Philosophical Transactions. 227 



On the double Metamorphosis in the Decapodous Crustacea, 

 exemplified in Cancer Mamas. By J. V. Thompson, F. L. S. 



On the supposed existence of Metamorphoses in the Crustacea. 

 By J. O. Westwood, F. L. S., &c. — These papers are devoted, the 

 two first to the illustration of a remarkable discovery made by the 

 author, and the other to a refutation of the inferences deduced by 

 Mr. Thompson from certain facts to which he has been eye witness. 



According to the latter, the Crustacea undergo a metamorphosis, a 

 change not consisting merely in the periodical shedding of the outer 

 envelope, but by which certain organs are acquired. After keeping 

 a full grown zoe for more than a month, it died in the act of chang- 

 ing its skin, and of passing into a new form, but one by no means 

 similar to that expected; for its disengaged members, which were 

 changed in number as well as form, corresponded with those of de- 

 capoda (crabs, &c.) viz. five pair, the anterior of these furnished with 

 a large claw or pincer, and, from being natatory and cleft, became 

 simple and adapted to crawling only. He found also, that the com- 

 mon lobster undergoes metamorphoses, but less in degree; the change 

 being from a cheliferous schizopode to a decapode, its first stage being 

 a modified zoea with a frontal spine, a spatulate tail, and wanting 

 sub-abdominal fins, " in short, such an animal as would never be 

 considered what it really is, were it not obtained by hatching the 

 spawn of the lobster." He hatched the ova of the common crab 

 {cancer pagurus), which presented exactly the appearance of the 

 Zoea Taurus with the addition of lateral spines to the corslet. 

 Numerous genera are subject to similar metamorphoses, among 

 which Mr. Thompson enumerates Pagurus, Porcellana, Galathea, 

 Crangon, Palemon, Homarus, Astacus. 



Mr. Westwood, in endeavouring to refute the conclusions of the 

 author, objects to the vagueness of his descriptions, and asserts, that 

 in the plates which he has given (which he allows to be beautiful) 

 of the metamorphoses of the zoes into crabs, there is no change in 

 reality, — that the zoe has not lost a single character which it possessed. 

 2. That the appearance of the limbs (represented by Mr. Thompson 

 as perfectly disengaged), is totally at variance with the principles 

 of ecdysis. 3. Mr. Thompson states, that his large zoes diffei-ed 

 from the smaller ones in the greater degree of developemcnt of all 

 their organs. This is precisely what would happen if the large zoes 

 were perfect animals, and precisely what would not occur if the zoes 

 were incipient crabs. 4. The elongated tail, rostrated cephalothorax, 

 but especially the structure of the mandibles, and two pairs of 

 maxilla;, peculiarities of the zoes, and so evidently partaking of the 

 macrourous type, tend to negative the opinion, that they would ever 

 become brachyurous. 5. The cray fish, according to Rathke, undergoes 

 no metamorphoses. Hence, Mr. Westwood considers he is warranted 

 to conclude, that the other decapods likewise undergo no change. 



Mr. Thompson lias extended his researches also to the Cirripedes, 

 and has come to the conclusion, 1. That the Cirripedes do not con- 

 stitute a distinct class of animals, as they have been considered by 

 all late naturalists, being connected with the Crustacea Decapoda 

 through the /{ii/inii, and with the Entomostraca by means of the 



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