GENERAL ZOOLOGY 



by the calcareous shell plates, and the segmental appendages are highly 

 modified for sweeping small particles and microscopic organisms from the 

 surrounding water into the mouth. To paraphrase Huxley, we may think 

 of the barnacle as a small shrimp-like animal, standing on its head in a 

 little house, kicking food into its mouth with its feet. 



The Malacostraca. These higher Crustacea include the forms popularly 

 known as sow bugs, scuds, shrimps, prawns, crayfishes, lobsters, and crabs 

 (Figs. 15.11, 15.12, 15.13). The great majority are marine animals, but 

 many occur in fresh water; and a few, like sow bugs and the land crabs of the 

 tropics, are terrestrial. The general characteristics of the typical malacostracan 

 have been adequately described in connection with the crayfish and need not 

 be repeated. Although internal organization is often modified in correlation 

 with the habits of particular species, and external features are highly specialized 

 in many instances, the fundamental plan of structure remains the same. 



The crabs are examples of highly specialized malacostracans. In the fa- 

 miliar "blue crab," Callinecles sapidus, the eggs begin their development, as in 

 the crayfish, attached to the abdominal appendages of the female. These eggs 

 hatch, producing pelagic larvae of the so-called zoaea stage; these larvae drift 

 with the plankton of the ocean, eventually transforming into a megalops form 

 (Fig. 15.13). Interestingly, the zoaea larva, although it lacks a full comple- 



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