270 SIDNEY F. HARMER. 



("b"). 'J'lie zocEcia to wliicli the two kinds of opercula 

 respectively belono- also show some dimorphism. In the 

 A-zooecia the condyles for the aiticnlation of the operculum 

 ore long- recurved teeth (fig. 21), while in the B-/ooecia they 

 are shoi't tubercles. The distal calcareous wall of the 

 A-zooecium is at the same time the proximal wall of the next 

 zooecium in the same longitudinal line. In the B-zooccium 

 this is not the case. The distal zoa3cium has a proximal 

 wall (j). u'.) of its own, from which some of its calcareous bars 

 may spring, and this is mnch thinner than the distal wall 

 (d. w.) of the proximal zooecium, from which it is separated 

 by a narrow cresceiitic space, ])assing about half round the 

 operculum of the proximal zooecium. This suggests that 

 the B-zocEcia possess a vestigial ovicell. The condition of 

 my specimens unfortunately prevents Tue from ascertaining 

 whether the production of ovaries is limited to B-zooecia. In 

 two cases counted at random, about three A-zooecia occurred 

 for every B-zooecium, no regular arrangement of the two 

 kinds being apparent. The additional breadth of the 

 B-operculum is correlated with a slightly increased trans- 

 verse diameter of the zooecium itself, immediately on the 

 proximal side of the operculum. This makes an appi-eciable 

 difference in the capacity of the zocccium, a fact which is in 

 favour of the view that the B-zocecia are female. Similar 

 differences in the opercula are commonly met with in other 

 Cheilostoraes, in which the operculum of an ovicell-beai'ing 

 zoa3cium may be wider than that of the ordinary zooecia. 



Two features in E. clathrata demand especial attention, 

 namely, the compensation-sac and the operculum. For the 

 discovery of the compensation-sac Jullien (1888, 1) is 

 entitled to full credit,^ although his results have been re- 



1 See my preliminary paper (1901) on tliis subject. Jullicn's accounts of 

 tlie compensation-sac were very short, and liis figures were not adequate. He 

 nowhere brinf,'S out the importance of the sac in tlie discussion of the mor- 

 Iiholoiry of the Ciieilostomala. So far as I know, he mentions its parietal 

 muscles in only one phicc Tiiis is in the ex|)lanation of a figure of Cribri- 

 lina figularis (1888, ii, p. 272), iu which he uses words which seem to 



