THE CRAYFISH, 



191 



chapter, enable the author to sketch the probable pedigree of cray- 

 fishes, that is, to refer them to their causes, viz., to the action of such 

 physical agencies as flowing i-ivers, 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 3Iesozoic 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 Potamo- 

 hiidce), and the other to the crayfishes of the southern hemisjihere (the 

 Parastacidai). 



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. Thi-ee 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 occu- 

 pied by the three pairs of 

 maxillipedes and the five 

 pairs of walking and nipping 

 legs). But no such realiza- 

 tion of the ideal can be found 

 in Astacine nature, any more 

 than in that of the higher 

 Catarrhines. In some cray- 

 fish more or less of the leg- 

 gills are suppressed ; in oth- 

 ers, the body-gills ; in oth- 

 ers, the joint-gills ; and, so 

 ringing the changes on the 

 combination of these ele- 

 ments, it is possible to con- 

 struct clearly distinguished 

 groups among the crayfishes 

 of many climes, which at first 

 sight seem to differ very little 

 from one another. Further, 



Professor Huxlev shows that FiG.4.-CA5n3ARrs Ci.auku, male (hair natural size), 

 -' . atler Haijen. 



crayfishes and lobsters differ 



from prawns, shrimps, and crabs, in having villous gills instead of lami- 

 nated gills, in being " trichobranchiate " in place of " phyllobranchiate." 



