156 A ren icolidae 



the largest of which is chaetiferous and branchiferous are, however, 

 alone sufficient, in the writer's opinion, to render necessary the 

 maintenance of the two genera. 



While Brancliiomaldanc presents some primitive characters, for 

 instance, a simple conical prostomium, and homonomy of its segments, 

 there is considerable evidence of its having undergone secondary 

 modification and retrogression. Its small size, the simple form of 

 its gills, the absence or great reduction of certain sense organs 

 (statocysts and nuchal organ), the reduction in the number of 

 nephridia, its hermaphroditism, the large size of the eggs and the 

 absence of a free-swimming larval stage features in which Branchio- 

 maldane departs from Arcnicola are probably to a large extent 

 correlated with the much more sedentary life of the former. 



The systematic position of Branch iomaldane mncenti may be 

 summarised thus it is an Arenicolid worm, most nearly related to 

 the ecaudate species of Arcnicola, to the young stages of which it 

 presents some points of similarity, both in form and habits, but from 

 which it differs in several important structural characters. Although 

 their habits are at first similar, the species of Arenicola soon assume 

 a more wandering mode of life, which they maintain henceforward, 

 whereas Brancliiomaldanc remains sedentary, is, in fact, tubicolous, 

 and exhibits certain retrogressive changes which are often associated 

 with that mode of life. 



Near Cherbourg . . . Ashworth Coll. 1912. 4. 9. 46. 



THE INTER-BELATIONSHIPS OF THE MEMBERS 

 OF THE FAMILY AEENICOLIDAE. 



The presence of homonomous segments, as in the ecaudate species 

 of Arenicola., is undoubtedly the primitive condition, while the 

 differentiation of the worm into two regions, an anterior in which 

 the segments bear parapodia and a posterior which has become 

 achaetous, and in which the segments are feebly marked, as in the 

 caudate species, is clearly secondary and is probably correlated 

 with a more sedentary mode of life. Of the ecaudate species 

 A. ecaudata possesses the greater number of segments, namely, about 

 sixty, compared with about forty in A. branchial?' s, and the former 

 may be regarded on this ground, as well as on account of its much 

 larger number of nephridia, as more nearly retaining the original 

 condition. The fact that A. ecaudata has, when complete, about 



