Page:Popular Science Monthly Volume 16.djvu/827

 chapter, enable the author to sketch the probable pedigree of crayfishes, that is, to refer them to their causes, viz., to the action of such physical agencies as flowing rivers, land and climatic barriers, brought to bear upon successive generations of the offspring of marine lobster-like ancestors which had a wide distribution in the earlier Tertiary and later Mesozoic periods, and before taking to fluviatile life had separated into two distinct races characterized by differences of form, the one giving rise to the crayfish of the northern hemisphere (the Potamobiidæ), and the other to the crayfishes of the southern hemisphere (the Parastacidæ).

The novel portion of this book (novel at least to those who do not study the transactions of learned societies) is that in which Professor Huxley details the very interesting results which he has obtained by a minute examination of the gills attached to the bases of the legs and sides of the body in all crayfish and allied forms. Three series of these gill-plumes may be distinguished according as they are attached to the leg, to the joint-membrane, or to the side of the body (Fig. 5).An ideally perfect crayfish would have all three series complete on each ring of the body in the branchial region (including the region occupied by the three pairs of maxillipedes and the five pairs of walking and nipping legs). But no such realization of the ideal can be found in Astacine nature, any more than in that of the higher Catarrhines. In some crayfish more or less of the leg-gills are suppressed; in others, the body-gills; in others, the joint-gills; and, so ringing the changes on the combination of these elements, it is possible to construct clearly distinguished groups among the crayfishes of many climes, which at first sight seem to differ very little from one another. Further, Professor Huxley shows that crayfishes and lobsters differ from prawns, shrimps, and crabs, in having villous gills instead of laminated gills, in being "trichobranchiate" in place of "phyllobranchiate."