ANATOMY AND CLASSIPIOATrON OF THE AHENIOOLIDiE. 435 



the more obscure points, such as the prostomium, the mor- 

 phological value of the buccal segment, the nature of the 

 gills, the presence of cirri, the presence or absence of scat- 

 tered sense-cells in the skin of the body. 



I. Segmentation. — The strictly morphological com- 

 ponents of the entire worm are the prostomium, the pygidium, 

 and the long intermediate region composed of segments, the 

 parapodia of which mark out certain regions. Thus there 

 is always a chgetigerous but abranchiate region up to the 

 seventh segment;^ then in the ''marina" section of the 

 genus there are from eleven to thirteen segments, bearing 

 plumose, highly vascular gills attached to the dorsal element 

 (notopodiura) of the parapodia. Gills and parapodia then 

 cease, and behind this point there is merely a caudal region, 

 varying in length, and, with the exception of some varieties 

 of A. cristata, unprovided with any parapodial outgrowths, 

 though still preserving traces of its segmentation. In these 

 specimens of A. cristata there is a double paired row (one 

 pair more dorsal and one more ventral) of hollow processes 

 placed at segmental intervals along the sides of the tail (PL 

 22, fig. 1, and PL 24, figs. 30—32). We discuss this matter 

 more fully later on (p. 442). In the "ecaudata" section 

 the gills and setee are continued to the hinder end of the body. 

 The presence of branched, strongly developed gills, and 

 their arrangement on at any rate all the middle segments, 

 together with their absence from the first seven, is a cha- 

 racter which distinguishes the genus Arenicola. Almost 

 equally diagnostic and constant is the secondary annula- 

 tion of the skin, so that the space between two cheetigerous 

 annuli is subdivided by three grooves into four rings. The 

 second of these grooves posterior to each of the chaetigerous 

 annuli is the intersegmental groove, and at this level the 

 septa, if present, are inserted. The segments of the caudal 



> The first bundle of setse is placed in the adult worm in the middle of the 

 first chseligerous segment, and this segment is referred to in the text as the 

 first segment. Consideration shows, however, that it is probably really the 

 third segment (see p. 441). 



VOL. 43, PART 3. NEW SERIES. HH 



