ANATOMY OF ARENICOLA ASSIMILIS. 741 



littoral specimens of A. marina. There are in most of them 

 about eight main stems, each 2 "5 mm. to 3 mm. long, bearing- 

 five or six dichotomously subdivided branches on each side. 

 The gills of tlie specimen from Puuta Arenas are larger and 

 of a somewhat more regular form. The eight or nine stout 

 main stems are about 4 mm. long, and are regularly arranged 

 in radiating fashion; each bears six or seven pairs of 

 branches which divide dichotomously. Although at first 

 sight this gill seems to approach to the pinnate type, the 

 lateral branches are neither so numerous nor so regularly 

 arranged as in pinnate gills, and the gill may be regarded 

 as merely a well-developed example of the dendritic type. 



The skin is subdivided into annuli. Between the prosto- 

 mium and the first chnetigerous annulus there are five rinsrs 

 (see fig. 20). The first four of these represent the region 

 found in other species of Arenicola which has been shown 

 to be composed of the peristomium (here represented by the 

 first two of these rings) fused with the first body-segment of 

 the post-larva, the sette in which disappear very early (see 

 tigs. 19, 20). The fifth ring is the first annulus of the first 

 chaetigerous segment, this segment being composed of three 

 annuli, viz. a chistigerous one and the annulus preceding 

 and following it. The second and third chastigerous seg- 

 ments also consist of three annuli, the middle one bearing the 

 setas. The fourth and succeeding segments up to the end of 

 the branchial region are composed of five annuli, the fourth 

 of which is chaetigerous. The region between any two 

 chastigerous annuli behind the third is therefore subdivided 

 into four rings. 



The epidermis of the tail is raised into numerous papillae. 

 The segmentation of this region is only feebly marked, but it 

 is indicated, especially in the anterior portion of the tail, 

 by the presence of somewhat larger annuli placed at reo-ular 

 intervals, upon which the epidermal papilla) are distinctly 

 larger than those on the intervening annuli. Each of these 

 larger rings is followed by a slight constriction, denoting the 

 presence internally of a septum, best seen in those parts of 



