SACK, MUSCLES OF. 61 



I need only further remark, that in some species of Balanus 

 and of Chthamalus, the under side of the shell is penetrated 

 by irregular pores, large enough to be visible to the naked 

 eye, into which threads of corium penetrate ; but these can 

 hardly be said to appertain to the microscopical structure ; 

 and are more nearly related to those pores and furrows, 

 formed by the greater or less development of the longitu- 

 dinal septa, and in which the threads of corium deposit, 

 or rather become changed into, transverse septa, or solid 

 shelly matter, as previously described. 



Sack ; muscles of, 8fc. 



In the pupa, the thorax, as we shall hereafter more fully 

 see, is continuous with, and opens into the large anterior 

 end or front part of the head ; but during the metamorphosis 

 (PL 30, fig. 2), the thorax of the Cirripede becomes, owing 

 to the almost transverse position occupied by the young 

 animal within the pupa, to a great extent internally sepa- 

 rated from the anterior end, — which anterior end forms, 

 as we know, either the peduncle or the basis. Hence 

 it comes to pass that the body or thorax (PL 25, fig. 1) 

 is lodged within a sack (/) within the shell. The chitine 

 membrane lining this sack is excessively thin and transpa- 

 rent, but less so in Xenobalanus and Tubicinella ; it is 

 obviously continuous with that investing the body of the 

 animal ; it is also essentially continuous with the opercular 

 valves and membrane, and consequently with the whole shell. 

 It is periodically moulted. It is lined by corium, as is 

 likewise the surrounding shell ; hence the corium is double 

 round the sack, as indeed might have been expected from 

 the shell and opercular valves (at least their upper parts) 

 being formed by the prolongation, as is obvious in the 

 pupa, of the posterior edges of the carapace. Between the 

 two folds of corium, which are united together by trans- 

 verse ligamentous fibres, branching out at both extremities, 

 like the roots and branches of a tree, we have the longitu- 

 dinal muscles, which go to the opercular valves ; and likewise 

 a layer-like mass of branching ovarian tubes (PL 25, fig. 1 g) : 

 the ovarian tubes, however, are often confined to the base 



