ANATOMY OP MUSSA AND EUPHYLLIA. 39 



basal plate becomes consolidated, and its central vacuity filled 

 up. No doubt it was the earliest formed ring-shaped part of 

 the basal plate which de Lacaze Duthiers took for the theca. 

 The first signs of septa are twelve radially disposed ridges of 

 the basal endoderm, which soon project upwards into the 

 cavity of the polyp as a series of folds, the mesogloea and 

 ectoderm being included in the folds. Between the limbs of 

 the folds of the ectoderm are deposited calcareous nodules, 

 similar to, and continuous with, those of the basal plate (see 

 fig. 13). The septa increase in height and thickness as growth 

 proceeds, but always remain covered over by a triple layer of 

 ectoderm, mesogloea, and endoderm, the ectoderm forming the 

 calicoblast layer of the adult polyp. The peripheral extremi- 

 ties of the septa become forked, and, according to von Koch, 

 the forked extremities of adjacent septa unite with 

 one another to form the porous theca. 



These statements of von Koch accord very well with certain 

 peculiarities of structure previously observed by him in Caryo- 

 phyllia, Galaxea lampeyrana, and Mussa, corals belong- 

 ing to the Madreporaria aporosa. He found that in these 

 the theca lies apparently within the body of the polyp, free 

 from the lateral external body wall, and separated from the soft 

 tissues outside by a space which is a part of the coelenteron. 

 The theca is formed by the thickening and fusion of the peri- 

 pheral ends of the septa and is not a separate structure ; where 

 costse are present they are nothing more than continuations of 

 the septa external to the theca. The septa, theca, and costse 

 are everywhere covered by the triple layer of ectoderm (calico- 

 blasts), mesogloea, and endoderm described above. From these 

 relations it will be understood that a portion of the coelenteron 

 lies external to the theca, and this portion may conveniently 

 be described as extrathecal coelenteron. It is divided into 

 chambers by mesenteries corresponding to those which divide 

 the intrathecal coelenteron into radical chambers, the originally 

 continuous mesenteries having been, according to von Koch, 

 cut in two by the fusion of the peripheral ends of the septa, 

 and thus divided into extrathecal and intrathecal portions. The 



