ARTICULATA. 



223 



2.) Entomostracans, which have a horny shell and no 

 abdominal limbs ; represented by the little Water-fleas, 

 (Cyclops^ (Fig. 125,) of 

 our ponds, the King- 

 crabs (Limulus] and the 

 extinct Trilobites. The 

 abdomen of the King- 

 crab is reduced to a 

 mere spine, the append- 

 ages about the mouth 

 are used for locomo- 

 tion, and their eyes are 

 smooth. 



3.) Tetradecapods, 

 small fourteen - footed 

 species ; as the Wood-louse, or Sow-bug, (Oniscus?) found 

 in damp places, and the Sand-flea, (Gammarus,) seen in 

 summer on the sea-shore. 



FIG. 125. Water-fleas : i. Cyclops communis. 

 2. Cypris unifasciata. 3. Daphnia pulex. 



FIG. 126. Metamorphosis of Crustacea, (Carcinus mcenas.} a. Larval or first form. 

 b. Second stage, c. Third stage, d. Final stage, in which the metamorphosis is complete. 



4.) Decapods, having ten legs, as the Shrimp, (Crangon^) 

 Cray-fish, Lobster, (Astacus^) and Crab, (Cancer^ Crabs 

 differ from Lobsters chiefly in being formed for creeping 



