CRABS AND SHRIMPS 211 



pair of legs, which terminate each in a hooked spine. 

 You now and then see these awkwardly thrust out from 

 beneath the hinder part of the shell, but locomotion is 

 principally effected by the pencilled antennae. There is, 

 however, a second pair of legs, but these do not usually 

 make their appearance outside the shell, being curved 

 backward to sustain the ovaries. 



About thirty years ago an Irish naturalist, Dr. J. 

 Yaughan Thompson, announced a discovery, which, over- 

 setting conclusions previously received by all, caused no 

 little dissent and opposition, and gave rise to a lengthened 

 and widespread controversy. A very minute crustaceous 

 animal was known, as inhabiting the open sea, to which 

 the name of Zoea had been given. It had sessile eyes, and 

 was remarkable for having a long 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 independ- 

 ent forms of animal life, distinct from each other, and 

 equally distinct from every known genus of Crustacea be- 

 sides. It was supposed that no animal of this class under- 

 went 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, 



