. Catalogue of the Chaetopoda in the British Museum (Natural History). Oligochaeta; Polychaeta. 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
. Catalogue of the Chaetopoda in the British Museum (Natural History). Oligochaeta; Polychaeta. 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, pa
Size: 1006px × 2484px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpublisherlondo, bookyear1912
