lly coiled in numerous close turns,
motile, but apparently owing to flexile movements, as no cilia are
found.

II. ORDER--TRICHOBACTERINAE. Vegetative body of branched or unbranched
cell-filaments, the segments of which separate as swarm-cells (_Gonidia_).

1. _Family_--TRICHOBACTERIACEAE. Characters those of the Order.

(a) Filaments rigid, non-motile, sheathed:--_Crenothrix_ (Cohn),
filaments unbranched and devoid of sulphur particles; _Thiothrix_
(Winogr.), as before, but with sulphur particles; _Cladothrix_
(Cohn), filaments branched in a pseudo-dichotomous manner.

(b) Filaments showing slow pendulous and creeping movements, and
with no distinct sheath:--_Beggiatoa_ (Trev.), with sulphur
particles.

The principal objections to this system are the following:--(1) The
extraordinary difficulty in obtaining satisfactory preparations showing the
cilia, and the discovery that these motile organs are not formed on all
substrata, or are only developed during short periods of activity while the
organism is young and vigorous, render this character almost nugatory. For
instance, _B. megatherium_ and _B. subtilis_ pass in a few hours after
commencement of growth from a motile stage with peritrichous cilia, into
one of filamentous growth preceded by casting of the cilia. (2) By far the
majority of the described species (over 1000) fall into the three
genera--_Micrococcus_ (about 400), _Bacillus_ (about 200) and _Bactridium_
(about 150), so that only a quarter or so of the forms are selected out by
the other genera. (3) The monotrichous and lophotrichous conditions are by
no means constant even in the motile stage; thus _Pseudomonas rosea_ (Mig.)
may have 1, 2 or 3 cilia at either end, and would be distributed by
Fischer's classification between _Bactrinium_ and _Bactrillum_, according
to which state was observed. In Migula's scheme the attempt is made to
avoid some of these difficulties, but others are introduced by his
otherwise clever devices for dealing with these puzzling little organisms.

The question, What is an individual? has given rise to much difficulty, and
around it many of the speculations regarding pleomorphism have centred
without useful result. If a tree fall apart into its constituent cells
periodically we should have the same difficulty on a larger and more
complex scale. The fact that every bacterial cell in a species in most
cases app