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 {329} are terminated by great chelæ, not unlike those of the crayfishes. Hence these fluviatile prawns (known in many places by the name of “Cammarons”) are not unfrequently confounded with true crayfishes; though the fact that there are only three pair of ordinary legs behind the largest, forceps-like pair, is sufficient at once to distinguish them from any of the _Astacidæ_. [Illustration: FIG. 79. _Palæmon jamaicensis_ (about 5/7 nat. size). A, female; B, fifth thoracic appendage of male.] Species of these large-clawed prawns live in the {330} brackish water lagoons of the Gulf of Mexico, but I am not aware that any of them have yet been met with in the sea itself. The _Palæmon lacustris_ (_Anchistia migratoria_, Heller) abounds in fresh-water ditches and canals between Padua and Venice, and in the Lago di Garda, as well as in the brooks of Dalmatia; but its occurrence in the Adriatic or the Mediterranean, which has been asserted, appears to be doubtful. So the Nile prawn, though very similar to some Mediterranean prawns, does not seem to be identical with any at present known.[40] In all these cases, it appears reasonable to apply the analogy of the _Mysis relicta_, and to suppose that the fluviatile prawns are simply the result of the adaptive modification of species which, like their congeners, were primitively marine. [40] Heller, “Die Crustaceen des südlichen Europas,” p. 259. Klunzinger, “Ueber eine Süsswasser-crustacee im Nil,” with the notes by von Martens and von Siebold: Zeitschrift für Wissenschaftliche Zoologie, 1866. But if the existing sea prawns were to die out, or to be beaten in the struggle for existence, we should have, scattered over the world in isolated river basins, more or less distinct species of freshwater prawns,[41] the areas inhabited by which might hereafter be indefinitely enlarged or diminished, by alteration in the elevation of the {331} land and by other changes in physical geography. And, indeed, under these circumstances, the freshwater prawns themselves might become so much modified, that, even if the descendants of their ancestors remained unchanged in structure and habits in the sea, the relationship of the two might no longer be obvious. [41] This seems actually to have happened in the case of the widely-spread allies and companions of the fluviatile prawns, _Atya_ and _Caridina_. I am not aware that truly marine species of these genera are known.