ON A NEW HISTRIOBDELLID. 325 



this view has been generally accepted by later writers, 

 though frequently with some reservation. 



Harmer (12, p. 22) assents to Fcettinger's views as to 

 affinities, and adds, " In the number of segments, in the 

 segmentation of the ventral nervous system, and in the 

 arrangement of the muscular system Dinophilus more nearly 

 approaches Histriodrilus than any of the other Archiaune- 

 lida." Hatschek (15) suggests that Histriobdella may be a 

 degenerate Eunicid. Bisig (5) expresses the opinion that 

 in Histriobdella we have to do with a strongly modified and 

 degenerate animal, and not with an Archiannelid. 



Though I have not been able to trace a closer affinity 

 between the Histriobdellidae and any other group of the 

 Annulata, I have come to the conclusion that to class them 

 as nearly related to Protodrilus and Polygordius is 

 altogether unjustifiable. Of the common features which 

 Foettinger adduces as affording evidence in support of his 

 view, nearly all are general annulate characteristics. To the 

 alleged primitive condition of the nervous system no weight 

 can be attached, since, as I have shown above, this condition 

 scarcely obtains in Stratiodrilus ; and since, as has been 

 pointed out by various observers, it is a condition which is 

 by no means confined to the Archiannelida, but which occurs 

 also in various Chaetopoda that do not, in other respects, 

 present any primitive features.^ To the purely negative 

 feature of the absence of setse, also, it is impossible to attri- 

 bute much importance. 



In both the Polygordiidse and Histriobdellidse the 

 body is segmented ; but the nature of the metamerism differs 

 greatly in the two cases. In the former group the segments 

 are numerous ; they are not very sharply defined externally, 

 but internally their cavities are separated by transverse par- 

 titions or mesenteries. In the Histriobdellidae the segments 

 are few in number : they are defined externally by constric- 

 tions, and, in the case of Stratiodrilus, by the occurrence of 

 the paired appendages ; but only one intersegmental partition, 

 > See Mensch, 21. 



