1912. | J. STEPHENSON: Indian Aquatic Oligochaeta. 233 
With the discovery of B. menont the case becomes stronger. 
I was in doubt at first as to whether it would not be advisable to 
unite it with Bqurne’s worm under the same species, B. sempert ; 
since the points of structural agreement are many, and in some 
cases extend into detail; and there can at least be little doubt 
of a close genetic connection. Yet, allowing the usual value to 
cephalization, the difference between some specimens of B. menon 
and B. semperi would be generic; and indeed the same might be 
said of specimens of B. menoni alone, when compared one with 
another. 
The conclusion to be drawn is that the degree of cephalization 
in the Naididae is correlated with the behaviour of the segments of the 
budding zone, and that variations in both, of considerable extent, 
may occur within the same species. The form of cephalization which 
is characterized by the absence of dorsal setae from a number of the 
anterior segments of the body has been evolved repeatedly, in different 
croups of the Naididae, and at different times. The degree of 
cephalization is not necessarily a generic character, since differing 
degrees of cephalization may coexist with a remarkable sinularity in 
general organization. 
One point of a more speculative nature remains. In the case 
of Branchiodrilus hortensis, which usually has five prebranchial 
segments, specimens are occasionally met with which possess 
only four such segments ; in these cases the ‘first gilled segment 
would seem to be homologous throughout, i.e. the sixth segment 
of some individuals is homologous with the fifth of others. 
Similarly, according to Piguet, in Nais communis (v. ant.) :— 
‘“mais tous ces individus (i.e. those with only four segments in 
front of that on which the dorsal setae began) avaient la région 
antétieure absolument normale, sauf que leur 5me segment était 
Vhomologue du 6me des autres.” 
Similar considerations must be extended to Branchiodvilus 
menont ; the first gilled segment is homologous, throughout the 
individuals of the species, though these individuals vary as regards 
the number of segments intercalated in front of this fixed point 
and behind the mouth. 
But while some specimens of B. menont agree in the arrange- 
ment of their segments (as determined by the setal bundles) with 
B. semperi, others agree with B. hortensis. It follows that the 
first gilled segment is homologous in these two species, i.e. 
segment vi of B. hortensis is homologous with segment ii of 
B. semper. 
If this is the case, then we must infer that the same holds 
generally in the Naididae, and that the segment immediately 
succeeding those produced in the budding zone,—usually, but not 
always, that on which the dorsal setae begin,—is homologous 
thoughout the group. 
_! Compare however what was said previously (footnote p. 230) regarding 
Pristina. 
