NOTES ON THE NAIDIFORM OLTGOCH^TA. 337 



of the group; and Mr. E. C. Bousfield ^ has still more recently 

 in a valuable paper given a systematic account of the various 

 species of the genus Dero at present known. 



1 am induced, even after this lapse of time, to publish some 

 of my uncompleted notes, partly because no one has since 

 described the new species which were discovered by Professor 

 Lankester and myself,^ and partly because no writer on the 

 group has adequately dealt with the importance of the number 

 of cephalized segments as a generic character. 



A monograph of the British species is still a desideratum. 

 I received from Mr. Bolton on several occasions specimens 

 other than those herein described, which were certainly not 

 referable to any existing species, but for which my notes are 

 insufficient to warrant the creation of new species. It has 

 frequently been pointed out, but very little stress has been 

 laid upon the fact even by Vejdovsky,^ that the dorsal setae 

 are often wanting in several of the anterior segments of the 

 body, while ventral setse are present in these segments. It is 

 this character which chiefly marks what I term, at Professor 

 Lankester^s suggestion, a cephalization. There is almost 

 always, if not always, a certain amount of cephalization in the 

 Oligochseta; that is to say, there is in the anterior region 

 a segment or number of segments which differ in their 

 organization from the segments which follow, these latter 

 being usually similarly developed throughout the remainder of 

 the worm. This may be exhibited by peculiarities of the 

 alimentary canal, the circulatory system, the arrangement of 

 septa, the absence of nephridia from the most anterior 

 segments, and so on. In most if not all Oligochseta there is a 

 peristomial segment which is devoid of any setse, and in many 

 Naids the dorsal setae are absent from three, four, or six of the 



' ' Journ. Linn. Soc. Zoology,' vol. xx, 1887, p. 91. 



2 Professor Lankester referred to Pterostylar ides macrocbseta (under 

 the name of Pterygonais macrochseta) and exhibited my drawing of this 

 worm, which is here published, at the meeting of the British Association at 

 Southport in 1883. 



8 ' Oligochaeten,' 1884. 



