THE MORPHOLOGY OF THE OHEILOSTOMATA. 283 



Cribrilina is vei'y sti-iking. Tliey arch over a greatly 

 reduced frontal membrane, into which parietal muscles are 

 inserted in the immature condition of the zooscium (fig. 17). 

 The bars are calcareous tubes, opening by a foramen into the 

 body-cavity just external to the frontal membrane, and each 

 bar has a minute membranous fenestra near its tip. After 

 the complete formation of the bars there is fomid (in the 

 fertile zooecium) a stage in the development of the com- 

 pensation-sac precisely like that shown in fig. 15 in a 

 zooecium without an ovicell, and it can now be seen that the 

 operculum is continuous with the floor of the compensation- 

 sac, a wide opening into which is left between the operculum 

 and the first pair of bars. In the mature fertile zooecium the 

 compensation-sac extends under the greater part of the 

 frontal shield, as in the ordinary zooecia. 



There is little difference between the relations of the frontal 

 bars in the fertile zooecia of E. episcopal is and that found 

 in certain species of Cribrilina (e. g. C. figularis), in 

 which the frontal membrane surrounded by the bars is of 

 reduced extent. 



In the immature ovicell of E. episcopalis (fig. 17) the 

 inner calcareous wall is a concave plate (^'. w.) lying on the 

 surface of the zooecium next distal to that to which the ovicell 

 belongs. The outer calcareous layer (o. tt-.) rises up con- 

 centrically outside it, and between the two is a mass of living 

 tissue. It is impossible not to be struck by the resemblance 

 between the development of the ovicell and that of the frontal 

 bars. The ovicell may be compared with two greatly ex- 

 panded bars, composed, like the others, of two layers of 

 calcareous matter surrounding living tissue. 



The median keel of the mature ovicell represents the 

 line along which these bars meet, and corresponds, I believe, 

 with a complete septum between their cavities. It may thus 

 be suggested that the ovicell is formed by the fusion of a 

 pair of greatly expanded oral spines, the bases of which 

 should communicate with the fertile zooecium on each side 

 of the operculum. I cannot claim to have pi'oved this to 



