126 CASTALIA ARCTICA. 



of the ceratophore of the dorsal cirrus. The ceratophore is proportionately shorter than 

 in Psamathe fusca. The cirrus shows crenations towards the base and is more distinctly 

 segmented towards the tip. The setigerous region diminishes distally, and has a conical 

 papilla in front of the tip of the spine. The bristles form a vertical fan and have a 

 distinct downward curve towards the tip of the shaft, which is somewhat dilated and 

 furnished with a shoulder from which the terminal piece proceeds, the margin sloping 

 from this upward to the pointed end of the shaft (Plate LXXVIII, fig. 3). The shaft 

 is marked similarly to that in G. fusca, but the lines are much more delicate and less 

 easily observed. The terminal blade is shortest ventrally and gradually increases in 

 length superiorly, the blades of those on the dorsal edge, however, being shortened, 

 though they are longer than those of the ventral edge. All are slightly curved and 

 diminished distally, and end in a minute claw with a secondary process beneath. The 

 serrations on the edge are finer than in G. fusca, yet there is a tendency to a more 

 distinct series inferiorly, and then they gradually become finer and fade away distally. 

 The bristle is thus intermediate in character between G. punctata and 0. fusca. 



The ventral cirrus is comparatively short and subulate. 



In all probability a flap existed at the tip of the setigerous process, but the state of 

 the preparation was unfavourable for determination. It may also be smaller than in 

 allied forms. 



Reproduction. — The specimen is a male with the reproductive elements well developed 

 in June. 



There are certain discrepancies between this form and the Nereis aphroditoicles of 

 0. Fabricius, especially in the structure of the foot, for, in addition to the single terminal 

 papilla, Fabricius adds that there are two smaller papillae inferiorly. The general 

 greenish hue with red lines make the species described by Fabricius conspicuous, and 

 it may be that further investigation will show that the British form differs from it. 

 In the uncertainty as to the minute characters of the older form it has been deemed 

 best to include the present species under Gastalia arctica. 



In the structure of the foot this form differs both from Gastalia Fabricii, Malmgren, 

 and G. midtipapillata, Theel, for, instead of the three papillse which characterize the tip of 

 the setigerous region in these, the British species has only one conical papilla in front of 

 the tip of the spine. 



Malmgren observes that in G. arctica the dorsal division has only a single spine, and 

 in the British species the spine is so slender in contrast with those of G. Fabricii that 

 Malmgren may have overlooked the fact that in some there are two. No details of the 

 structure of the feet are given by Malmgren, and none of the bristles or of the proboscis, 

 so that considerable doubt must remain as to the nature of these. 



Wiren amalgamates the G. Fabricii and G. arctica of Malmgren as the G. aphroditoides 

 of Fabricius, the former, G. Fabricii, having two acicules dorsally and three or four 

 ventrally, whereas G. arctica has only one dorsally and two ventrally. He considers the 

 latter a variety of the former, but he gives no details of the structure of the foot and no 

 figures. 



Marenzeller (1898) alludes to this form from Spitsbergen as having three spines, as 

 in 0. Fabricii, Malmgren. 



