THE MORPHOLOGY OF THE CHEILOSTOMATA. 295 



of its tentacle sheath are connected the two " opercular 

 glands/'^ while somewhat more proximally are inserted the 

 conspicuous paiieto-vaginal bands {p. r. ?m.). The operculum 

 is of delicate Flustrine structure^ and has no special divari- 

 cators. Of the strong occlusor muscles (occL), that of the 

 (apparent) left side is seen to originate from the distal vertical 

 wall, and that of the right side from the lateral wall. A 

 series of groups of parietal muscles (p. vi.) originate from the 

 lateral and proximal walls, and are inserted into the frontal 

 membrane. 



There can, I think, be little doubt that the space below the 

 calcareous roof is the homologue of the similar space in 

 Cribrilina. The main difference between the two spaces is 

 that that of Cribrilina is covered by a series of originally 

 separate bars, while in Umbonula it is from the first a 

 continuous crescentic film. The marginal pores." which are 

 the only ones that occur in the film, have, however, precisely 

 the same relations as the communications between the cavities 

 of the hollow Cribrilina spines and the general body-cavity. 

 I regard the covering of the couipensation-sac in Umbonula 

 (fig. 50) as having been derived from a Cribrilina-like con- 

 dition (fig. 44) by the lateral fusion of the spines with one 

 another, the edges of the spines being indicated by the 

 buttresses, and their cavities by the marginal areola? and by 

 the pores leading from those spaces to the general body- 

 cavity. This involves the necessity of assuming that the 

 outer calcareous layer of the Cribrilina spines is now I'epre- 

 sented by an uncalcified membrane, the epitheca. The very 

 common occurrence of marginal areolas iu Escharine forms 

 may be appreciated by turning over the plates of almost any 

 paper dealing with a collection of recent or fossil forms. 



' Cr. Waters (1892), ]). 272 ("gland-like bodies"), and Calvet (1900), 

 p. 200. 



2 The relations of these pores a'e well siiown in a figure given by Neviani 

 (' Boll. Soc. Geol. lul.,' xv, 1S9G, p. 21). 



