274 SIDNEY V. HAHMER. 



This is a part of tlio i2-shaped figure which is seen in a 

 surface view (fig. 21) of tlie operculum, an appearance which 

 results from the fact that the thickness of the chitin is not 

 everywhere the same. The whole of the region included 

 within the two limbs of the 12 is much thickened (figs. 

 26, 27), while the rest of the free surface, as far as the edge, 

 is composed merely of thin cuticle (fig. 29). From the 

 median thickening a curved chitinous buttress diverges 

 (figs. 28 — 31) on each side, passing down into that part of 

 the body-cavity which immediately underlies the free surface 

 of the operculum. In young zocEcia, the space between the 

 buttress and the outer lamella of the operculum contains a 

 conspicuous epithelium, which secretes the cuticular sub- 

 stance of which the operculum is composed. Remains of this 

 cellular material may be seen, in the same position, in the 

 adult operculum. 



The greater part of the thickness of the operculum is con- 

 stituted by a superficial layer of cuticle which readily stains 

 yellow with picric acid. The whole of the deeper surface of 

 this layer (and therefore both sides of the strengthening 

 buttress which depends from the free surface), is lined by a 

 much thinner layer of chitin (not indicated in the figures), 

 which takes a red colour in sections stained with borax car- 

 mine and picric acid. A two-layered chitinous cuticle has 

 been described in certain Gymnolasmata by Calvet (1900, 

 p. 164). 



The buttress of the operculum is crcsceutic in shape, as 

 seen from above (fig. 21, hiittr.). Thus, if the thin lateral 

 parts of the outer lamella of the operculum were removed, 

 there would be exposed a crescentic cavity, the limbs of the 

 crescent being directed towards the proximal side of the 

 zooecium, the floor of the cavity being constituted by 

 the slightly concave upper surface of the buttresses. Distally 

 the buttress meets and fuses with the vertical flange of the 

 operculum (fig. 26). Laterally the fusion is incomplete 

 distally (figs. 27, 28), so that the cavity above the buttress is 

 here continuous with the general body-cavity. lu the 



