GROWTH OF SHELL. 59 



being converted into shell, whereas under the opercular 

 membrane it has been converted and condensed into fine 

 constituent laminae of chitine. Inasmuch as the successive 

 layers of shell, during each period of growth, go on en- 

 croaching on those of the membrane, the line of junction 

 between the shell and chitine becomes oblique or bevelled. 

 The membrane on this bevelled line of junction assumes a 

 slightly different aspect to what it has elsewhere; it be- 

 comes yellowish or brown, thicker and very much tougher. 

 In many genera it is also furnished with a row of small 

 bristles. At the period of exuviation the opercular mem- 

 brane separates just outside this modified portion, leaving 

 the latter adherent, as a rim or slip, on the valves. If, how- 

 ever, the opercular membrane be rudely torn off before its 

 proper period of exuviation, it carries with it the as yet con- 

 tinuous, but already modified, slip. A slightly indented 

 line may sometimes be traced before the period of exuvia- 

 tion, showing where the separation will take place : what 

 produces this line I know not. The coloured, thickened, and 

 modified slips of opercular membrane, which are thus retained 

 adhering to the valves, and which together form an investing 

 membrane, have been considered by most authors as the 

 epidermis ; but they have no more right to be thus called 

 than has any other part of the opercular membrane. Exactly 

 similar slips of membrane are left investing the sheath. So, 

 again, the membrane which, when well preserved, invests 

 the walls of the shell, is made up, as already stated, of 

 successively adherent slips, which originally covered the 

 lines of suture.* 



The little bristles above alluded to, which arise from the 



* In the case of Coronula there is a peculiarity, described in the last section 

 of this Introduction, (under the head of Cementing Apparatus), namely, that the 

 two or three last-formed, exterior zones of the Basal membrane continue for a 

 period to increase in width ; being, as I believe, dragged one from over the 

 other, with fresh laminae of membrane continually thrown down. In this same 

 genus, and inTubicinella, the walls of the shell are invested by membrane, which is 

 doubled inwards under their basal edges ; and as the latter grow, the investing 

 parietal membrane splits and separates from the basal membrane, and is pulled 

 outwards and downwards. This inflected, often broad border of membrane, 

 seems to me more strictly comparable with the opercular membrane, than with 

 those narrow, thickened rims of yellowish membrane which in other Cirripedes 

 cover the suture between the basal edges of the walls and the basis. 



