BORRADAILE— ON THE PONTONIIN^ 343 



second maxilliped and reduced in varying degrees the arthrobranch of the third 

 maxilliped {i in figs.). In Peridimenes, however, as SoUaud has shown, there 

 is often a vestige of the podobranch of the second maxiUiped in the form of a finger- 

 shaped process at the base of the mastigobranch (Plate 52, fig. 1 h). I have not been able 

 to satisfy myself of the presence of this process in some of the species I have examined, 

 but it is possible that some trace of it is always present. A very minute protuberance 

 found in the angle between the mastigobranch and the exopodite in Urocaris, Har- 

 piliopsis, PericlimencBus and Anchistus may have the same significance. Curiously enough, 

 Palcemonella, in other respects the most primitive of these genera, shows a greater 

 reduction of the gill of the third maxilliped than Coralliocaris or Harpiliopsis, and has 

 no vestige of that on the second maxilliped. In PericlimencBus and Anchistus the 

 arthrobranch of the third maxilliped is represented by a slightly folded lobe. In 

 Harpilius, Pontonia, Conehodytes and Typton the gills of both maxillipeds are entirely 

 lost, save for a hardly recognizable vestige at the base of the third maxilliped of 

 Pontonia. 



It will be seen that, while all Pontoniidse have undergone some reduction of the 

 gill formula as compared with other Palaemonidse, within the subfamily a loss of gills, 

 roughly speaking, accompanies the. adoption of a less active mode of life. Where there 

 is less need for active respiration there is less provision of respiratory organs. In some 

 cases, however {Urocaris, Palcemonella, Peridimenes), the loss of gills is in advance 

 of the change of habits, at least in so far as that be indicated by the build of body and 

 limbs. From this fact SoUaud draws the conclusion that the reduction of the gill 

 apparatus is the cause rather than the effect of the altered mode of life. In that case, 

 it has presumably come about by meristic variation, more or less fortuitously, and the 

 animals have survived by the adjustment of their habits to their anatomy rather than 

 by the selection of variations in their anatomy which were suitable to their habits. 

 The theory is attractive, and consonant with a good deal in modern biological speculation, 

 but before it can be adopted more must be known about the habits of the prawns. The 

 build of body is not always an indication of the mode of life, either among the Pontoniinae 

 or in other prawns. Metapenceus commensalis, for instance, lives in the zone of protection 

 of a large sea-anemone, though its general appearance is that of allied species which are 

 believed to be free-living, and various gracefully built species of Palcemonella and 

 Peridimenes are commensal with feather-stars. It may be after all that in the Pon- 

 toniinse a change of behaviour has been the forerunner of structural change, and that 

 the long series of modifications which we have traced in all parts of the body of these 

 prawns, by whatever process of variation or selection it may in detail have been brought 

 about, is but the result of congenital laziness in an environment which offers to the 

 sluggish endless opportunities of retreat. 



The Colours of the Pontoniinse. 



Most species of Pontoniinge have been described on data obtained only from spirit 

 specimens, and information concerning the colouration of the subfamily is consequently 



