ANATOMY AND CLASSIFICATION OP THE ARENICOLIDiE. 441 



with any certainty, but we are able to produce additional 

 evidence in favour of this view. In A. marina and A. 

 Grubii there are certain "giant-cells " which occur usually 

 in couples near the hinder limit of each segment (see PL 29). 

 In A. ecaudata and A. cristata we have also found them 

 in the anterior segments, and always in the same relative 

 position. Now at the meeting-point of the connectives there 

 is a single giant- cell in the first three of the species named. 

 This giant-cell lies in the region which bears the vestigial 

 seta, and it may be taken as confirming the view first sug- 

 gested by Benham (1893) that this is the second segment, 

 the first parapodium being attached to the third. 



The limits of the second segment then include a number 

 of annuli up to the dorsal insertion of the first septum, exactly 

 one annulus in front of the first parapodium of the adult. 

 Accordingly the first chsetigerous segment of the full-grown 

 Arenicola is really the third, and is preceded by a segment 

 bearing a giant-cell and vestigial seta, and this again by the 

 peristomium. 



The conical notopodium and vertical rows of neuropodial 

 crotchets lie about, or slightly behind, the middle of each of 

 the succeeding segments. In all the species the surface of 

 the body is areolated ; the areolae are composed of glandular 

 pigmented cells, the interstices being made up of finer non 

 glandular elements. 



V. The Gills. — These structures, so characteristic of the 

 genus, are hollow branched outgrowths of the body-wall, and 

 contain an extension of the coelom by which the afferent and 

 efferent blood- capillaries are brought into close contact with 

 the thin and delicate epidermis. The gills are attached to the 

 inner side of the notopodium, and their branches radiate from 

 this point like the parts of a fan. In the " marina " section 

 the type of gill is in every species the same, though, as we 

 showed in our previous paper, there are two distinct varieties 

 of gill (the dendritic and the larger feathery or pinnate) in 

 A. marina. In this type the branches are only connected 

 together at their bases by a kind of webbing, they are not 



