56 BALAN1D.E. 



the upper part of the shell : I have seen instances of this in 

 Tetracliia porosa, and purpurascens, and in Balanus per- 

 for aim : it appeared, but of course erroneously, as if the 

 lateral growth of the compartments had been subjected to 

 the will of the animal. 



Considering the strength of the shell of sessile Cirripedes, 

 the separation of their compartments one from another and 

 from the basis, during growth, has justly been thought a 

 surprising circumstance. In most Chthamalinae and in 

 some species of Balanus, however, if the shell be boiled in 

 caustic potash, the compartments fall apart with a touch ; 

 this shows that their union is due to animal and probably to 

 organised matter, and the growth of such matter between 

 the opposed edges of the compartments, and their conse- 

 quent gradual separation, offers no particular difficulty. 

 But in many Balani, boiling in potash for hours does not 

 seem even to weaken, in the least degree, the sutures, which 

 are wonderfully strong — the shell oftembreaking rather than 

 yield on these lines ; if, however, the shell be dissolved in 

 acid, the animalised tissue which is left easily separates on 

 the lines of suture, and if this tissue be boiled in potash, the 

 remnants of the compartments fall quite separate. These 

 facts seem to me to show, that the compartments in such 

 cases are joined along the lines of suture by tissue, which 

 must be in a calcified state, but which, nevertheless, con- 

 tinues to grow by intersusception ; in other words, I believe 

 that the tips of the complicated ridges and points inter- 

 locking on the lines of suture, are not separated from each 

 other by films of corium or simple animal matter, but are 

 actually united by corium in a calcified, yet still growing 

 condition. 



In ordinary Crustaceans, the growth is periodical and 

 sudden ; a new and larger carapace, for instance, is formed 

 under the old one, and after the exuviation of the latter, the 

 new one soon hardens, and does not subsequently increase 

 in size ; so it is in the case of Cirripedes, with the membranes 

 of the body, and even with certain parts, as the opercular 

 membrane, of the external covering. But a Cirripede cannot, 

 like a crab, crawl into some crevice and remain protected 

 till its shell becomes hardened ; hence, probably, it is that 



