34 Arenicolidae 



does not bear any sensory processes tentacles or palps and though 

 eyes are present they are minute and sub-epidermal, and are not 

 visible on external examination, except in the case of specimens less 

 than 10 mm. in length. 



The proportionate size of the median and lateral lobes of the 

 prostomium varies considerably in the different caudate species, and 

 forms a useful specific character for instance, in A. loveni the 

 median lobe is large, while in A. pusilla the lateral lobes are very 

 highly developed (cf. Figs. 1, 2). In the two ecaudate species the 

 prostomium forms a transverse ridge, the mid-dorsal portion of which, 

 in some specimens, is slightly elevated above the rest (Fig. 3). In 



these species the prostomium merges ante- 

 riorly with the dorsal oral papillae, which 

 form an " upper lip," while posteriorly 



it is bounded by the nuchal groove. The 



N.GR- --^^MD^R conical anterior end of each of these 



species is therefore not entirely prostomial 

 in nature, it is constituted also, in fact 

 largely, of the " upper lip." A. pusilla 

 Fig. S.-A. branchMis. Anterior possesses the most complex and highly 

 e chaigtot;TK e . c prosSu m Nu ' developed prostomium, while the two ecau- 

 date species stand, in this respect, at the 



opposite end of the series, the prostomium being feebly developed. 

 The prostomium is large in the larval and post-larval stages (see 

 PI. X), and in these its pre-oral nature is well seen. In the adults 

 of all species, however, the prostomium is much smaller, relatively 

 to the peristomium and following segments, than in larval and post- 

 larval stages, and is not pre-oral in position, for it is situated behind 

 the " upper lip." 



In Arenicola the segmentation of the body is somewhat obscured 

 by the subdivision of each segment into rings or annuli. The 

 annuli are generally five in number, and one, larger than the rest, 

 bears the parapodia. 



The parapodia are obviously much reduced, as compared with 

 those of Nereis (PI. XII, Fig. 38), no doubt in correlation with the very 

 different habits of the worms ; the " feet " of Nereis are well adapted 

 for use in rapid crawling or in swimming, while the parapodia of 

 Arenicola are more fitted to its burrowing habits. The two rami of the 

 acirrate parapodia of Arenicola (PI. XII, Fig. 39) are dissociated : they 

 arise, not from a common basal piece, but directly from the body wall. 

 The notopodium, which is situated dorso-laterally, is a cylindro- 



