66 THE ESSEX NATURALIST
Volvocales—Stephanosphaera pluvialis, Spondylomorum quarter-
narium, Chlamydobotrys spp., Pandorina morum, Mastigosphaera
Gobii, Stephanoon Askenasii, Eudorina elegans, E. illinoisensis,
Valvulina spp., Pleodorina californica, Volvox globator, V. aureus,
V. tertius, and other species.
ROTIFERA.
Conochilus volvox, C. unicornis.
SYSTEMATIC POSITION
It will be evident from the foregoing list that, with the excep-
tion of Conochilus (a comparatively highly organised animal
belonging to the phylum Rotifera), free-swimming spherical
colonies are only formed by flagellated unicellular organisms.
There seems to be no case of a ciliate producing such a colony
and even among the flagellate types there is a very marked
restriction of occurrence; many important groups, such as the
Cryptomonadaceae, Dinoflagellatae and Euglenineae, being
without any representatives of such colonies. Why this should
be so is not at present clear, but it probably depends upon some
special combination of characters the elucidation of which would
yield valuable results.
STRUCTURE
There is much variation in the arrangement of the units
forming these spherical colonies. In many cases they are closely
packed together radiating from the centre of the sphere (e.g.
Pelosphaera, Synura, Pandorina, etc.), or the radiate arrangement
may be retained with more or less open spacing (e.g. Sphaeroeca,
Conochilus, etc.). In other cases the cells may be arranged in
an equatorial ring (e.g. Stephanosphaera and Stephanoon). In
still others in a series of rings (e.g. Eudorina) or in very large
numbers just under the surface of the sphere (e.g. Volvox).
One feature which is probably common to most if not all
motile spherical colonies, although this to be sure is also shared
by many of the non-motile and non-spherical forms, is that the
individual units are held in their places by a gelatinous or
mucilaginous matrix. This jelly may be very small in amount,
only surrounding perhaps the inner ends of closely packed
radiating cells, but usually it is more abundant than this and
may be so copious that the cells are widely separated and brought
near the surface of a voluminous gelatinous ball as is so par-
ticularly well shown in Volvox.
A more direct connection than merely being embedded in
jelly is seen in some cases where the individuals are attached to
stalks radiating from the centre of the sphere (e.g. Uroglena) or
are connected by protoplasmic strands (e.g. some, but not all,
species of Volvox).