HEKATEROBRANCHUS SHRUBSOLII. 189 



tion of the nerve-cord, the distinctness of the two cords from 

 one another^ and the absence of ganglionic swellings. 



It differs from all other genera of the family Spionidse in the 

 possession of only one pair of dorsal branchiae. ^ These are on 

 the first or head-segment. They are found in this position as 

 well as on the following segments in some species of some of 

 the other genera (e. g. Spio fuliginosus/ Scolecolepis 

 vulgaris, and Scolecolepis cirrata^). The single pair 

 of branchiae of Hekaterobranchus are much larger and more 

 developed than are any of the numerous branchiae of other 

 Spionidae. This reduction in number of the branchiae^ their 

 increase in size, and their position on the head together seem 

 to indicate that the worm once led a more sedentary life than 

 it now does. Other facts leading to the same conclusion are — 

 the presence of a ventral collar, the single pair of modified 

 thoracic nephridia, the reduction of the parapodia, and, 

 as in all other sedentary annelids, the possession of crotchet 

 chaetae. 



It is true that most genera of the family Spionidae, with 

 a much greater number of dorsal branchiae, inhabit now 

 much more distinct tubes than Hekaterobranchus. It 

 would appear, therefore, either that they have not yet dege- 

 nerated so far as Hekaterobranchus from the ancestral Spio, 

 or that Hekaterobranchus has developed other modes of respira- 

 tion which other Spionids have not, e. g. the anal respiration 

 and that of the cephalic tentacles. For although in other 

 genera the cephalic tentacles do serve as respiratory organs, 

 they do so to a very slight extent only: they are ciliated only 

 on one side, and they serve mainly as prehensile and tactile 

 organs ; whereas here they are ciliated all over, and probably 



' See, however, note at the end of this paper. 



2 Claparede, 'Ann. Chset. du G. de Naples,' 1868, part ii, pi. xxiii, fig. 1. 

 In the text (p. 63) Claparede says the branchiae begin on the second segment ; 

 but this is evidently a slip, as, in his definition of the species (p. 62), he says, 

 rightly enough, they begin on the first setigerous segment ; and here, as in 

 most, if not all the Spionidse, the first segment is setigerous. 



3 Malmgren, 'Ann. Polych.,' 1SG7, pi. x, figs. 54a and a'. 



