828 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES. 



plies to Lakes Superior and Michigan, or whether the 

 Mysis oculata has not passed into these masses of fresh 

 water by channels of communication with the Arctic 

 Ocean which no longer exist, is a secondary question. 

 The fact remains that Mysis relicta is a primitively 

 marine animal which has become completely adapted to 

 fresh- water life. 



Several species of prawns {Palcemon) abound in our 

 own seas. Other marine prawns are found on the coasts 

 of North America, in the Mediterranean, in the South 

 Atlantic and Indian Oceans, and in the Pacific as far 

 south as New Zealand. But species of the same genus 

 {Palcemon) are met with, living altogether in fresh water, 

 in Lake Erie, in the rivers of Florida, in the Ohio, in 

 the rivers of the Gulf of Mexico, of the West India 

 Islands and of eastern South America, as far as southern 

 Brazil, if not further ; in those of Chili and those of 

 Costa Eica in western South America ; in the UjDper 

 Nile, in West Africa, in Natal, in the Islands of Johanna, 

 Mauritius, and Bourbon, in the Ganges, in the Molucca 

 and Philippine Islands, and j)robably elsewhere. 



Many of these fluviatile prawns differ from the marine 

 species not only in their great size (some attaining a foot 

 or more in length), but still more remarkably in the vast 

 development of the fifth pair of thoracic appendages. 

 These are always larger than the slender fourth pair 

 (wliich answer to the forceps of the crayfishes) ; and, in 

 the males especially, they are very long and strong, and 



