OF METAMORPHOSES IN THE CRUSTACEA. 319 



for the latter opinion in their comparatively less perfect organization, a circumstance 

 to which a completely analogous case exists amongst the Hyperiidce in the order Am- 

 phipoda, &c. Zoe, indeed, is not the only animal respecting which this kind of para- 

 sitic obscurity exists ; the genus Meloe amongst the coleopterous insects is perfectly 

 analogous, the young of which, according to some authors, are Acari, whilst others 

 state them to resemble the perfect insect. I am the more anxious to offer this 

 explanation of Mr. Thompson's argument, in as much as the facts subsequently 

 stated respecting the ova and young of the Brachyura are totally at variance with 

 Mr. Thompson's assertions. 



Thirdly, As respects Mr. Thompson's statements relative to the young of the com- 

 mon Lobster, we have again to regret the slightness of the information given to us 

 upon this branch of the subject. The young is called a modified Zoe, a cheliferous 

 Scliizopode, with a frontal spine, a spatulate tail, and wanting subabdominal fins, 

 undergoing a metamorphosis less in degree than the other mentioned genera. We 

 are left in uncertainty whether there are eight pairs of locomotive organs, as in the 

 true Schizopods, or whether these organs are all divided into two parts ; the only 

 evidence of such Schizopod nature being the chelate limb figured ; and yet this is 

 precisely where information was required. Examine the other characters given of 

 this " modified Zoe" without reference to its undescribed legs, and we are able to 

 trace (notwithstanding Mr. Thompson's assertion to the contrary) precisely such 

 an animal as might be expected for an immature Lobster. But if we examine the 

 nature of the cheliferous member figured, we shall find the strongest reason for con- 

 sidering that this " larva" is not a Schizopode. Fig. a. represents the cheliferous 

 member of the perfect Lobster, as well as of its larva ; but this organ is not provided 

 in the perfect state with any lateral appendage. And Mr. Thompson himself does 

 not attempt to prove the connexion of the lateral appendage which he figures with 

 the cheliferous limb of the perfect Lobster, since he describes this lateral appendage 

 as thQ future Jiagrum of the Lobster, that is, the lateral division of the exterior pair 

 of foot-jaws ; consequently, unless Mr. Thompson is prepared to prove that the lateral 

 appendage of one organ in the immature state becomes the lateral appendage of a 

 totally distinct organ in the perfect state of the same animal, it must follow that this 

 gentleman has erred in his dissections of the immature Lobster, and mistaken the 

 lateral appendage of the outer foot-jaw for a Schizopodous appendage of the Cheli- 

 ferous limb. 



Fourthly, As respects the explanation or " excuse" which this principle of meta- 

 morphosis enables us to give for the annual migrations of the land Crabs of the 

 West Indies to the ocean to deposit their spawn, the young produced from which 

 being natatory animals — Zoes, in fact, — are incapable of living in the same element 

 as their parents in their early stages, I can very well agree with Mr. Thompson that 

 if any exception existed amongst the Crustacea, in which the young should not un- 

 dergo any change from aquatic to terrestrial habits, accompanied of course by a 



MDCCCXXXV. 2 T 



