cnusTACEA. 339 



Avliole series of the suboesopliageal ganglia, which clearly indicates 

 that the brachyurous Crustacea are more highly developed than the 

 macrourous species; contrary, however, to the opinion of Rathke. 

 The eyes are at first sessile in all Crustacea. 



It is certain that the moult of the young craw-fish is not at any 

 period accompanied by a marked change in the form of the body, or 

 in the structure and functions of the locomotive members ; this 

 Crustacean, in short, undergoes no sudden metamorphosis. The 

 progress of development is not interrupted by arrested phases. 



A series of less complete observations on the ova of a species of 

 land-crab {Gecarciiius), more recently published by Westwood*, lead 

 to the same inference in respect of that species ; although the ma- 

 crourous proportions of the abdomen, and the exposed position of the 

 gills at tlie base of the thoracic legs, obviously unfitted the larva for 

 land life, and demonstrated the necessity for the parent's migration 

 to the sea. Nevertheless, this accomplished entomologist coincides 

 with Rathke in the general conclusion, that the Crustacea undergo 

 no metamorphosis, and that the contrary evidence adduced by 

 Slabber and Mr. Thompson must depend on some erroneous ob- 

 servation. 



The opposite conclusions of both parties from the phenomena 

 afforded them by the solitary species examined, may be comjiared 

 Avith analogous premature generalizations which might have been 

 drawn, in reference to the class of Insects, by the observer of the 

 development of a cockroach on the one hand, and by the observer of 

 the metamorphoses of a butterfly on the other. As reasonably might 

 the one, after detailing the progressive development of the orthop- 

 terous insect, broach the inference that insects underwent no other 

 metamorphosis than the gradual acquisition of wings; and, with 

 equal reason, might the other observer of the wonderful changes of 

 the lepidopterous insect affirm them to be characteristic of all insects. 

 It needs only that each theorist should question the reality of the 

 other's observation to make the parallel complete. The failure of 

 both to arrive by so short and easy a route at the entire truth incul- 

 cates the necessity of acquiring a sufficient foundation, by careful and 

 extensive induction of facts, before proceeding to erect the super- 

 structure of general theory. 



With regard to the metamorphosis of the common crab, valuable 

 testimony in confirmation of Mr. Thompson's discovery has been 

 contributed by Capt. Du Cane, R. N.f This gentleman obtained 

 crabs with ova under their tails in the month of December, from 



* CCXXXVII. p. 311. t CCXXXVIII. p. 438. 



z 2 



