CRUSTACEA. 219 



foramen near the centre of this body; but tliere is no vestige 

 of either muscle or tendon^ the attachments to which are at 

 each extremity. In fact^ this body is perfectly defined, and 

 can be turned out of the shell without being much injured. 

 When the limb is thrown off, the blood-vessels and nerve 

 retract, thus leaving a small cavity in the new-made surface. 

 It is from this cavity that the germ of the future leg springs, 

 and is at first seen as a nucleated cell. A cicatrix forms 

 over the raw surface, caused by the separation, which after- 

 wards forms a sheath for the young leg.^' 



Metamorphoses. 



The changes of form, which take place in Crustacea pre- 

 vious to their adult condition, are not the least interesting 

 part of their history, which, although hinted at long ago, 

 has only been clearly brought to light, and investigated, 

 within a comparatively recent period. There were certain 

 forms of Crustacea which were not well understood, but con- 

 stitute the genus Zoea of authors. These have been ascer- 

 tained to be nothing but larva conditions of so many forms 

 of higher members of the class. 



In the year 1778, a Dutch Naturalist, named Slabber , pub- 

 lished a work, in which is described and fiijured a crustacean 



