210 EVENINGS AT THE MICEOSCOPE. 



It had sessile eyes, and was remarkable for having 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 Megalopa : in 

 which the body was broad, the eyes stalked, and the 

 abdomen projecting behind. This was also small, but 

 somewhat larger than the preceding. 



Nobody 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 egg. 



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, 

 which therefore undergo a metamorphosis as complete 

 as that by which the catei-pillar 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 he kept in captivity. 

 Some of these passed into the Megalopa form, which in 

 turn changed to the most abundant of our larger Crus- 

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

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

 Crab was proved to pass through two temporary condi- 



