294 



ZOOLOGY. 



step upwards. Dana's law of cephalization, or transfer of 

 parts headwards, is more strikingly manifested in the Crus- 

 tacea than in any other animals. 



Xearly all Decapods undergo this decided metamorphosis ; 

 in only a few forms, such as the craw- fish, lobster, and a few 

 shrimps and crabs, do the young leave the egg in the general 



form of the adult, the 

 Zoea stage being rap- 

 idly assumed and dis- 

 carded during em- 

 bryonic life. !Most 

 Crustacea bear their 

 eggs about with 

 them ; in only a few 

 cases, as the Squilla 

 and the land-crab of 

 the West Indies, are- 

 the eggs left by the 

 parent in holes or on. 

 the sea-shore. 



Thoracostraea in- 

 clude Stomapods, 

 the Scliizojxjda, rep- 

 resented by My sis ; 

 the Cumacta, repre- 

 sented by C«7?iay the 

 long-tailed Decapods, 

 such as the shrimps 

 and lobster, called 

 Macrura, and the genuine short-tailed Decapoda, or Bra- 

 chyura. Most of the species of the crabs are confined to 

 tropical seas and live in shallow water. 



The Decapods appeared in the Coal Period, and were rep- 

 resented by somewhat generalized forms, such as AntTira- 

 palcemon (Fig. 262) from the coal measures of Illinois. 

 Recently a genuine shrimp {PalcBopalmmon) has been de- 

 scribed by Whitfield from the Upper Devonian formation of 

 Ohio. 



Crustacea, especially shrimps and crabs, are sensitive to 



Fig. 261.— Megalops of the Crab.— After Smith. 



