TUE MORFHOLOI.JY Oi' THE CHEILOSTOMATA. 275 



proximal half of the operculum the buttresses agaiu meet the 

 lateral flanges (figs. 29 — 31). 



The distal half of an operculum is shown in fig. 27, which 

 represents a transverse slice of a specimen with its calca- 

 reous parts, cut freehand ^ after embedding in paraffin. By 

 focussing deeply are seen the labium {Ih.) and the curved slit 

 between it and the edge of the operculum. Into the ends of 

 the lateral flanges of the operculum, and at the same time 

 into the two sides of the base of the labium, are inserted the 

 occlusor muscles [occl.]. By focussing less deeply the free 

 edges of the lateral buttresses are seen ; while the cut surface 

 passes through the region of the condyles {coud.). Fig. 26 

 shows that the operculum works on its condyles as a lever of 

 the first order, the occlusors {occl.) and the divaricators {div.) 

 passing on opposite sides of the fulcrum. 



Fig. 28 shows a transverse section of the distal part of the 

 operculum where the buttresses have a free edge. Fig. 29 

 is through the hinge-line. The vertical flange is still deep 

 (cf. fig. 26), and is here strengthened by the buttresses, 

 which have completely fused with it. On the proximal side 

 of the condyles the depth of the flange rapidly diminishes. 

 In the entire operculum a small circular mark (fig. 21, a') 

 appears on each side near the proximal end of the 12-shaped 

 figure. In transverse section (fig. 30) this is seen to be due 

 to the fact that at this point the buttresses leave the vertical 

 flanges and are inserted into the free wall of the operculum, 

 so that at this point the body-cavity is separated from the 

 outside by a single layer of chitin. The small tube thus 

 formed often stains deeply in carmine preparations. I am 

 unable to explain the meaning of this arrangement. The 

 tube does not communicate with the space above the 

 buttresses. On its proximal side the operculum becomes 

 very shallow, as indicated in fig. 31, a section just on the 

 distal side of the basal sclerite. This forms, with the median 

 thickening, a horizontal J.-shaped figure, the outline of which 



' The hard nature of Uie cliitiu makes it difiicull to pifpaie iiucrotoine 

 sections of the opercula. 



