THE MOKFHOLOGY OF THE OHEI I.OSTOMATA. 297 



The existence of the epitlieca is responsible not only for 

 tlie ordinary form of secondary calcification^ but it explaius 

 the condition so commonly found in Ce Hep or a, Avhere new 

 zocccia originate on the free surfaces of the old ones. This 

 appears to be the result of the separation of the epitheca from 

 the calcareous wall, the subjacent space (which is morpho- 

 logically part of the body-cavity) increasing in size and 

 formiug the body-cavity of a new zocecium. 



Lepralia pallasiana,^ Moll (PI. 17, fig. 41). 



The frontal Avail of the young zooscium is at first 

 Membranipora-like. Calcification begins at the proximal 

 end by the formation of a thin calcareous film, which is per- 

 forated by large, uniformly spaced pores. This film is 

 formed in situ beneath a delicate epithecal membrane. The 

 process proceeds with considerable rapidity until the whole 

 of the frontal wall is calcified, with the exception of the 

 operculum, between which and the distal end of the zooccium 

 occurs a portion of the frontal wall, with a single row of large 

 pores. In the earlier part of this process the polypide bud 

 is small, and is at the proximal end of the zocecium, being 

 connected with the membranous frontal wall by a cord of 

 cells. This becomes holloAved out to form the tentacle sheath 

 (fig. 41), at the distal end of which appear two thickenings, 

 the opercular glands [op.gl.). When the calcification is com- 

 pleted, the compensation-sac (fig. 41, c. s.) begins to be obvious 

 as a well-marked cavity, extending from the base of the 

 operculum beneath the calcareous frontal wall, and at first 

 nnich shorter than the operculum. Into the floor of the sac 

 are inserted the parietal muscles {p. in.), which, in the im- 

 mature zooccium, radiate even fi-om distant parts of the 

 lateral walls of the zocecium. 



The sac rapidly grows to such an extent that it underlies 

 the whole of the frontal wall, the parietal muscles thereby 

 acquiring an arrangement similar to those of Cribrilina. 



1 lliiicks (.ISSO), p. 2'J?. 



