124 SECRETS OF EARTH AND SEA 



legs, already known in the red-footed crayfish. It is only 

 two millimetres long about one-fifteenth of an inch ! 

 But its presence serves very distinctly to separate the red- 

 footed crayfish, Astacus fluviatilis of French and German 

 rivers, thus discovered to have four pairs of rudimentary 

 gill-plumes, from the Astacus leptodactylus of the Danube 

 basin and East Europe, which has only three pairs, and 

 still more to emphasize the difference between it and our 

 British species, the " white-foot " or Astacus pallipes, 

 which has only two ! 



This little history is noteworthy, firstly, because it 

 shows that a young student may, to use an appropriate 

 term, " wipe the eye " of an expert observer and rightly 

 venerated teacher (who would have delighted in the 

 little discovery had he been alive), as well as the eyes of 

 tens of thousands of students and teachers (including my- 

 self) who have studied the red-foot crayfish year after year, 

 and missed the little gill. It is also interesting as showing 

 us a good sample of a specific mark or character which 

 has no survival value; that is, could not advantage the 

 crayfish in the struggle for life. The fact is, that this one 

 particular very minute forward pair of gill-plumes is like 

 the other rudimentary gills a survival in a reduced con- 

 dition of a pair of gill-plumes which were well-grown, 

 useful plumes aerating the blood, in the prawn-like 

 ancestors of all crayfishes, lobsters, shrimps, and prawns, 

 and is, owing to circumstances of nutrition and growth 

 which we know nothing about but can vaguely imagine, 

 retained by the red-foot species of crayfish, but lost by 

 all other crayfishes, lobsters, common prawns and 

 shrimps, and, in fact, only retained besides by a very few 

 out-of-the-way kinds of marine prawns. That is the sort 

 of thing which frequently has to serve as "a specific 

 character " or mark, distinguishing one " species " from 

 another. 



