CIBRHIPODA CRUSTACEA. 250 



CLASS II. CIBEHIPODA. 



This class was for a long time considered to belong to the 

 Mollusca ; but the structure of the nervous system, and the 

 physiological facts which have been ascertained relative to 

 the mode of their development, prove that the cirrhipedes 

 belong to the Articulata. They are marine animals, attached 

 in the adult state either by a peduncle or by a shell to sub- 

 marine bodies. The appendages are in the form of jointed 

 cirrhi, and the body is enclosed in a multivalve calcareous 

 shell. In recent genera the valves are either united or 

 Separated by membranes. In. the fossil forms, the valves of 

 the shell are generally detached. The class comprises two 

 families : the peduiiculated, or AKITIFID^, and the sessile, 



Or BALANID2E. 



The ANATIFIDJE have a multivalve shell, attached by a 

 fleshy peduncle; the valves are united by a membrane. A fossil 

 species of the genus Anatifa is found in the miocene stage of 

 the tertiaries of France. Pollicipes occurs in the Stonesfield 

 slate of England, and in the cretaceous and tertiary rocks. 

 Aptyclms, which was considered by some to be a tooth, by 

 others to be the operculum of an ammonite, is now describe'd 

 by D'Orbigny as an extinct genus of this family nearly 

 allied to anatifa. This singular form lived in the palaeozoic, 

 but attained its greatest development in the oolitic period : 

 it is likewise met with in the chalk. 



The BALAINTD^; have a complicated, many-valved shell, 

 attached to sub-marine bodies; the aperture through which the 

 cirrhi project is closed by an opercular apparatus, composed 

 of several pieces. "We find fossil species of Balanus, Acastra, 

 Coronula, and other existing genera, in the tertiary strata. 



CLASS III. CRUSTACEA. 



This class comprehends all the articulated animals with 

 articulated feet, which have a heart for circulation, and 

 branchiae for respiration. The crabs and lobsters are the true 

 types of the class ; but it comprehends a great many orders, 

 which differ widely from these. The skeleton is in the form 

 of an external crust, or shell, partaking of the" character 



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