Crotchets of Arenicola 



49 



Fig. 18.— 4. marina. 

 Crotchet from a 

 young adult, 17 

 mm. long. 



A crotchet from a young A. marina, 17 mm. in length, which 

 had assumed the adult form and mode of life, is shown in Fig. 18. 

 The teeth are proportionately smaller, and the rostrum 

 is placed at a greater angle to the shaft — about 

 120°, instead of about 90° in the younger crotchet 

 of Fig. 17. Specimens about 100-130 mm. long have 

 crotchets of the form shown in Fig. 19 ; there are 

 four or five minute teeth behind the rostrum, which 

 latter is placed at an angle of about 135° to the 

 shaft. 



Crotchets from very large specimens have a 

 markedly elongate rostrum, still more nearly in line 

 with the shaft, and are entirely without teeth. The 

 absence of teeth is not due to their having been 

 worn away by use, as may be 

 shown by isolating the entire 

 series of crotchets and selecting 

 for examination those which have not yet 

 come into use at the ventral end of the 

 series. The ventral portion of such a pre- 

 paration is shown in Fig. 20. The tip of 

 the crotchet on the left is projecting from 

 the lip of the chaetal sac, the outline of 

 which is shown, and would soon have come 

 into use.^ Various stages in the formation 

 of crotchets are seen, and it will be noted 

 that teeth are not developed. New crotchets 

 are being constantly produced at the ventral 

 end of the series and the old, worn ones cast 

 out at the dorsal end of the chaetal sac. 

 Occasionally small relict crotchets are found 

 in the bottom, and usually near the dorsal 

 end of the chaetal sac, from which they 

 should liave been cast along with their con- 

 Fig. i9.—^.»iari/ia. a, crotchet temporaries, but for some reason — probably 



from a specimen 125 mm. long ; , i . . , . 



B, Tip of another crotchet entanglement m the tissue or mucus — they 



from the same specimen. i i ^ • i 



have been retained. 

 The principal changes in the crotchets of A. marina, pari passu 

 with the increase in size of the worm, are (1) increase in size and 



* This chaetal sac contained 117 other crotchets, practically identical in 

 size and form with that shown on the left of the figure, 



E 



