GROWTH OF SHELL. 57 



the shell is never* wholly moulted. Even if the margins 

 of the opposed compartments and of the basis were to grow 

 rapidly, the shell would necessarily be much weakened on 

 the lines of suture, and unable to withstand the heavy 

 breakers, to which so many species of sessile Cirripedes are 

 exposed. On the other hand, although the margins are 

 thus compelled to grow slowly, they do not grow con- 

 tinuously, as may be seen in the zones of increment on all 

 the valves, corresponding, I believe, with the periods of 

 exuviation of the membranes of the body. A layer of shell, 

 often very thin, seems to be generally deposited over the 

 whole internal surface of the several valves, at the same 

 time that the marginal zones are added ; so that the only 

 essential difference in the growth of the external covering, 

 in Cirripedes as compared with ordinary Crustaceans, is that 

 the old shell is not cast off, but adheres to the outside of 

 the new shell, and that the margins are added to (in certain 

 definite directions) slowly yet not continuously, instead of 

 the whole being formed at a single period. 



If, now, a section of one of the shelly zones of growth be 

 carefully examined, it can in some cases be distinctly seen 

 to be formed of successive, excessively fine laminae; but 

 the animalised tissue (which differs much in amount in 

 different Cirripedes) left after the shell has been dissolved 

 in acid, exhibits, in most cases, neither laminae nor any other 

 structure whatever. The shell seems to be the actual pulpy 

 corium, or true skin, in a calcified condition, but generally 

 with its cellular structure modified and much reduced : 

 I have taken a bit of recently-formed shell of Tetraclita 

 and of Coronula, with the corium still adherent to its under 

 surface, and after dissolution in acid, I could not distin- 

 guish the part, which had just -before existed as shell, from 

 the corium itself. In the case of Coronula, immediately 



* In the genus Alcippe, and in Cryptophialus, the whole of the external 

 membranes are moulted, excepting the surface of attachment ; but then these 

 Cirripedes live in cavities which they form for themselves, and are thus pro- 

 tected. In Lithotrya the membrane of the peduncle, with its little valves or 

 scales, is moulted, but here, again, this very part is protected by the tubular 

 cavity, which the animal forms and inhabits. Neither of these three genera 

 belong to the Balanidae, or sessile Cirripedes, which we are now more especi- 

 ally describing. 



