XXXVIU INTRODUCTION. 



phalanx. The author has found that a small glandular- 

 like body exists at this spot in each of the limbs, which 

 supplies the germs for future legs. This body completely 

 fills up the cavity of the shell for the extent of about half 

 an inch in length. The microscopic structure of this glan- 

 dular-like body is very peculiar, consisting of a great 

 number of large nucleated cells, which are interspersed 

 throughout a fibro-gelatinous mass. A single branch of 

 each of the great vessels, accompanied by a branch of 

 nerve, runs through a small foramen near the centre of 

 this body, but there is no vestige of either muscle or 

 tendon, the attachments of 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. Tt 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 separa- 

 tion, which afterwards forms a sheath for the young leg." 



METAMORPHOSIS. 



One of the most marked characters by which this class 

 was long considered as distinguished fi'om that of insects, 

 was the supposed absence of any such change of form, 

 during the progress of development after exclusion from 

 the egg, as is ordinarily understood by the term metainor- 

 pliosis; and Dr. Leach, in his definition of the class,* 

 formally adopts this character, which has been repeatedly 

 recognised by others. 



* Encycl. Brit., Art. Crustacea. 



