The Crayfish: An Introduction to the Study of Zoology.

by Thomas Henry Huxley

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.

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.

[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.

[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.

These considerations appear to me to indicate the direction in which we must look for a rational explanation of the origin of crayfishes and their present distribution.

I have no doubt that they are derived from ancestors which lived altogether in the sea, as the great majority of the Mysidæ and many of the prawns do now; and that, of these ancestral crayfishes, there were some which, like Mysis oculata or Penæus brasiliensis, readily adapted themselves to fresh water conditions, ascended rivers, and took possession of lakes. These, more or less modified, have given rise to the existing crayfishes, while the primitive stock would seem to have vanished. At any rate, at the present time, no marine crustacean with the characters of the Astacidæ is known.

As crayfishes have been found in the later tertiaries of North America, we shall hardly err in dating the existence of these marine crayfishes at least as far back as the miocene epoch; and I am disposed to think that, during the earlier tertiary and later mesozoic periods, these Crustacea not only had as wide a distribution as the Prawns and Penæi have now, but were differentiated into two groups, one with the general characters of the {332} Potamobiidæ in the northern hemisphere, and another, with those of the Parastacidæ, in the southern hemisphere.

The ancestral Potamobine form probably presented the peculiarities of the Potamobiidæ in a less marked degree than any existing species does. Probably the four pleurobranchiæ were all equally well developed; the laminæ of the podobranchiæ smaller and less distinct from the stem; the first and second abdominal appendages less specialised; and the telson less distinctly divided. So far as the type was less specially Potamobine, it must have approached the common form in which Homarus and Nephrops originated. And it is to be remarked that these also are exclusively confined to the northern hemisphere.

The wide range and close affinity of the genera Astacus and Cambarus appear to me to necessitate the supposition that they are derived from some one already specialised Potamobine form; and I have already mentioned the grounds upon which I am disposed to believe that this ancestral Potamobine existed in the sea which lay north of the miocene continent in the northern hemisphere.

In the marine primitive crayfishes south of the equator, the branchial apparatus appears to have suffered less modification, while the suppression of the first abdominal appendages, in both sexes, has its analogue among the Palinuridæ, the headquarters of which are in the southern hemisphere. That they should have ascended {333} the rivers of New Zealand, Australia, Madagascar, and South America, and become fresh water Parastacidæ, is an assumption which is justified by the analogy of the fresh-water prawns. It remains to be seen whether marine Parastacidæ still remain in the South Pacific and Atlantic Oceans, or whether they have become extinct.