324 MR. WESTWOOD ON THE SUPPOSED EXISTENCE 



posed point of transformation into a Crab, is nearly four lines long between the tips 

 of the spines. Now if Mr. Thompson's views be correct, and these latter Zoes are to 

 be regarded as the larvae of Crabs, they must be considered as having acquired the 

 maximum of their Zoe form ; but so far is this from being the case, that I have ob- 

 tained from the collection of the late Rev. Lansdown Guilding, specimens of a spe- 

 cies of Zoea ten lines long between the points of the spines ; a size far too large to 

 allow us to suppose that they would subsequently put off their Zoe form, and appear 

 as Crabs ; bearing at the same time in mind the minute size of the latter animals in 

 the very young state, although possessing their ordinary form. 



Of this West Indian species I have given, in the accompanying sketch, figures in 

 detail of the various organs, which I shall not describe at length. The palpigerous 

 mandibles, the two pairs of antennae, one pair of which is bipartite, the multilobed 

 inner maxillae, are all characters found in the Macroura and Schizopoda, but not in 

 the Brachyura. The natatory apparatus of the tail, observed in my species and un- 

 noticed by Mr. Thompson, is also similarly characteristic, but the locomotive organs 

 are those to which the highest importance attaches with respect to the real nature of 

 the animal. At first sight, in addition to and immediately succeeding the two pairs 

 of maxillae, there appear only two pairs of large locomotive bipartite organs. These 

 therefore, on the supposition that the Zoe is the young of a Decapod animal, must 

 either be legs, or outer foot-jaws greatly developed ; and from their bipartite struc- 

 ture, the latter may be partly assumed ; but upon carefully dissecting the animal, a 

 series of organs were found, which not only fully proved this to be the case, but also 

 led at once to the discovery of the real nature of these animals, and gave a clue for 

 the correction of Mr. Thompson's ideas upon the supposed disengagement of the tho- 

 racic limbs. Immediately succeeding the outer pair of the natatory organs, and, in 

 fact, lying between them when at rest, was discovered a pair of slender minute 

 organs, composed apparently of two joints, one long and one short, and furnished at 

 the base with a still more minute lateral appendage. Beyond these, in succession, 

 were found the five pairs of organs precisely similar to Mr. Thompson's " limbs of 

 the future Crab disengaged from beneath the clypeus." Moreover, a number (unde- 

 termined) of minute fleshy elongated masses were found near and attached to the 

 base of these limbs. Are we therefore, with Mr. Thompson, to suppose that in this 

 Zoe (and all the specimens were alike) metamorphosis had commenced, whilst not 

 the slightest trace of such a process could be observed beyond this acquisition of 

 rudimental limbs, which can otherwise be much more satisfactorily accounted for ? 

 The researches of recent authors, and those particularly of M. H. Milne Edwards, 

 have clearly proved that in some species of Decapods {Acetes, Sergestes,) one or more 

 pairs of legs become rudimental, and that their place is supplied by highly developed 

 foot-jaws. 



Now upon applying this theory, to the correctness of which Mr. Thompson bears 

 witness, to Zoea, we find that the two large pairs of natatory organs represent the 



