48 Arenicolidae 



the ecaudate species have, along one side, a narrow lamina (2 to 4 /x 

 wide), which, however, soon breaks up into fine teeth (see the left 

 proximal portion of Tig. 16 a). The opposite side of the chaeta 

 bears longer spines. Towards the tip the shaft of the chaeta appears 

 to be encircled by series of spines (Sageblatter), which can be resolved 

 only with difficulty (Fig. 16 b). 



The only species of Arenicola in which the uotopodial chaetae 

 present characters sufficiently striking and definite to be of real 

 service in systematic work is A. loveni, the chaetae of which are 

 distinguished Ijy their well- developed Sageblatter, and the transverse 

 striation of their distal portion. The notopodial chaetae in the case 

 of a specimen of any other species would serve, at most, to indicate to 

 which of three groups — marina-pitsilla-assimilis, cristata-glacialis, 

 ccaudata-branchialis — the specimen belonged. 



Neuropodial Crotchets. — The successive generations of crotchets 

 exhibit progressive changes which, although small, result in the end 

 terms of the series being of markedly different form, that is, the 

 crotchets of the larval and post-larval stages differ widely from those 

 of the adult. 



A. marina. — The crotchets of a post-larval specimen oi A. marina, 



5 mm. long, have the form shown in Fig. 17. The proximal end of 



the shaft is fixed to the bottom of the setal sac and the greater part 



of the length of the shaft, up to and including the well marked 



dilatation present upon it, is enclosed in the sac. 



The distal portion of the crotchet is bent almost at 



a right angle to the shaft and forms a beak-like 



structure — the rostrum. Immediately proximal to 



the rostrum, on the convex {i.e. ventral) side of the 



curvature of the crotchet,^ there are three (or four) 



teeth, of which that nearest the rostrum is the 



largest. On the shaft, just under the rostrum, there 



is generally visible a small pointed process— the 



sub-rostral process — which is more highly developed 



* ciotchet" from""a in somc crotclicts than in others. Careful focussing 



nifil's^'mm. long"' sliows that the scrics of teeth is continued round 



the sides of the rostrum, and that the sub-rostral 



process is the expression of the smallest of these ; thus the rostrum 



projects from a series of teeth encircling its base. This is more 



clearly seen in the crotchets of post-larval stages of A. assimilis 



(Fig. 21, p. 50) and also in the adult crotchets of certain species, 



e.g. A. branchialis (Fig. 30, p. 57). 



^ The position of the crotchets in the neuropodium is shown in PL XII, Fig. 39. 



