276 SIDNEY F. HAKMEJR. 



gives ribo to thu 12-sliaped mark seen in an external view of 

 the operculum (fig. 21). 



In decalcified sectious the calcareous part of the condyle is 

 seen to be surrounded by a strong layer of chitiu. In longi- 

 tudinal sections this has the appearance indicated in tigs. 25, 

 2G (co^id.), the chitin being developed especially on the 

 proximal side of the condyle. Fig. 24, which represents an 

 open operculum, shows that this chitinous investment is pro- 

 longed into the sharp edge {scl.) Avhich surrounds the cavity 

 in which the operculum lies (see also figs. 27, 29). This edge 

 is continuous with a layer of chitin forming the outer wall of 

 the cavity (figs. 28 — 31) ; and this, in its turn, becomes con- 

 tinuous with the deep end of the vertical flange of the oper- 

 culum. The flange is strongly bent inwards so as to lie along 

 the surface of the condyle. 



I think there is great probability that these arrangements 

 are in the nature of a spring, and that if the partially opened 

 operculum shown in fig. 29 be imagined to be closed by the 

 occlusor muscles, the shape of the lateral flanges and of the 

 buttresses will be somewhat distorted by the surfaces of the 

 condyles. The elasticity of the operculum would thus tend 

 to commence the opening of the orifice and of the compensa- 

 tion-sac. This supposed elasticity probably gives a reason 

 for the free ends of the buttresses in the distal part of the 

 operculum, an arrangement which would facilitate the slight 

 alterations in the vertical flanges during the movements of 

 the operculum. 



The whole apparatus, consisting of the operculum, the 

 labium, and the chitinous cutting edge which surrounds the 

 cavity containing the operculum, is obviously one of remark- 

 able perfection, and is well adapted to defend the entrance 

 to the tentacle-sheath. 



The most external part of the vestibule is lined by a 

 distinct layer of cuticle (fig. 26), which is continuous with 

 the cuticle of the operculum and with that of the labium. 

 The actual " biting " surface of the jaws is constituted by a 

 thickened ])art of the cuticle, as shown in the same figure. 



