314 



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. 



Nearly 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 

 Zoe'a 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. 



The Decapoda are 

 divided into the 

 Schizopoda, repre- 

 sented by My sis j 

 the Cumacea, repre- 

 sented by Cuma ; 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 AntJira- 

 palcemon (Fig. 267) from the coal measures of Illinois. 

 Eecently a genuine crab (BracJiypyge carlonis) has been 

 described by "Woodward from the carboniferous formation 

 of Belgium. 



Crustacea, especially shrimps and crabs, are sensitive to 



Pig. 266. Megalops of the Crab. After Smith. 



