182 EVENINGS AT THE MICROSCOPE. 



pendent forms of animal life, distinct from each other, 

 and equally distinct from every known genus of Crustacea 

 besides. It was supposed that no animal of this class 

 underwent metamorphosis, or that change of form in 

 different periods of life which distinguishes Insects ; but 

 that these creatures retained through life the general shape, 

 slightly modified by development of parts and organs, 

 which they each displayed when hatched from the egg. 



But these conclusions were quite set aside by the bril- 

 liant discovery of Thompson, that Zoea and Megalopa 

 were the same animal in different stages of existence ; and 

 that, moreover, both were but the early states of well- 

 known and familiar forms of larger Crustacea, which there- 

 fore undergo a metamorphosis as complete as that by 

 which the caterpillar changes to a chrysalis, and the chry- 

 salis to a butterfly, and in every essential point parallel to it. 



In the Cove of Cork this naturalist met with a con- 

 siderable number of Zoeas, which he kept in captivity. 

 Some of these passed into the Megalopa form, which in 

 turn changed to the most abundant of all our larger Crus- 

 tacea, the common Shore-crab (Carcinus manias'). " Thus, 

 in its progress from the egg to its final development, the 

 Crab was proved to pass through two temporary condi- 

 tions, which had previously been regarded as types, not of 

 general only, but of different families ; and both strikingly 

 dissimilar from the group to which, in its perfect state, it 

 belongs." 



I have not myself examined the transformations of this 

 species ; but, as they have been well worked out, and as 

 the animal is so abundant everywhere on the coast tbat 

 you may easily verify what has been observed, I will cite 

 you the elaborate account of Mr. R. Q. Couch, of Pen- 

 zance, who has investigated the subject with great skill, 

 zeal, and success. 



Having procured some specimens of the Shore-crab 

 laden with eggs just ready for shedding, he goes on to say, 



