THE COLONIAL HABIT IN SOME LOWER ORGANISMS 5
colonies are built up; but colony formation here is a very
ephemeral affair, for any cell may develop a flagellum and leave
the colony to start a new one.
Turning now to colonies which are formed by the apposition
of cells, we find it difficult indeed to draw a dividing line between
this type of colony and those which have just been considered.
Thus in Chrysodendron the union is between the tips of tenuous
protoplasmic prolongations, while in Synura the individuals are
united only by their posterior extremities. Probably the sim-
plest colonies formed by apposition of cells are those of Chromu-
lina Hokeana Pascher, where two to eight cells remain associated
for a short time as a temporary colony. More permanent are
the spherical colonies of Synura, composed of numerous in-
dividuals. In Chrysosphaerella longispina Lauterb. the in-
dividuals also appear to be united by their posterior ends, but
the colony is embedded in a copious mucilage through which
the flagella protrude. The spherical colonies of Syncrypta
volvox Ehr. are supposedly of similar form, but there is reason
to believe that our knowledge of this genus is erroneous.
Lateral attachment of the individual cells leads to the forma-
tion of colonies of diverse forms, such as the ring-like ones
of Cyclonexis and the mulberry shaped colonies of Chrysobotrys
spondylomorum Conrad. Clusters of cells forming more or less
spherical colonies by the apposition of their posterior ends may
arise from a stalk which is to be regarded as a localised secretion
of the cells ; such colonies are seen in Codonosiga botrytis
(Ehrenb.) Kent and in Anthophysa. In the. latter it has been
demonstrated recently that the stalks, which were regarded as
mucilage stained brown with iron salts, are not, in fact, mucilage
and that the brown colour is due to manganese and not iron.
In a number of flagellates the individual is enclosed within a
cup-like structure, the lorica, and such loricate forms may be
colonial. In the solitary species of Dinobryon the protoplast
divides longitudinally and one daughter cell remains within the
lorica while the other swims away and forms a new case for
itself. The behaviour in the colonial species is similar except
that the daughter cell which escapes settles upon the rim of the
parent lorica and there forms its own case ; both daughter cells
may behave in this manner. In the related genus Hyalobryon
we may observe a series of stages from the solitary species,
through solitary but gregarious ones and others which form the
loricae in clumps in which individual loricae are generally free
from one another, to the colonial species H. ramosum Lauterb.
where the escaping daughter cells form their loricae on the
outside of the parent one. In Dinobryon and Hyalobryon it
would seem that the determining factor in colony formation is