Chaetae of Arenicola 



39 



ST.O. 



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 1 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 1 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 

 ' 



ot the neuropodia of the segments 

 on which they occur. In A. marina, 

 assimilis 2 and glacialis the first 

 nephridiopore is on the fourth segment, in A. cristata, pusilla, loveni, 

 branchialis and ecaudata the first pore is on the fifth segment. 

 There are six pores on either side in A. marina, assimilis,' 2 ' glacialis 

 and cristata, five in A. pusilla, loveni and branchialis, and thirteen in 

 A. ecaudata. 



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 

 " Clymenides ecaudatus" afterwards shown to be post-larval speci- 



1 These apertures are minute, and can be seen only in those specimens in 

 which the peristomial region is well extended. 



2 In examples of A. assimilis var. affinis from South Africa there are only 

 five pairs of nephridiopores, the first of which is on the fourth segment. 



Fig. 4. A. assimilis var. affinis. 

 end, dorsal aspect ; showing prostomium 

 very fully protruded, exposing to view the 

 median posterior part (P.), which is usually 

 hidden in the nuchal organ ; M. Median, 

 L. Lateral lobe of prostomium ; ST.O. 

 External opening of statocyst. 



