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 

 brilliant discovery of Thompson, that Zoea and Megal- 

 opa 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 therefore undergo a metamorphosis, as complete 

 as that by which 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 he kept in captivity. 

 Some of these passed into the Megalopa form, which in 

 turn changed to the most abundant of all our larger 

 Crustacea, the common Shore-crab (Carcinus mcenas). 

 " Thus, in its progress from the egg to its final develop- 

 ment, the Crab was proved to pass through two tem- 

 porary conditions, which had previously been regarded 

 as types, not of genera 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 that you may easily verify what has been ob- 

 served, I will cite you the elaborate account of Mr. 

 R. Q. Couch, of Penzance, who has investigated the 

 subject with great skill, zeal, and success. 



