360 MR. THOMPSON ON THE DOUBLE METAMORPHOSIS 



PI. VIII. fig. 1 . It is, however, certainly much smaller, and of a greenish tinge, with 

 a few darker spots. 



If the above facts do not warrant the conclusion I have drawn, what other proof 

 can be required ? Is it necessary that the young Crab should be traced through its 

 subsequent changes until the character of the species becomes more apparent ? or 

 that the grown Zoe should be actually seen to change into a Megalope ? Neither of 

 these is impracticable, but may yet for a long time elude the most zealous and scru- 

 tinizing observers. 



It appears, then, that the animals of this division of the Crustacea not only undergo 

 metamorphosis, as formerly stated, but that they even undergo a double metamor- 

 phosis, being hatched from the ova under the singular and grotesque form of Zoea, 

 then assume that of Megalopce, and finally that of their parent Crab. How long they 

 remain in each of these two intermediate states it may be difficult to determine with 

 exactitude ; but judging from the very considerable size of the Zoe I observed about 

 changing its condition *, compared with their very minute size when first hatched, 

 and also from Megalopce not appearing before May or June, while Zoea are seen so 

 early as March and April, I think a month may be assigned as the probable duration 

 of the Zoe stage. The other, or Megalope stage, is less within the scope of observa- 

 tion ; but as there is not that very great disparity of size between young and full- 

 grown Megalopce, it is likely that it does not exceed half that of the former. 



If further proof of this double metamorphosis be desired, I have been so fortunate 

 as to trace it, but not in quite so satisfactory a manner, in one of the Swimming Crabs, 

 or Portuni, and also in Inachus, belonging to the section of Triangular Crabs. These 

 examples, derived from some of the principal groups of the Brachyura, may be sup- 

 posed quite sufficient to satisfy the most scrupulous, as to their metamorphosis ; I 

 propose, however, in future memoirs, to bring under the notice of the yet sceptical, 

 proofs of the same thing in the following genera, viz. Eriphia, Thelphusa, Gegarcinus, 

 and Pinnotheres -f . The three former genera, it may be observed, are foreign, which 

 friends in the East and West Indies have enabled me to add to the first proofs of 

 metamorphosis, by having females with ova on the point of hatching, sent home in 

 spirits : the larvae of these, consequently, have not been seen in the living state ; but 

 by examining such as have burst from their envelopes, without being completely de- 

 veloped, it is quite evident that they are Zoea. 



With regard to the other great division of the Decapoda, viz. the Macroura, or 

 those with extended tails, I shall only now say, that as far as my observations have 

 gone they also undergo metamorphosis, being Ipizopoda when first hatched, and 

 during the whole of their progress to the perfect animal ; such is the case in Astacus 

 marinus, Palinurus, Palcemon Squilla, Crangor, Galathea, Pagurus, and Porcellana. 



To return to the immediate subject of the present memoir, the Carcinus Mcenas. 

 In its first or Zoe stage it is wholly natatory from structure, while in its second it 



* Zoological Researches, p. 8. t This has since been published in the Entomological Magazine. 



