Brkisli Species o/Siphonostoma. 191 



S. affinis (specimens from S. diplochaUos (Naples 



Diiiard). specimens). 



Length 20-40 niillim. Lengtli 70-80 millim. 



Breadth 2-3 millim. Breadth 7 millira. 



Segments 30-47. Segments 39-40. 



Gills 40-.')0. Gills 80-100. 



Cephalic bristles CO-80. Cephalic bristles 200 or more. 



Notopodial bristles 4-5. Notopodial bristles 12-14. 

 Neuropndium with 1-2 honks and Neiiropodium with 4-5 hooks and 

 6 simple bristles. 6 simple bristles. 



Thus, apart from size, the differences chiefly consist in the 

 disparity in numbers of the various appendages. These 

 distinctions are somewliat vitiated by the fact that St. Joseph 

 is inclined to regard ChlorcBma Dujard'inn^ Q%«> ^s tlie 

 young of S. affinis^ and it differs in the diminished number 

 of segments, gills, and cephalic bristles. It is obvious that if 

 in *S'. affiiiis these structures increase in number throughout 

 life, they may also do the same in S. diplochaitos^ and the 

 young stages of the latter may thus resemble the adults 

 of S. affihu. In fact this is precisely what the iMillport 

 specimens show. St. Joseph, however, further seeks to 

 differentiate the two species by the minute characters of the 

 bristles, and states that these are of much importance in 

 distinguisiiing the species in the genus. The special points 

 upon which he relies are the following : — 



In S. affinis the distance between the successive annuli of 

 the cephalic bristles is 0*025 millim. The hooks have the 

 following structure : immediately below the hooked region 

 there are ^Q annuli, the intervening spaces gradually 

 increasing, but never exceeding 0'02 millim. ; there is then a 

 long gap of O'J millim. in length, followed by 10-20 annuli 

 separated by spaces of about 0'02 millim. 



In H. diplocha'iios the distance between the successive rings 

 of the cephalic bristles is only 0'0168 millim. In the hooks 

 the hooked region is less curved ; it is followed by 50-60 

 annuli placed very close together, then by 4 annuli separated 

 by distances of 0'048 millim., finally by 50 closely aggre- 

 gated annuli. 



As to my own observations, the Plymouth specimens agreed 

 very closely with St. Joseph's description of S. affinis. The 

 length varied from 15-25 millim., with a usual breadth of 

 2 millim. The number of segments varied from 39-45, and 

 there were about 40 gills. In regard to the cephalic bristles, 

 there were some slight differences ; usually numbering about 

 80, in one case at least a very careful count gave over 100 

 though some of these were short and small. As to the 



