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XXI. On the Double Metamorphosis in the Decapodous Crustacea, exemplified in 

 Cancer Msenas, Linn. By J. V. Thompson, F.L.S. Deputy Inspector-General of 

 Hospitals. Communicated by Sir James Macgrigor, Bart. M.D. F.R.S. 



Received May 21, — Read June 4, 1835. 



XN the Memoir published in my Zoological Researches, p. 1, with its sequel, p. 63, 

 having first made known the fact of the Brachyura of the Decapoda (Crabs) passing 

 through the intermediate form of Zoea ; I have now to announce that they undergo 

 another metamorphose, no less singular and unlooked for, in which they assume the 

 form of the genus designated by the name of Megalopa by Dr. Leach, from the dis- 

 proportionate size of the eyes. This second stage we may therefore consider analo- 

 gous to that of pupa in the class Insecta. 



By the former memoir it appears that the young of Cancer Pagurus, the common 

 market Crab, first presents itself as a Zoe *, and that a full-grown Zoe was observed 

 passing into some other more perfect form -f-, which at that time was considered to 

 be that of some species of Crab : the discovery now first detailed, however, shows 

 that it must have been only passing into that of a Megalope. 



The first proof I had of this new and extraordinary fact, which cancels another 

 anomalous genus of the Crustacea, was obtained by keeping in regularly renewed sea- 

 water a number of individuals of a Megalope % which makes its appearance in the 

 river Lee, just below the city of Cork, in considerable abundance every summer : 

 these, to my very great surprise, began, after a short time, to change into a mi- 

 nute Crab §, until the whole of them, to the amount of about two dozen, were so 

 metamorphosed. I have frequently since observed the same circumstance, and came 

 to the conclusion that these must be the progeny of the only Crab that is ever found 

 in the higher parts of the river, where these Megalopce were taken, viz. Carcinus Mcenas, 

 our common Shore Crab. The young Crab, it will be noticed, has not the distinctive 

 characters of its parent, which it probably acquires only after several casts of its 

 shelly covering. 



To complete the series of metamorphoses in this species of Crab now became a 

 matter of research ; and I have been so fortunate as to succeed in hatching its mature 

 spaw^n, so as to be enabled to give a representation of its Zoe ||, or first stage^ and 

 thereby render complete its natural history. In this stage it does not appear to differ 

 materially from that of Cancer Pagurus, formerly figured in Zoological Researches 



* Pages 9 and 64. f Page 8. % Fig. 2. § Fig. 6. || Fig. 1. 



MDCCCXXXV. 3 A 



