38 balamhj:. 



recipient compartments are thickened all round, excepting 

 where they receive the alae. This thickened, upper, internal 

 portion of the walls or shell, together with the alae them- 

 selves, form the part called the sheath. The sheath some- 

 times blends insensibly into the lower parts of the compart- 

 ments, and then perhaps it would not be thought to be a 

 distinct element ; but often it is abruptly separated by an 

 overhanging edge (see PL 9, fig. 5 b, 9 b ; PL 20, fig. 1 b ; 

 PL 25, fig. 1, k') from the lower part, and then the 

 sheath greatly complicates the internal appearance, but 

 not the essential structure of the shell. The sheath acts 

 beautifully, like an internal hoop, in strengthening the 

 shell round the orifice, where it is naturally weaker than at 

 the lower or basal end, where it adheres to the surface of 

 attachment : in the upper part of the shell, moreover, the 

 sutures between the compartments do not go straight 

 through, but owing to the aloe projecting and being over- 

 lapped, are extremely oblique ; or the joints, in the language 

 of carpenters, may be said to be broken. 



There is one other point of structure in the shells of the 

 Balanidae, more especially of species like Balanus tintinna- 

 bulum, which adds to their apparent complexity, namely, 

 that the rim or orifice of the shell formed by the upper ends 

 of the compartments, projects considerably above the oper- 

 cular valves. In a young Balanus, immediately after the 

 metamorphosis, the operculum is attached by the opercular 

 membrane all round to the summits of the compartments, 

 and there cannot be said to be any orifice to the shell 

 itself, but only an orifice or slit between the opercular 

 valves ; but during growth, as the compartments are added 

 to at their basal edges, their upper ends are deserted, and 

 cease to enclose the sack, within which lies the animal's 

 body. J [ence the upper ends come to project freely, either 

 quite separately as in some species of Pollicipes, where they 

 cannot be said to form an orifice; or more or less united 

 into a ring so as to form an orifice, as in the different species 

 of Balanidae. It follows, that to understand the real shape 

 of a Balanus, or rather of the cavity enclosing the animal's 

 body, all that part of the shell which projects above the 

 opercular membrane, may, in imagination, be removed as 



