194 On British Species of Siphonostoma. 



varied from 140-150, as contrasted with over 200 in the 

 adult ; the number of gills varied from 60-80, as compared 

 w'itli about 100 in tlie adult. The distance of the annuli 

 apart varied from 0'0158-0'034:2 millim. in the cephalic 

 bristles. The notopodial papillai bore usually 6 long pro- 

 jecting bristles (cf. S. ajinis), but there were 8-9 small 

 additional bristles buried in the substance of the papilla. 

 These small bristles are not visible except after careful search, 

 and under a low power the general aspect of the papilla 

 closely resembles that of S. qfflnis. The only constant 

 difference which I could find lies in the fact that, in specimens 

 of the two species of the same body-length, the dorsal bristles 

 of S. diphchaitos are longer than those of S. affinis. Thus 

 in specimens about 25 millim. long those of the latter species 

 are 1"6-1'2 millim. long, tliose of the former 2*l6-2"37 millim. 



The small specimens of IS. diphcha'itos displayed on their 

 neuropodial papillae 1-2 hooks whose bases were surrounded 

 by 6 simple bristles. The hooks did not agree closely in 

 their minute cliaracters with those of St. Joseph's specimens. 



This description shows that S. diplocha'itos changes in 

 character during growth, the changes occurring in those 

 special characters which serve to differentiate the adult from 

 S. affinis. Thus the young as compared with the adult show 

 a diminished number of segments, gills, cephalic bristles, 

 ordinary noto] odial bristles, and neuropodial hooks ; and it is 

 precisely the numbers of these structures which constitute the 

 distinguishing marks of the species. In other words, the 

 young S. dijlochaitos approximates in character to the adult 

 tS. affinis. The present observations do not support St. Joseph's 

 suggestion that the minute characters of the bristles are of 

 specific importance. In view of the approximation of the 

 young S. diphcha'itos to the characters of the adult S. affinis, 

 it must be doubtful how far the two are entitled to rank as 

 separate species. 



It may seem remarkable that the Plymouth specimens of 

 S. offinis should agree so closely with St. Joseph's descrip- 

 tions, while the Millport specimens should differ so markedly 

 from his descriptions of S. diplocha'itos ; but it is to be noticed 

 that in the case of S. affinis the two sets of specimens are 

 both Channel forms, while the specimens of S. diplocha'itos 

 come from such widely separated localities as the Bay of 

 Naples and the Firth of Clyde. 



It is interesting to note that St. Joseph describes from 

 Saint-Jean-de-Luz * a new Siphonostoma under the name of 



* " Ann^lides PolychetPS des Cotes de France," Ann. Sci. Xat. (Zool.) 

 V. (1897) p. 363. 



