Classification of some colonial Chlamydotnonads. 155 
orientation of these two contractile vacuoles, and the fact they 
often exceed the two in number certainly points to an affinity with 
Sphccrella rather than with Chlamydomonas. It may be noted that 
even in the former genus there is a reduction in the vacuole 
system, for Sphccrella Butschlii only possesses two or at the most 
three [22]. 
A single chromatophore is found in each cell of Volvox. It 
has the form of a peripheral sheet, comparable with that found in 
Chlorogouium. Young cells of Sphccrella also exhibit this structure, 
although at a later stage in their development their chromatophore 
frequently becomes lacunated and forms a network. The chroma¬ 
tophore of Volvox is characterised by its angular outline; it may 
he protruded deep into the protoplasmic strands which run out 
rom the protoplast. The same irregular outline is seen in the 
chromatophore of Sphccrella, and in 5. droebakensis the normal 
zoospore has a continuous peripheral chromatophore protrusions 
of which run deeply into the pseudopodial outgrowths [22]. The 
pyrenoid is too variable throughout the Chlamydomonadales to be 
of any great systematic importance. 
The cell-wall of Volvox gives this genus its chief claim to be 
ranked apart from the other colonial Chlamydomonads. Each cell 
possesses an individual envelope of two layers, a thin outer lamella 
and a thick highly refractive and mucilaginous inner layer which 
often renders the wall very massive in proportion to the protoplast. 
The species of Volvox differ somewhat amongst themselves in the 
form of their separating lamellae. In V. tertius [Fig. 2 C] the 
globular cells are completely surrounded by the outer lamella, 
adjacent ones being in contact at their equatorial regions, but 
otherwise separated by an intercellular jelly. During development 
[12] the products of division of the mother cell are at first only 
separated by particularly refractive protoplasmic layers. As soon 
as division is complete and flagella have been formed the cells 
separate from one another and a membrane which at an early 
stage appears as two layers, is seen between them. The mucilage 
layer is at first relatively thin ; it grows much more rapidly than 
the other parts however, becoming a thick investment. The cells 
of V. tertius thus have fundamentally the same structure as in 
Sphccrella. The other species of Volvox are slightly modified but 
possess the same essentials. In V. globator [Fig. 2 D] the lateral 
lamellae of the hexagonal prismatic cells are common to adjacent 
elements. In V. aureus [Fig. 2 EJ the lamella of each cell is 
