THE CIRRIPEDIA. 
471 
Lepas , and their absence may be remarked in many instances. 
They are enclosed within the cavity formed by the white looking 
shell, which, however, in Nature is occasionally coloured with a blue 
and even purple tint. When a sessile barnacle is examined — and 
they can readily be kept in the aquarium — the cirri will be seen to 
project through a trap-door apparatus called the “ operculum,” or 
lid of the shell. The shell is made up of five or more side pieces 
connected together with strips of membrane, which are often bril- 
liantly coloured. The pieces or valves are composed of carbonate 
of lime ; but in some kinds of Cirripedia they are formed, like 
AN ADULT SESSILE BARNACLE. ( BalatlUS .) 
the membranes, of chitine, and the whole is lined internally by 
the sac which contains the living barnacle. When there is a 
peduncle one end forms the base of the shell, and the other is stuck 
fast to timber, stones, and sea-weeds, and the living animal is con- 
tinued down into this long neck -like attachment. This peduncle 
varies in length in different kinds ; it is usually flattened, but often 
quite cylindrical, and is composed of very strong, thick, trans- 
parent membrane, which covers a thick true skin, which is often 
coloured in long bands. The outside membrane is either naked 
or clothed with minute pointed spines, or it is penetrated by scales 
frequently of carbonate of lime, which are more or less symme- 
trically placed over parts or the whole structure. The peduncle 
is lined within by three layers of muscles, longitudinal, transverse, 
and oblique, and they produce a gentle swaying movement, which 
is beyond the will of the animal, for the muscular fibres are not 
of that class which is within the influence of volition. The interior 
