134 



A COURSE ON ZOOLOGY. 



FIG. 72. 



/T 



CRAYFISH SEEN FROM BELOW (letters have the 

 same significance as in the previous figure). 

 M, mandibles ; pm 3 , jaw feet ; ov, eggs held 

 between the abdominal false feet pa. 



which it comes as 

 out of a case. The 

 moulting is accom- 

 panied by an in- 

 crease in size. After 

 this has taken place, 

 the new shell re- 

 mains soft for a cer- 

 tain length of time ; 

 then it also gradu- 

 ally becomes hard, 

 and the animal re- 

 sumes its habitual 

 life. The hardening 

 of the new shell is 

 effected by the aid 

 of a reserve of cal- 

 careous matter that 

 the animal secretes 

 in its stomach, in the 

 form of white, calca- 

 reous disks, convex 

 on one face, and con- 

 cave on the other, 

 and formerly used in 

 medicine under the 

 name crab's-eyes or 

 erabstones. When 

 the new carapace is 

 formed, these con- 

 cretions have disap- 

 peared, having been 



absorbed for the formation of the new shell. 



