Phylum Arthropoda 507 



that it literally stands on its head and kicks food into its mouth with 

 its modified thoracic appendages. Balaniis, the common acorn barnacle, 

 lacks a stalk. Both of these barnacles are hermaphroditic. They do 

 considerable damage to submerged portions of wharves and boats to 

 which they become attached. Their weight may become so great as to 

 slow down a boat. The greatest expense involved, of course, is the 

 dry docking and removal of the accumulation of barnacles from large 

 boats. Sacculina is a peculiar root-headed barnacle which is parasitic 

 on crabs. In the adult, it is little more than a much-branched mass 

 of cells that penetrates to all parts of the crab's body. Its identity as a 

 barnacle was not known until its larval stages were identified. 



Fig. 167. — Some marine crustaceans, a, Squilla, the mantis shrimp; b, Homarus, 

 ihe lobster; c, Crago, the edible shrimp; d, Balanus, the acorn barnacle; e, Lepas, 

 the goose barnacle; /, Panulirus, the rock lobster; g, Pagurus, the hermit crab; 

 h, Uca, the fiddler crab, i, Cancer, the edible crab. 



The last subclass of the crustaceans is that containing the larger 

 forms, the Malacostraca. This again represents a varied assemblage 

 of forms ranging in size from a millimeter to crabs of more than 12 feet 

 in diameter. It includes the small fresh-water shrimp, Mysis; the 

 highly modified Nehalia; the marine prawn Penaeus; the dorsoventrally 

 flattened isopods such as the fresh- water Asellus and the terrestrial 

 Armadillium; the marine Idothea; and the laterally flattened amphipods 

 such as Gammarus. The larger forms are all included in the order 

 Decapoda. These include the structurally similar crayfish Cambarus 

 and Astacits, the lobster Homarus, a vast assemblage of highly modified 



