CEMENTING APPARATUS. 137 



in a line exactly conformable to the edge of the last-formed 

 slip of basal membrane; and likewise in straight, medial 

 lines under the spoke-like (cut off in fig. 1 a) doubled 

 walls. I have seen, under a high power, the line of splitting, 

 very shortly after its formation, with the two edges ragged 

 and near together, with an extremely narrow, new cir- 

 cumferential slip just formed, between and over the edges 

 of the previously formed slip. What causes the cir- 

 cumferential slip to split so symmetrically, I cannot say : 

 the opposed edges, after a time, become thickened, appa- 

 rently from adhering to the underlying layer of cement, as 

 will presently be described. The circumferential slip con- 

 tinues increasing in breadth till the period of its splitting 

 arrives, by which time it has become much broader than 

 the last-formed slip of basal membrane ; and after the 

 splitting takes place, the interior half towards the basal 

 membrane, forms a new basal slip all round the basis, and 

 the exterior half adds a new slip to the membrane invest- 

 ing the walls. This latter membrane being inflected under 

 the basal edges of the walls, is, during the growth of their 

 edges, drawn straight down, the newly-formed portion 

 taking the inflected position. 



In the sectional diagram, (1 b) the circumferential slip is 

 not yet broad enough to split ; when it has become so, it will 

 split under the letter (6). The slips of basal membrane are, as 

 may be seen in fig. L a, narrower towards the circumference ; 

 but the two or three last-formed slips, are out of proportion 

 narrower than the others; and it is certain, from the com- 

 parison of the basal membranes of specimens of different 

 ages, that these will afterwards increase in width.* I 

 have seen no other instance, in Cirripedes, of growth in 

 membranes, except at their extreme margins : I suspect 

 that these last-formed slips are pulled, during the downward 

 and outward growth of the shell, a little from over the last- 



* In the case of one young shell, I found that the previously-formed circum- 

 ferential slip must have split, long before it had assumed its proper and ordinary 

 width ; for the last-formed slip of basal membrane was of extreme narrowness, 

 and would have to be considerably added to in width, whilst the new and 

 narrow circumferential slip was likewise being added to in width. This slip 

 of basal membrane, though so extremely narrow, had its own cement-ducts and 

 glands. 



