210 EVENINGS AT THE MICEOSCOPE. 



It had sessile eves, and was remarkable for liaviuor a 

 Jong spine projecting from the face, and a similar one 

 standing up from the centre of the back. Another form 

 was known, which constituted the genus Jtlegalopa : in 

 whicli the body M'as broad, the eyes stalked, and the 

 abdomen j^rojecting behind. This was also small, but 

 somewhat larger them tlie preceding. 



Kobody suspected that these were other than inde- 

 pendent forms of animal life, distinct from each other, 

 and equally distinct from every known genus of Crus- 

 tacea 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 eo:2:. 



But these conclusions were quite set aside by the 

 brilliant discovery of Thompson, that Zoea and Mega- 

 lopa were the same animal in different stages of exist- 

 ence ; and that, moreover, both were but the early states 

 of well-known and familiar forms of larger Crustacea^ 

 whicli therefore undergo a metamorphosis as complete 

 as that by Avhich the caterpillar changes to a chrysalis, 

 and the chrysalis 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 lie kept in captivity. 

 Some of these passed into the 2icgalopa form, which in 

 turn chansred to the most abundant of our larirer Cnis- 

 tacea, the common Shore-crab {Carcimis moinas). " Thus, 

 in its progress from the egg to its final develoj^ment, the 

 Crab was proved to pass through two temporary condi 



