CIRRIPEDIA. 497 



anterior extremity of the body, with the larval antennae, usually 

 cemented to some foreign body. The capitulum is supported upon 

 the peduncle, and consists of a case composed of several calcareous 

 plates, united by a membrane, enclosing the remainder of the 

 animal. 



The group of the Rhizocephala differs in many respects from that 

 of the typical Cirripedes, cement-glands being absent, and the animal 

 being fixed parasitically to the bodies of other Crustaceans by means 

 of branched root-like processes derived from the metamorphosis of 

 the antennae, while the alimentary canal and limbs are absent. 



The order of the Cirripedia is divisible into four principal divi- 

 sions — the Thoracica, the Abdominalia, the Apoda, and the Rhizo- 

 cephala — of which only the sub-order of the Thoracica is known to 

 possess fossil representatives. This sub-order includes the Acorn- 

 shells and Barnacles, in which the body is protected by a more or 

 less complete calcareous shell ; and as it is with the shell that the 

 palaeontologist has to deal, some details must be given as to its 

 general structure. 



As regards the microscopic structure of the shell, the calcareous 

 tissue of the exoskeleton is exceedingly hard and compact, and 

 shows in thin sections a finely granular structure. In the Lepa- 

 doids the shell-structure is very dense, with an obscure fibrous 

 arrangement, but in many Balanoids a coarse cellular or tubular 

 structure is present, the tubes being of large size, and being crossed 

 by transverse partitions. 



In the symmetrical Sessile Cirripedes or JBalanidce, 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 membranous plate, which 

 closes the lower aperture of the shell, and which is termed the 

 "basis" (fig. 355, a, /). The "basis" is fixed by its outer surface 

 to some foreign object, and is sometimes 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 pyramidal lid or " operculum." Leaving the operculum out of 

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

 posed of from four to eight separate pieces or valves, or, as they 

 are technically called, compartments. These compartments are 

 usually closely contiguous by their lateral margins, and are separ- 

 ated by lines of division or " sutures " ; but they are sometimes 

 anchylosed together. Each compartment consists of a main central 

 portion, which is termed the "paries" (fig. 355, B, p), and which is 

 attached below to the "basis" of the shell. The "paries" grows 

 downwards, so that the whole shell increases by additions made 



vol. 1. 2 1 



