126 HISTORY OF CRUSTACEA. CHAP. XII. 



by the adult Prawns, in the Zoeae of which it exists 

 still in a jointed form, like the outer branch of the 

 second pair of feet of the Nauplius or Pewews-Zoea. 



The long, spiniform processes on the carapace of the 

 Zoeae of the Crabs and Porcellanse are not to be ex- 

 plained in this way, but their advantage to the larvae is 

 evident. Thus, for example, if the body of the Zoe'a of 

 Porcellana stellicola (fig. 24), without the processes 

 of the carapace and without the abdomen, which how- 

 ever is not rigidly extensible, is scarcely half a line in 

 length, whilst with the processes it is four lines long, a 

 mouth of eight times the width is necessary in order to 

 swallow the little animal when thus armed. 2 Conse- 

 quently these processes of the carapace may be regarded 

 as acquired by the Zoe'a itself in the struggle for 

 existence. 



The formation of new limbs beneath the skin of the 

 larvaa is also to be referred to an earlier occurrence of 

 processes which originally took place at a later period. 

 The original course must have been that they sprouted 

 forth in a free form upon the ventral surface of the 

 larva in the next stage after the change of skin ; whilst 

 now they are developed before the change of skin, and 

 thus only come into action a stage earlier. In lame 

 which, for other reasons, must be regarded as more 

 nearly approaching the primitive form, the original 



2 PersepTione, a rare Crab, belonging to the family Leucosiidae, is 

 served in the same manner by its long chelate feet. If we seize 

 the animal, it extends them most obstinately straight downwards, 

 so that in all probability we should more easily break than bend 

 them. 



