150 ANNULOSA. 



The Cirripedia include three orders, of which only the order 

 Thoracica has ever been found in a fossil condition, or is ever 

 likely to be so. In this order are the common Acorn-shells 

 (Balanidcz) and Barnacles (Lepadidtz), in which the body is 

 protected by a more or less complete calcareous shell. The 

 Acorn-shells are generally known as the " Sessile Cirripedes," 

 because the shell is directly attached by its base to some 

 foreign body, whereas the Barnacles are commonly known as 

 the " Pedunculated Cirripedes," because the shell is supported 

 upon a stalk or " peduncle." Besides these, the order Thor- 

 acica comprises a third family, that of the Verrucidce, in which 

 the shell resembles that of the Balanidcz in being sessile, but 

 differs in being unsymmetrical, and in some other particulars. 



It is with the shell of the Cirripedes that the palaeontologist 

 has to deal ; and we may, therefore, consider briefly the chief 

 parts of the shell in the Sessile and Pedunculated Cirripedes 

 respectively. It will not be necessary, however, to enter into 

 minute details on this complicated subject, and it will be 

 sufficient to indicate the leading facts of importance. 



In the symmetrical Sessile Cirripedes or Balanidcz, commonly 

 known as Acorn-shells, the animal is protected by a calcareous 

 shell formed by calcifications within the walls of the first three 

 cephalic segments. The animal is placed within the shell, 

 head downwards, and is fixed to the centre of a shelly or mem- 

 branous plate, which closes the lower aperture of the shell, and 

 which is termed the "basis" (fig. 96 A, /). The "basis" is 

 fixed by its outer surface to some foreign object, and is some- 

 times compact, sometimes porous. Above the basis rises a 

 limpet-shaped, conical, or cylindrical shell, which is open at 

 the top, but is capable of being completely closed by a pyra- 

 midal lid or " qperculum." Leaving the operculum out of 

 consideration at present, the sides of the shell are seen to be 

 composed of from four to eight separate pieces, valves, or, as 

 they are technically called, compartments. These compart- 

 ments are usually closely contiguous by their lateral margins, 

 and are separated by lines of division or " sutures ; " but they 

 are sometimes anchylosed together. Each compartment con- 

 sists of a central portion, which is termed the "paries 5 ' (fig. 

 96, B/), which is attached by its base to the "basis" of the 

 shell. The "paries" grows downwards, so that the whole 

 shell increases by additions made round the base. The paries 

 of each compartment is flanked by wing-like portions, which 

 differ from the paries in appearance, and are called "radii" 

 and " alag," according to their shape (fig. 96, B, C). Some- 

 times the paries has a "radius" on both sides, sometimes 



