. Catalogue of the Chaetopoda in the British Museum (Natural History). Oligochaeta; Polychaeta. Chaetae of Arenicola 39 ^- i4tfi^- invagination at the junction of the prostomium and peristomium, and into it, in the caudate species, the prostomium may be with- drawn so as to be almost or quite hidden from view. In A. marina, assimilis and glacialis the statocysts open to the exterior by pores situated on ^ the dorso-lateral wall of the peri- stomium. In A. marina the pore ^ is present, on each side, near the point at which the metastomial groove crosses the first inter-annular groove (PL
. Catalogue of the Chaetopoda in the British Museum (Natural History). Oligochaeta; Polychaeta. Chaetae of Arenicola 39 ^- i4tfi^- invagination at the junction of the prostomium and peristomium, and into it, in the caudate species, the prostomium may be with- drawn so as to be almost or quite hidden from view. In A. marina, assimilis and glacialis the statocysts open to the exterior by pores situated on ^ the dorso-lateral wall of the peri- stomium. In A. marina the pore ^ is present, on each side, near the point at which the metastomial groove crosses the first inter-annular groove (PL XII, Fig. 40). In A. assimilis the pore ^ is situated at the origin of the metastomial groove, that is, close to the prostomium on each side (Fig. 4). The position of the aper- ture in A. glacialis has not been definitely ascertained, but it is pro- bably similar to that of A. marina. The nephridial apertures are situated immediately dorsal and slightly posterior to the upper ends of the neuropodia of the segments on which they occur. In A. marina, assimilis'^ and glacialis the first nephridiopore is on the fourth segment, in A. cristata, pusilla, loveni, hranchialis and ecaudata the first pore is on the fifth segment. There are six pores on either side in A. marina, assimilis,'^ glacialis and cristata, five in A. pusilla, loveni and hranchialis, and thirteen in A. Fig. 4 assimilis var. affinis. Anterior end, dorsal aspect; showing prostomium very fully protruded, exposin<^ to view the jin'di;ui ]Kistt'rior part(P.), which is usually hiddLii in the nuchal organ ; JI. Median, L. Lateral lobe of prostomium ; External opening of statocyst. Chaetae. In adult specimens the notopodia never contain crotchets, but a crotchet, either alone or accompanied by one or more capillary chaetae, was observed by Prof. E. B. Wilson (1883) in the notopodia of larval A. cristata, and by Prof. Mesnil (1897) in specimens of " Clymcnidcs ccaudatus," afterwards shown to b
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