SOME SPECIFIC CHARACTERS 121 



named and distinguished, which do not come into the 

 London market. 



Crayfishes, lobsters and the like have groups of plume- 

 like gills (corresponding in the most ancient forms to the 

 number of the legs and jaw-legs) overhung and hidden by 

 the sides of the great shield or "head" of the animal. 

 The common lobsters and crayfishes retain most of these 

 in full size and activity, but have lost in the course of 

 geologic ages the original complete number. These 

 plume-like gills each half an inch or so in length 

 are attached, some to the bases of the legs and some 

 to the sides of the body above the legs. In the 

 ancestral form there were thirty-two plumes on each 

 side, twenty-four attached to the bases of the legs, 

 and eight placed each at some distance above the con- 

 nection of one of the eight legs with the side of the body. 

 It is those on the side of the body which have suffered 

 most diminution in the course of the development of 

 modern crayfishes (and lobsters) from the ancestral form 

 provided with the full equipment of thirty-two gill-plumes 

 on each side. In fact, only one well-grown gill-plume, out 

 of the eight which should exist on each side of the body- 

 wall, is to be found and that is the one placed above the 

 insertion of the hindermost or eighth of the eight legs 

 (eight when we reckon the three jaw-legs as " legs " as 

 well as the five walking-legs). In front of this the side or 

 wall of the body is bare of gill-plumes though they are 

 present in full size on the basal part of most of the legs. 

 Nevertheless, when one examines carefully with a lens 

 the bare side of the body overhung by the head-shield or 

 " carapace," one finds in a specimen of the common 

 English " pale-footed crayfish " a very minute gill-plume 

 high above the articulation of the seventh leg and another 

 above the articulation of the sixth leg. They are small 

 dwindled things, as though on the way to extinction, and 



