6 THE ESSEX NATURALIST
the period of motility of the swarmer ; if this period be very
short, the new cell will form its lorica on that of the mother cell,
if long then solitary individuals are formed.
Colonies comparable to those of Dinobryon are those which
occur in the non-motile Ankistrodesmus falcatus var. stipitatus
(Chod.) Lemm. among the Chlorophyceae, and in the Sciadium
section of Ophiocytium (e.g. O. arbusculum) among the Xantho-
phyceae, for here daughter cells develop at the mouth of the
mother cell and thus build up branched structures representing
several successive generations.
Among the chrysophycean algae there is found a series of
amoeboid, or rhizopodial forms. Of these, Rhizochrysis Scherf-
felii Pascher may exist as solitary individuals with long thin
rhizopodia, but often several individuals form a little cluster
which is probably to be regarded as a colony, although I have
never been able to demonstrate any connection between in-
dividual cells either by the presence of investing mucilage or by
anastomoses between the rhizopodia; nevertheless, the cells
of such a group in some way remain together. In Chrysidiastrum
the cells are united by their rhizopodia into a chain, while in
Chrysarachnion a number of individuals form a net-like expanse
which serves to trap relatively large organisms. Finally we
have Myxochrysis, which exists as a multinucleate plasmodium;
plasmodia may fuse together: reproduction involves, directly or
indirectly, the formation of amoebae which are able to fuse and
build up the characteristic multinucleate plasmodium.
Apart from the algae the formation of colonies of rhizopodial
individuals is seen in the Heliozoa where, in Raphidiopsis elegans
H. and L. individuals are united, much as in Chrysidiastrum,
to form more or less spherical colonies. Among the slime
moulds the Acrasieae pass their vegetative stage as independent
amoeboid cells (myxamobae); at spore formation these cells
collect together in large numbers to form a pseudoplasmodium,
which changes into a fruit body. In Dictyostelium, for example,
the pseudoplasmodium becomes heaped up in its centre, and the
amoebae in this region become walled; a tapered stalk is formed
from this column by the movement of more amoebae while
yet others migrate up this stalk and form a mass at its apex;
these last constitute the fertile part of the fruit body or sorophore
and secrete walls, remaining thus as spores until they are dis-
persed, when, under favourable conditions the spore wall breaks
and an amoeba is liberated. In the Myxomycetes the spores
give rise to gametes which fuse in pairs, the resulting amoeboid
zygotes fusing in large numbers to form a true plasmodium,
which may be compared to a huge multinucleate amoeba and
from which, eventually, sporangia arise.