530 LECTURE XXI. 



is a stunted homologue of one of the valves of the bivalve, and espe- 

 cially of the smaller and flatter valve in the inequivalve bivalves. 



Both valves are simultaneously and equally developed in the oyster, 

 as in the mussel, and form the peripheral layer of the germ-mass, 

 prior to the formation of any muscular organ : the recognisable basis 

 of each valve is prior, indeed, to the fission of the germ-mass, which 

 is preliminary to the formation of the digestive and circulating 

 systems of the two primarily incomplete individuals. The oper- 

 culum, in the operculated univalves, is the subject of a much later 

 growth ; it does not appear simultaneously with the proper univalve 

 shell, nor is it ever equal to that shell in size or similar in shape. 

 Both the external temporary locomotive organs, I mean the vibra- 

 tile cilia, and certain internal organs, are developed before there is 

 a rudiment of the operculum in the univalve : even the permanent 

 locomotive organ, the foot, is plainly established, before the oper- 

 culum makes its appearance ; and when it does so, it is not as a 

 metamorphosis and coalescence of certain of the peripheral cells of 

 the germ-yolk, but as a thickened epithelial layer upon the back part 

 of the previously formed foot. 



And whilst upon the subject of Oken's ideas of the homology of 

 the operculum, we may as well test, by the light of molluscous em- 

 bryology, some others which he has propounded relative to the nature 

 of the bivalve shell. 



" In the mussel," says Oken, " a structure originates for the first 

 time which can be compared with a thoracic cavity. What covers 

 the branchi^ must stand in the signification of the thorax. The 

 pallium or mantle of the mussels is pleura. Their shells are branchial 

 opercula (as in fishes). They are secretions from the mantle, and 

 everywhere accompany the branchiae. The hinge of the shell-valves 

 corresponds to the rachis or spina dorsi. The shells of bivalve 

 mollusks are a calcareo-thoracic box, open in front, inverted behind, 

 and moveable like ribs." * 



Each valve in the Bivalve pre-exists, as we see, to any trace of 

 gill : the first organs which it pi-otects are the stomach, the liver, 

 and the whole length of the intestine ; it does not defend a thoracic 

 or pectoral cavity, but an abdominal one. If the two valves are 

 comparable to opercula they are ventral, not thoracic or pectoral ; 

 and the pallium, or layer of germ-mass lining the valves, in which 



whole body ; the other remains as the mere opercukim, which shuts the opening 

 of the tube." And in the edition of the same work, translated by the " Ray 

 Society," Oken says, " With the one-sided evolution of the mantle, one shell also 

 is only developed, while the other is stunted or placed under arrest. The snail's 

 shell is one of the bivalve mollusk's shells, its operculum is the other. This last 

 is stony, horny, and finally is entirely wanting." 

 * lb. Th. iii. p. 254. 8vo. 1811.* 



