THE WOKPHOLOGY OF THE CHEHOSTOTMATA. 833 



parietal muscles at first radiate to the bnse of the operculum 

 (figs. 13, 14), but their insertions become shifted by the 

 growth of the compensatiou-sac, until in the fully formed 

 zooecium (figs. 33, 46) they liave much the snme relations as 

 those of Unibonula (fig. 11) or of Cribriliua philomela 

 (fig. 9). It seems difficult to avoid the conclusion that the 

 floor of the compensation-sac represents in all cases the 

 original frontal membrane, as indicated in the diagrammatic 

 figs. 43, 44, 50. From this it would appear to follow that in 

 the Lepralioid mode of development the phylogenetic liistory 

 of the compensation-sac implied by those figures has been 

 modified ontogeneticall}'^, in such a way that the apparent 

 frontal membrane of the young zooecium is in reality an 

 epitheca (cf. fig. 12, ep.), that the calcareous frontal shield 

 is next formed beneath the epitheca, and that the frontal 

 membrane only makes its appearance when the compensa- 

 tion-sac is invaginated. The fully developed frontal wall of 

 a Lepralia would, on this view, be represented by a com- 

 pletely fused series of Cribrilinid spines, of which the outer 

 layer is membranous ( ^ epitheca) and the inner layer is 

 Cidcareous (fig. 50) ; the marginal pores on this view corre- 

 sponding with the original communications between the 

 Cribrilinid frontal bars and the general body-cavity. 



Another view is, however, possible. The membranous 

 covering of the young zooecium may be, as it appears to be, 

 the frontal membrane, and the frontal shield may be a cryp- 

 tocyst which grows beneath the membrane towards the distal 

 end of the zooecium. 



Thus in fig. 34 the calcareous frontal wall from which the 

 papilla {cede, p.) is seen to arise would be the cr3'ptoc3"st, which 

 has grown distally beneath the frontal membrane [ep.). The 

 invagination of the compensation-sac would then represent 

 an ingrowth of a fold of the frontal membrane round the 

 free distal border of the cryptocyst. The actu.il ontogeny 

 of the organ would then agree completely with its supposed 

 phylogeny in such cases as Lepralia, Schizoporella, and 

 Euthyris. The Microporoid series of genera would tlius 



