Crustacea — Xiphosura and Cirripedia. 79 



2. — In the Xiphosuea (king-crabs) the head-shield forms a Crustacea, 

 large semicircular buckler (Fig. 138(?), bearing a pair of compound GALLERY 

 eyes and two minute ocelli upon its upper surface, and covering East Side, 

 beneath its convexity (Fig. 138 J) the mouth with its rudimentary Wall-cases 

 lower lip (metastoma) and six pairs of feet, all but the first of }^^ ^^' 

 which act as masticatory organs at their bases, and are also the cases 

 only locomotory organs. The body-segments in living king-crabs 80-82. 

 are fused together, and carry six leaf-like plates upon their 

 under-side, each bearing the branchiae or gills attached to their 

 inner surface. In the larval king-crab, and in some fossil species, 

 these posterior segments of the body were freely articulated, not 

 soldered together into one piece. The last segment of the body 

 forms a long, sword-like, movable telson, or tail-spine. King- 

 crabs first appear in the Upper Silurian ; they are found in the 

 Carboniferous period, in the Oolitic, Cretaceous, and Tertiary 

 periods, and living on the coasts of America and China at the 

 present day. (See Table-case 80 and Wall-case 13.) 



(Ill) Anchoeacephala.. — This section includes the Ehizo- Cirripedes, 



CEPHALA and the Cieeipedia. The Rhizocephala and Cirripedia ^^l^^^^®? 



and 

 have free-swimming larvae, resembling ordinary bivalved Entomo- Lepadidae. 



straca ; but the former, when adult, have no mouth, lose all their 



limbs, and attach themselves by root-like processes to some living Table-case 



Crustacean, upon the juices of which they subsist; the latter attach case 12c. 



themselves, when adult, to rocks, shells, drift-wood, ships, etc., 



and develop a peculiar multi valve shell, either fixed upon a stalk 



(pedunculated, Lepadidce) — "Barnacles " — or attached directly to the 



stone or wood by the surface of their own shell (sessile, Balanidce) 



— "Acorn-shells." These animals possess long cirri, which they 



protrude from their shell, and by the constant movements which 



they keep up convey in a current the food-particles to their 



mouth within the valves of their shell. The Rhizocephala are 



not found fossil ; but the remains of Cirripedia occur both in 



Secondary and Tertiary strata, and have been admirably described 



by Charles Darwin. One form referred to the Cirripedia, named 



Turrilepas, has been met with in the Upper Silurian of Dudley, etc. 



(lY) Malacosteaca. — Passing from the Entomostbaca and their 

 allies, to which nearly the whole of the older fossil forms of Crustacea 

 belong, we arrive at the Malacosteaca, the next and higher division. 



