230 Records of the Indian Museum. LVOr.Vin: 
a number (usually four, i.e. segments ii—v) of segments which 
have ventral but no dorsal setae. It is frequently found more- 
over, e.g. in the genus+Nazs, that the ventral setae of these 
‘cephalized’ segments are different in type from the succeeding 
ones ; thus the distal of the two prongs of the forked end may be 
relatively longer, and the nodulus situated proximal, instead of 
distal, to the middle of the shaft. 
Other systems or organs are concerned. Beddard instances 
septa and nephridia, which may be absent from the anterior 
segments. In addition I may adduce pigmentation ; the cephal- 
ized segments may be lighter in colour, or the pigment may be 
differently arranged, as compared with the following segments. 
Chloragogen cells are absent from the alimentary tract in the 
cephalized segments. The gills of Branchiodrilus hortensis are 
similarly absent from this region. 
The next point which I wish to bring forward is that this 
cephalization is related in the Naididae to the manner of asexual 
division, and to the production, between two separating individuals, 
of a budding zone. When one of the Naididae divides asexually, 
the usual procedure is that at some spot near the middle of the 
length of the animal a rapid production of new segments takes 
place ; of these segments the larger number go to form the tail 
end of the anterior, the smaller number to form the head of the 
posterior, of the two resulting animals. This head commonly 
consists of fivesegments, with a prostomium, all newly formed ; 
i.e., it corresponds to the number of cephalized segments as 
determined by the examination of free-living specimens. In other 
words these segments, produced in the budding zone, and represent- 
ing the head of the (subsequently to be detached) animal, want 
the dorsal setae, and frequently have the ventral setae modified ; 
they are commonly, at first at least, less pigmented (as are also 
the newly formed segments at the posterior end of the anterior 
animal); they contain no chloragogen cells, have no nephridia, 
and in Branchiodrilus hortensis are without gilis. 
Since the predominant mode of reproduction in the Naididae 
is the asexual, by fission,—sexual reproduction being a compara- 
tively, or absolutely, rare occurrence,—by far the larger number 
of individuals of a species existing at any time will have been pro- 
duced asexually, and the cephalized segments will be those which 
have been produced in a zone of budding. 
1 This point with regard to the budding zone has not apparently received 
much attention from students of the Naididae and (in the somewhat scanty 
literature at my disposal) I cannot find any references as to how many of the 
segments produced in the budding zone go to the anterior end of the posterior 
animal in the different genera. My own observations on the genera Chaetogaster, 
Nais, Slavina, Stylavia, Aulophorus and Branchiodrilus show that therule just stated 
holds for these (in Chaetogaster, where there are no dorsal setae, cephalization is 
marked by the regular series of ventral setae beginning only in the sixth segment) ; 
and it apparently holds also for Aeolosoma (fam. Aeolosomatidae}, where the 
process is similar. It is to be noted however that Pristina is a remarkable 
exception; here no fewer than seven of the anterior segments are formed in the 
budding zone, though dorsal setae begin on the second segment, and cephalization 
is therefore confined to the first. 
