Gatty Marine Laboratory , St. Andrews. 47 



nuclei externally and a large pale area, probably chordoid, 

 with a well-defined ovoid outliue, in the centre of which is 

 the blood-vessel. Muscular fibres would thus act on the 

 base and up the stalk of the operculum, whilst its rigid 

 tissues distally arc fitted to perform the part of a plug to 

 the calcareous tube. Beyond the lateral subulate processes 

 the distal region of the decalcified operculum presents 

 externally a tough cuticular investment, then a layer of long 

 hypodermic cells with the nuclei near the external border, 

 the central area being occupied by a tough nucleated plasma 

 with small spaces near the external margin, where a thin 

 basement-tissue bounds the hypoderm. 



In vertical section the decalcified operculum has on its 

 convex side the thick cuticle very dense at the rim, then a 

 deep layer of long narrow granular cells, a thin connective- 

 tissue or chordoid centre, and on the concave surfaee 

 (anterior) a narrow belt of reticulated tissue, and externally a 

 cuticular coat about twice the thickness of that on the convex 

 side. When viewed externally the distal (calcareous) 

 region of the operculum presents a minutely reticulated 

 condition all over (after decalcification). 



It has generally been held that the operculum is developed 

 on a modified branchial filament, and hence the occasional 

 occurrence of one on each side, or the facility with which a 

 new organ is produced on the right when the other is lost. 

 "Without calling this view in question, the foregoing account 

 shows that about half the area of the body-wall behind the 

 branchial base is concerned in the production of the oper- 

 culum with its special differentiation of tissues, and that 

 the development -of the branchial filaments occurs in front 

 under different conditions, and rather in association with 

 the vestibule and mouth than with the protective, or it may 

 be in certain cases the reproductive, functions of the oper- 

 culum. The appearance of the inter-filamentar slits after 

 the formation and separation of the opercular stalk point 

 to a wide divergence both of structure and function, though 

 it may be argued that these radical differences may have 

 been evolved slowly in the history of the race. Yet eye- 

 specks or more complex visual organs are never found on 

 the opercula, while they are not infrequent on the branchial 

 filaments ; just as calcareous or other hard structures belong- 

 to the opercula, for the soft cellular thickenings of the tips 

 of the branchial filaments, which characterize certain varieties 

 of Filograna, and which some have supposed to perform 

 opercular functions, can scarcely be placed in this category. 

 Moreover, in some groups the opercula are very variable, 



