PROTOZOA 21 
the Radiolarian ; more recent research has, however, shown that 
they continue to live after the death of the animal, that they 
multiply in more than one way, occasionally forming mobile 
swarm-spores, that similar cells occur in the tissues of many 
Coelenterates, and that they contain chlorophyll, although this 
colouring matter is masked by a yellow pigment. A nucleus 
and a cellulose cell-way are also present. These features have 
caused these yellow cells to be regarded as unicellular algae, 
living in a state of commensalism with the Radiolarian, and 
they have received the name of Zooxanthella nutricola. They 
are not found in all species, and are usually absent in 
Acanthometra, and in the other species with a horny skeleton. 
The protoplasm contains, in addition to the yellow cells, 
numerous oil or fat globules, and crystals and concretions of 
unknown use. 
No conjugation has ever been observed in this class; repro- 
duction is sometimes by simple fission, which commences first In 
the central capsules. Spore formation in the central capsules 
also takes place, and results in the formation of mobile spores ; 
but the details are complex, and the exact sequence of events 
not thoroughly understood. 
Many form colonies by the fusion of their extracapsular 
protoplasm. That of Collozowm, the individuals forming which 
are devoid of skeleton, may be an inch or more long. The 
various members of the colony are held together by a gelatin- 
ous matrix. 
Group B. CorTIcaTa. 
The animals which are grouped together in this second 
division of the Protozoa have as a common feature a differen- 
tiation of the protoplasm into a more fluid central portion and 
a firmer cortical layer usually associated with a limiting mem- 
brane, which surrounds their body and gives it a definite shape. 
As a consequence of the presence of this cortical layer, these forms 
which take solid food have acquired one or more channels through 
which the nutriment is ingested, and usually a definite area 
whence the undigested remnants are extruded. The parasitic 
forms, which live in the nutritive fluids of their hosts, are 
usually devoid of any such cell mouth or anus. The presence 
