Supposing, however, that the ancestral forms of the Potamobiidæ obtained access to the river basins in which they are now found, from the north, the hypothesis that a mass of fresh water once occupied a great part of the region which is now Siberia and the Arctic Ocean, would be hardly tenable, and it is, in fact, wholly unnecessary for our present purpose.
The vast majority of the stalk-eyed crustaceans are, and always have been, exclusively marine animals; the crayfishes, the Atyidæ, and the fluviatile crabs (Thelphusidæ), being the only considerable groups among them which habitually confine themselves to fresh waters. But even in such a genus as Penæus, most of the species of which are exclusively marine, some, such as Penæus brasiliensis, ascend rivers for long distances. Moreover, there are cases in which it cannot be doubted that the descendants of marine Crustacea have gradually accustomed themselves to fresh water conditions, and have, at the same time, become more or less modified, {327} so that they are no longer absolutely identical with those descendants of their ancestors which have continued to live in the sea.[39]
In several of the lakes of Norway, Sweden and Finland, and in Lake Ladoga, in Northern Europe; in Lake Superior and Lake Michigan, in North America; a small crustacean, Mysis relicta, occurs in such abundance as to furnish a great part of the supply of food to the fresh water fishes which inhabit these lakes. Now, this Mysis relicta is hardly distinguishable from the Mysis oculata which inhabits the Arctic seas, and is certainly nothing but a slight variety of that species.
In the case of the lakes of Norway and Sweden, there is independent evidence that they formerly communicated with the Baltic, and were, in fact, fiords or arms of the sea. The communication of these fiords with the sea having been gradually cut off, the marine animals they contained have been imprisoned; and as the water has been slowly changed from salt to fresh by the drainage of the surrounding land, only those which were able to withstand the altered conditions have survived. Among these is the Mysis oculata, which has in the meanwhile undergone the slight variation which has converted it into Mysis relicta. Whether the same explanation {328} applies 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.
[39] See on this interesting subject: Martens, “On the occurrence of marine animal forms in fresh water.” Annals of Natural History, 1858: Lovèn. “Ueber einige im Wetter und Wener See gefundene Crustaceen.” Halle Zeitschrift für die Gesammten Wissenschaften, xix., 1862: G. O. Sars, “Histoire Naturelle des Crustacés d’eau douce de Norvège,” 1867.
Several species of prawns (Palæmon) 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 (Palæmon) 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 {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.
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]