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. 
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 
Zoéa 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. 
Thoracostraca in- 
clude Stomapods, 
! the Schizopoda, rep- 
AN resented by Mysis ; 
bln the Cumacea, repre- 
Wh sented by Cuma ; the 
WV long-tailed Decapods, 
jm 
\ 
N 
| \ 
\ 
i 
Fig. 261.—Megalops of the Crab.—After Smith. 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 Anthra- 
palemon (Fig. 262) from the coal measures of Illinois. 
Recently a genuine shrimp (Palwopalemon) has been de- 
scribed by Whitfield from the Upper Devonian formation of 
Ohio, 
Crustacea, especially shrimps and crabs, are sensitive to 
