448 THE OPERCULUM. 



it generally occupies the whole length of that lip ; but in 

 the orbicular and many-whorled opcrcula, it is directed to- 

 wards the hinder part of the inner lip, near the angle formed 

 by its junction with the outer. Now every time that the 

 animal adds a new layer on the end of the last whorl, the 

 ojDerculum, to allow of this part continuing to occupy the 

 same position, must make a slight turn backwards on its 

 centre, which is the nucleus of the spire, whether the spire 

 be placed either towards one end or in the centre of the 

 operculum. This rotation on the adductor muscle, although 

 it may at first sight appear improbable, bears a striking ana- 

 logy to several other phenomena of the same kind which 

 are continually taking place in the animal economy : I 

 need only add two well-known and apposite examples : — 

 1st, the gradual change of situation of the adductor muscle 

 as it passes down the pillar of spiral shells, which in some 

 of the long turreted species, such as Turritella archimedes, 

 where there are as many as thirty whorls, must have been 

 carried to the extent of thirty complete revolutions on this 

 part ; and, 2nd, the change of place of the adductor muscle 

 in bivalve shells. In the opercula of the Littorina3 and 

 Naticae, which consist of a few very rapidly enlarging whorls, 

 the motion and consequent alteration of position of the place 

 of attachment is very gradual and slow ; but in those or- 

 bicular opercula which are composed of many gradually en- 

 larging whorls, as in the Trochi and Monodontag, the place 

 of attachment must be continually changing, as many com- 

 plete revolutions being made as there are whorls in the 

 operculum. These are sometimes extremely numerous : in 

 a small specimen of Turbo pica now before me, there are 

 seventeen or eighteen, and in some Trochi I have counted 

 still more. In the spiral opercula, which thus rotate on 

 their axis, the nucleus, which is the centre of motion, is 

 always included in the scar, and adherent to the muscle of 

 attachment : it is often furnished internally with a small 

 spiral process, buried in the muscle, and resembling the end 

 of a screw. On the contrary, in the annular and subannular 

 opercula, which have no rotatory motion, the nucleus is often 

 removed from the point of attachment, as is well illustrated 

 in those of the Strombi. 



In many of those ovate or suborbicular spiral opercula 

 which are formed of a few rapidly enlarging whorls, as, for 

 example, those of the genus Littorina, the inner surface 

 exhibits the lines of growth as well as the outer, there being 

 in these cases no internal deposit. In these opercula the 

 adductor muscle is anterior, and occupies more than half the 



