METAMOKPHOSES OP CRUSTACEA. 199 



They retreat to the shells of the MOLLUSCA, not to feed 

 on the animal, as a true parasite does, but, as is sup- 

 posed, for protection, as other animals would take re- 

 fuge in a nest or cave. The shells of these Crabs, at 

 least of the female, are very soft and thin, and possibly 

 this is the cause of their singular habits. It is a pity, 

 at least for the poet, that truth obliges us to omit the 

 romantic stories once believed regarding the mutual 

 affection of the pair thus oddly consorted. 



A highly curious fact in the history of the CRUSTACEA 

 relates to their metamorphoses, the young animal pass- 

 ing through stages as wonderful as those observed in the 

 class of Insects. To Mr. J. Vaughan Thompson natu- 

 ralists are indebted for the 

 discovery of the metamor- 

 phic stages in the Common 

 Crab, and several others of 

 the order to which it belongs. 

 Other observers have since 

 witnessed the development 

 of many other species, and 

 thus rendered it probable 

 that all the higher Crustacea 

 pass through similar stages 

 of existence. Before Mr. 

 Thompson's observations the 

 little creatures, which are 

 now known to be young 

 Crabs, were considered as be- 

 longing to a distinct genus, called Zoea, placed in a dif- 

 ferent order of Crustacea, widely apart from the Crab- 



