ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 451 



or three teeth. In older cheetae there is often also a very- 

 slight process beneath the rostrum (see figs. 18, 22). 



The crotchets of a specimen of A. ecaudata 7*2 mm. 

 long are about '05 mm. long (fig. 19). Each bears a curved, 

 pointed, beak-like rostrum, placed at right angles to the 

 shaft of the chaeta, and two lai*ge teeth behind it. In some 

 specimens there is a short pointed process beneath the 

 rostrum, but this does not appear to be constant. On com- 

 paring these chsetae with those of older specimens it will be 

 seen that the same change takes place in the inclination of 

 the rostrum to the shaft as is described above in A. marina 

 and A. cristata (see figs. 18 and 19). 



The crotchets of A. ecaudata and of A. Grubii never 

 attain a very great length ; at any rate, the largest chtetas in 

 our specimens are only one third of a millimetre in length (in 

 worms 200 mm. long). 



In order that the characters of the chsetas as above 

 described may be accurately determined, it is necessary to 

 examine unworn specimens ; especially is this precaution 

 necessary in the neuropodial chsetee of medium-sized or large 

 specimens, in which the teeth behind the rostrum, being very 

 small, are worn away very soon after the chaeta3 come into use. 

 It is therefoi-e best to examine chaet^ which have not yet 

 come into use, and this may be readily done. After isolating 

 the entire band of neuropodial chaetasby theuse of warm caustic 

 potash solution, thechgetae in the ventral portion of the band 

 should be examined. Besides the chgetse in course of forma- 

 tion, one or two fully developed crotchets which have not yet 

 come into action may be usually seen, and these should be 

 selected for observation of their characters, since they are 

 uninjured by wear. Especial care is also required in the case 

 of the notopodial setae of A. ecaudata and A. Grubii, in 

 which the hair-like processes are very small and soon worn 

 away. 



VOL. 48, PART 3. — NEW SERIES, I I 



