OF METAMORPHOSES IN THE CRUSTACEA. 317 



unconnected with them, being-, as Mr. Thompson himself says, " disengaged from 

 beneath the clypeus", (and his Plate II. fig. 2. represents the same idea,) and having 

 no previous existence in the young Zoe. Now if this be the case, setting aside its 

 disagreement with the recognised conditions of development, we arrive at once at 

 this startling and important conclusion, namely, that the large natatory organs of 

 the Zoe are instrumenta cibaria, foot-jaws, in fact, and that the animal in its Zoe 

 state has no true legs. Without, however, asserting (which might reasonably be 

 done) that every annulose animal which in its immature state is furnished with loco- 

 motive organs, is also furnished with instrumenta cibaria, which latter legitimately 

 represent the instrumenta cibaria of the imago, whilst the former as truly represent 

 the true legs of the imago, we may assert, that where an immature annulose animal 

 is furnished with locomotive organs, these, or at least some of them, represent the 

 true thoracic legs of the imago, and are not, in such immature state, merely rudi- 

 mental trophi of the perfect animal. Now on applying this principle to the case in 

 question, we find the Zoe furnished both with trophi and natatory organs ; and if we 

 regard the trophi, although few in number, as representatives of the trophi, and the 

 two pairs of natatory organs as representatives of the locomotive organs of the future 

 Crab, we can only regard the five pairs of disengaged limbs either as representing 

 the subabdominal appendages of the Crab, or as simple thoracic appendages (distinct 

 from legs), or as supplemental limbs. But each of these suppositions is so contrary 

 to nature with reference to the organization of Zoe or the Crab as distinct animals, 

 that in order to show their futility it will be suflicient to notice the determinate leg- 

 like form of these disengaged limbs, the first pair of which is cheliferous ; the fact 

 that the Zoea has distinct subabdominal appendages ; that the Crustacea are not, like 

 the Myriapoda, furnished with auxiliary limbs ; and that true thoracic locomotive 

 organs (which in Zoe, according to the principles above stated, must still remain 

 undeveloped,) are constantly developed at the same time as, or even before, supple- 

 mental ones. 



Thirdly, We may imagine the disengagement of these "future limbs" to take 

 place in a mixed manner, by considering that the two pairs of natatory limbs of the 

 Zoe produce the first, or chelate, and second pairs of legs, and that the three posterior 

 pairs are simply disengaged from beneath the clypeus. Against this idea many of 

 the preceding observations may be conjointly adduced ; to which it may be added, 

 that the similar size of these disengaged limbs is sufficient to prove that they must 

 have undergone an equal degree of development. Moreover, in such case the che- 

 late members, which are larger than the following limbs, must be produced from the 

 first pair of natatory limbs of the Zoe, which are much smaller than the second pair. 



I have in these observations left unnoticed the small member anterior to the claws, 

 observed by Mr. Thompson, and considered by him as the rudiment of the outer foot- 

 jaw, which offers still greater difficulties as to its nature if we adopt Mr. Thompson's 

 views, but which, as I shall subsequently show, is a necessary organ of the Zoe. 



