THE MORPHOLOGY OF THE OHElLf^STOMATA. 279 



outer sides of tlie mnrginal zooocin. Each bar is at first 

 narrow, but soon dilates into a thickened, tuberculated, 

 rounded head, the heads being closely apposed to form a con- 

 tinuous calcareous border. From the border a longitudinal 

 vertical ridge stands up at right angles to the frontal 

 epitheca, which it meets. A similar ridge [l] connects the 

 border with the basal epitheca. In older branches each bar 

 may be dilated at two points, and a second longitudinal ridge 

 may be formed on each surface (fig. 37, I, I). 



The marginal thickening,^ thus constituted of a series of 

 calcareous thickenings, gives flexibility as well as strength to 

 the margin of the frond. There can be little doubt that the 

 space beneath the epitheca is a kind of colonial body-cavity. 

 The marginal bars of E. obtecta can thus be regarded as 

 directly comparable with the zooecial bars of E. clathrata. 

 From the irregularity and variability of the bars in the latter 

 I am inclined to regard this as a species in which the calcifica- 

 tion has been reduced from a condition like that in E. obtecta. 



The general arrangement of the viscera in E. obtecta is 

 shown in fig. 34. The operculum has conspicuous lateral 

 flanges, and there is a delicate labium (lb.). The occlusor 

 muscles [occL] are inserted into the apices of the lateral 

 flanges. Some of the fibres of the divaricator muscles {div.) 

 appear to reach the base-line of the operculum, but some are 

 probably inserted into the adjacent part of the floor of the 

 compensation-sac (c. s.). This structure is very large, and in 

 an adult zooecium it underlies the whole of the frontal 

 surface. Fig. 33 shows its appearance in a B-zooecium in 

 back view. The greater part of the basal wall {h. iv.) of the 

 zocecium and part of the polypide have been removed. The 

 tentacle sheath depresses the sac medianly, but the sac bulges 

 out on each side into a strongly convex lateral lobe. Each 

 lobe thus formed is rounded off distally, but by deep focussing 

 the two lobes can be seen to unite on the far side of the 

 tentacle sheath into a single cavity, which can be traced to 



1 A similar tliickeniug is well known in certain other flexible Clieilostomes. 

 See, for example, Levinsen's account (1891, p. 274) of Fiustra carbasea. 



