THE CIRRIPEDIA. 4/1 



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. {BaluftUS.) 



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 Lands. 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 



