ARTHROPOD A 173 



their sharp shells are sources of much annoyance to bathers. Bar- 

 nacles encrust the bottoms of ships. ^ Huxley described the barnacle 

 as a " crustacean fixed by its head and kicking food into its mouth 

 with its legs." One of the Cin-ipedia, a parasitic form known as 

 Sacculina, is considered to be the most degenerate of parasites. 

 (Figure 78.) It attacks the abdomen of crabs and causes degenera- 

 tion of the gonads. Barnacles are degenerate crustaceans.^ The 

 animals have lost their independence by attachment of the head or 

 back, and the readjustment of segmental plates to suit the new 

 needs. Embryology of the barnacles shows that they probably 

 arose from free swimming phyllopods. Ruedemann discovered a 

 primitive Balanus attached to the shell of a Cephalopod in the 

 Ordovician shale, which also bears out this hypothesis. The goose- 

 barnacles {Lepas) became fixed by the head, and developed several 

 valves. 



Subclass Malacostraca. — The Malacostraca are usually large 

 Crustacea, with five cephalic, eight thoracic and six abdominal 

 segments, and with a grinding " gastric mill " in the stomach. 

 The Malacostraca include lobsters, crabs, prawns, shrimps and pill 

 bugs. The orders which we shall consider are the Amphipoda, the 

 Isopoda and the Decapoda. The Amphipoda lack a carapace and 

 have an elongated abdomen with three pairs of posteriorly directed 

 feet, and three pairs of anteriorly directed swimming feet. 



The beach fleas ^ Amphipoda^ found on sea beaches, are important 

 food for other marine animals. A boring amphipod, Chelura, 

 attacks piles and wooden ships. The Isopoda have no carapace, a 

 broad flat body, leaflike legs, and seven free thoracic segments. 

 The wood-lice or sow bugs are Isopoda which live a wholly terrestrial 

 life. They feed on decaying vegetable matter but sometimes 

 damage plants in gardens and greenhouses. As they breathe by 

 abdominal gills, they seek moist places. Some of their relatives 

 live in fresh and salt water.'* 



^ Visscher, J. P. 1927. Nature and Extent of Fouling of Ships' Bottoms. Bull. 

 U. S. Bur. Fish., vol. 43, 1927, Doc. 1031. Visscher found that paint of lighter colors 

 should be used to prevent fouling of ships' bottoms while they are in port. Barnacles 

 are stimulated most by light in the field of green and blue. 



^ Clarke, J. M. 1921. Organic Dependence and Disease. N. Y. State Museum. 



* Allee, W. C, who for over fifteen years has been studying animal aggregations in 

 Isopoda and other forms, has recently given an excellent summary, Animal aggrega- 

 tions, Qu. Rev. of Biol., 1927, vol. 1, no. 3, pp. 367-398. Consult also his book, Animal 

 Aggregations, U. of Chi, Press, 1931. 



