138 THE CRUSTACEA 



are attached to rocks, seaweeds, etc., at the bottom, but the species 

 of Lcpas and other genera are found on floating timber and ships' 

 bottoms, while in L. fascicularis, the colonies of which gather 

 round small floating objects such as dead Velellae or Spirula 

 shells, additional buoyancy is gained by a vesicular mass of secretion 

 from the cement-glands. Many Operculata are found attached to 

 or embedded in corals, while the pedunculate Litholrya and the 

 Acrothoracica burrow into corals and the shells of Mollusca. 

 Various species of Pedunculate and Operculata are constantly found 

 attached to large marine animals such as whales, turtles, and sea- 

 snakes, or to the limbs and gills of large Decapod Crustacea. In 

 the case of the operculate 1'ubicinella, found on whales, the shell 

 becomes deeply embedded in the epidermis of the host. The 

 line dividing commensalism from parasitism is definitely crossed by 

 the pedunculate Anelqsma, in which the mouth-parts and limbs are 

 reduced, and the peduncle, embedded in the flesh of a shark, 

 absorbs nourishment by ramified " roots." From this it is but a 

 step to the parasitic and degenerate Ithizocephala, of which the 

 habits have already been indicated. 



The smallest Cirripedia are found among the Acrothoracica, some 

 species of which are only two or three millimetres in length. Most 

 of the Thoracica are much larger, the bulkiest being Balanus 

 psittaeus, of which the shell is stated to reach nine inches in height 

 by two or three inches in diameter. The peduncle of Lepas 

 aimtifera may grow to 16 or 18 inches long. 



PALAEONTOLOGY. 



The characters and phylogenetic importance of some of the 

 Palaeozoic Cirripedia have already been alluded to. It may be 

 added that, like Pollicipes, the still existing genus Scalpellum dates 

 back to the Silurian, and that both are well represented in the later 

 Secondary rocks. Of the extinct genera, the Palaeozoic Turrilepas 

 and the Cretaceous Loricula, already mentioned, are the most 

 important. The earliest undoubted Operculate is Verruca (Asym- 

 metrica) from the Upper Cretaceous. Many of the existing genera 

 of Pedunculata and Operculata are found fossil in Tertiary 

 deposits. 



AFFINITIES AND CLASSIFICATION. 



The great structural differences separating the Cirripedia from 

 the other Crustacea show that they must have diverged very early 

 from the main line of Crustacean descent. The simple biramous 

 form of the trunk-limbs and their number have been regarded as 

 indicating an affinity with the Copepoda, but there is little else to 



