516 NAINEREIS. 



segments, and it only needs the effect of friction to alter the form of the anterior ventral 

 bristles. Too rigid adherence to (Ersted's 1 description would give neither papillae nor 

 cirri to the tail. 



Mau (1881) furnishes a detailed account of the body-wall, bristles, circulatory, 

 nervous, muscular, digestive, and reproductive systems, and enables a comparison to be 

 made with southern examples. Certain corrections are necessary in regard to the nerve- 

 trunks, but he pointed out the position of the segmental organs which are at the base of 

 the ventral division of the feet. 



There is little in the description of Hansen's 2 Aricia arctica from Jan Meyen to 

 distinguish it from this species. 



Genus LXXIX. — Nainereis, JDe Blainville, 1828. 



Head button-shaped. Body of two regions, slender and elongated, terminated by 

 two rounded dorsal papillae and two short ventral cirri. Branchiae commence on the 

 sixth foot and continue nearly to the posterior end. The structure of the foot in the 

 second region is less complex than in Aricia. 



The generic description of 0. Fabricius was : — Nais, furnished with bifid lateral 

 processes, abdominal cirri, and a quadrifid tail. By subsequent authors, such as 

 Malmgren, the title was altered to Naidonereis, but there seems to be no special reason 

 why the original name of De Blainville should not be retained. 



The arrangement of the nerve-cords anteriorly seems to be similar to that in Aricia 

 and Scoloplos, and they take position under the raphe, where the circular bands and 

 oblique muscles meet. The muscles of the body-wall do not show the disproportion so 

 characteristic of the species just mentioned, for the ventral are only a little larger than 

 the dorsal. On the other hand the oblique and vertical are very strong, both uniting in 

 the median raphe over the nerve-cords, which are at the inner end of a short, broad area 

 passing inward from the hypoderm. In the middle of the body this area has externally 

 the cuticle, then a somewhat thick hypodermic coat, which in the middle line presents a 

 symmetrical arrangement of its cells and areolae. Across the inner border of this coat 

 pass fibres — probably from the circular coat (unless the basement tissue, which is not 

 visibly differentiated, has them). Other fibres from the circular coat clasp the inner 

 borders of the longitudinal layer. Deeply stained cells occur next, and then the nerve- 

 area with its rounded cords and a neural canal superiorly in the middle line. 



A new genus (Theodisca) was founded by Fritz Muller for Ariciidae, with strap-like 

 branchiae, and biramous feet with the ventral division bilobed. The proboscis is extruded 

 as a five-lobed foliaceous button. So far as can be observed all the known forms may 

 readily be placed under Nainereis. 



An allied group is the Levinseniens of Mesnil and Caullery. The prostomium has a 

 median tentacle, as in Aricidea, and this is always absent in the Ariciidae proper, which 

 have a palpode. In the Ariciidae the first body-segment is achetous, the branchiae com- 



1 ( Annul. Dorsibranch.,' p. 201, figs. 115, 117, 118. 



2 ' Norske Nord.-Exped./ 1876—8, vii, p. 34, Tab. v, figs. 20—26. 



