30 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 39. 



The nepliridia are apparently of the same type as those of A. 

 marina. The dorsal lip of the second one bears eight flattened 

 processes, rounded at their margins. 



A brain and nerve cord are apparently similar to those of ^. marina, 

 but a detailed examination of them is precluded. Sections of the 

 nerve cord demonstrate the presence of giant nerve cells and giant 

 nerve fibers, such as are found in the cord of all species of Arenicola 

 except A. daparedii 



Statocysts are present. A small piece of the wall of the peris- 

 tomium and the statocyst situated upon it were cut into serial sec- 

 tions, the examination of which shows that the statocyst is a pear- 

 shaped vesicle opening to the exterior by a very narrow tube situ- 

 ated in a position corresponding to that of the stalk of the pear. 

 The three diameters of the lumen of the statocyst are 0.144, 0.084, 

 and 0.084 mm., respectively. Its wall is 0.04 to 0.08 mm. in thickness. 

 The statoliths are numerous yellow or colorless sand grains, the 

 largest of which is about 25/^ in diameter. As seen in transverse 

 section, the lumen of the tube, which connects the statocyst to the 

 exterior, is a narrow oval slit, the diameters of which are 4/i by 20 to 

 25 fi. The statocyst and statoliths are similar to those of A. marina, 

 and differ entirely from those of A. cristata, which has a closed stato- 

 cyst containing a single large secreted statolith. 



We may now proceed to state the diagnostic characters of 



ARENICOLA GLACIALIS Murdoch. 



Seventeen chsetigerous segments ; eleven pairs of small gills, the fii'st 

 on the seventh segment; the gill axes are short and their branches 

 are not given off right and left but at or near the distal end of the axis, 

 each branch divides dichotomously once or twice, the resultant finger 

 or thumb shaped gill filaments form clusters at the end of each axis; 

 the median lobe of the prostomium is small, the lateral ones are well 

 developed but are not markedly dilated or lobate at their anterior 

 ends; neuropodia are clearly visible in each segment, those of the pos- 

 terior branchial segments are long dorso-ventrally and almost reach the 

 mid- ventral line; six pairs of neplu-idia which open on the fourth to 

 the ninth segments ; one pan* of conical oesophageal glands ; a pair of 

 small pouches projecting backward from the ventral region of the first 

 septum; a pair of statocysts in the peristomium, which open to the 

 exterior; statoliths, numerous, composed of sand grains. Types, the 

 only specimens known, in the Smithsonian Institution, from the beach. 

 Cape Smyth, Alaska, September 12, 1882. [Bottle No. 851.] 



Affinities of Arenicola glacialis. — The affinities of this species with 

 other caudate species of Arenicola may now be determmed. There 

 are really no points of agreement between this species and A. dapa- 

 redii. Only two features are common to A. glacialis and A. assimilis, 

 namely, the number of nephridia and the open statocysts, but these 

 are greatly overbalanced by weighty differences in the number of 

 segments, the number of segments bearing gills, the nature of the 



