 

 stance from which the latter is secreted (the calycoblasts of 

 von Heider), a very thin layer of mesogloea, and a layer of 

 endoderm], and is thus separated from the coelenteron by 

 the three layers of tissue which limit every part of the body. 

 Thus, in a cast of the coelenteron, the latter is seen to be 

 broken up into wedges by the spaces which are occupied by 

 the septa (vide fig. 16), and in Fungia these wedges are 

 further perforated by the apertures through which pass the 

 synapticula connecting adjacent septa. 



Further than this, outside the theca (which is basal and also 

 perforate in Fungia), there lies a portion of the ccelenteron, 

 communicating with the intra-thecal chambers by canals which 

 pass through the perforations in the theca, and, like the intra- 

 thecal chambers, divided by the continuations of the mesen- 

 teries into exocoeles and entocoeles {vide^igs. 15 and 16, coel.). 

 These complicated relations cannot, I conceive, admit of rational 

 explanation unless the theory of von Koch be admitted, namely, 

 that the corallum is derived primitively from the basal ecto- 

 derm, and that the theca is formed by the fused peripheral 

 parts of the septa, which in fusing divide the mesenteries, and 

 leave a portion of the coelenteron external to the theca. From 

 his account of the development of Astroides calycularis 

 (21) it appears that the skeleton first makes its appearance as 

 a ring of calcareous nodules situated between the ecto- 

 derm of the basal disc and the surface of attach- 

 ment. As development proceeds radial folds of the ectoderm 

 and mesogloea (mesoderm) are formed, beneath which are 



