328 DISTRIBUTION AND ZTIOLOGY 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 (Palemon) 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 
(Palemon) 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 Rica in western South America; in the Upper 
Nile, in West Africa, in Natal, in the Islands of Johanna, 
Mauritius, and Bourbon, in the Ganges, in the Molucca 
and Philippine Islands, and probably 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 
(which answer to the forceps of the crayfishes) ; and, in 
the males especially, they are very long and strong, and 
