122 SECRETS OF EARTH AND SEA 



are the mere vestiges of what were once well-grown gill- 

 plumes, and still are so in the rock lobster and some 

 prawns. In the red-footed crayfish of the Continent 

 (Astacus fluviatilis) yet another minute vestige of a gill- 

 plume is found, farther in front, on the body-wall above 

 the fifth leg on each side of the animal. This furnishes 

 a definite mark or character by which we can distinguish 

 the red-footed crayfish from the common English pale- 

 footed one. But these three rudimentary gill-plumes in 

 the red-foot species, and two in the pale-foot species are 

 all that until lately were recorded. The region of the 

 body-wall above the fourth, third, second, and first of the 

 legs was declared to be devoid even of a vestige of the 

 branchial plumes which were there in ancestral forms, and 

 have been retained more or less in some exceptional 

 prawn-like creatures allied to the crayfish. 



Zoologists take a special interest in the' crayfish 

 because it is found to be a most convenient type for the 

 purpose of teaching the principles of zoology to young 

 students, and with that end in view was made the subject 

 of a very beautiful little book by the great teacher 

 Huxley. The conclusions above stated in regard to the 

 gills are set forth in that book with admirable illustrative 

 drawings, and the striking fact of the dwindling and 

 suppression of the various gill-plumes is clearly explained. 



And now we come to an interesting discovery in this 

 matter of the gill-plumes of crayfishes. Some fifteen years 

 ago the daughter of my friend and colleague Professor 

 Moseley was a member of the class of Elementary Biology 

 at Oxford. She had to examine and identify these and 

 other points in the structure of the crayfish. The class 

 was supplied with specimens of the French red-footed 

 crayfish " Astacus fluviatilis," as it is more readily obtained 

 from fishmongers than our own " pale-foot " or " Astacus 

 pallipes." She found in her specimen far forward on each 



