12. _Anthophysa vegetans_, O.F. Muller (_Monadidae_). A typical,
erect, shortly-branching colony stock with four terminal
monad-clusters.
13. Monad cluster of the same in optical section, showing the
relation of the individual monads or flagellate zooids to the stem d.
14. _Tetramitus rostratus_, Perty (_Tetramitidae_).
a = nucleus.
b = contractile vacuole.
15. _Proterospongia Haeckeli_, Saville Kent (Choanoflagellata); A
social colony of about forty flagellate zooids.
a = nucleus.
b = contractile vacuole.
c = amoebiform cell sunk within the colonial gelatinous test
compared by S. Kent to a mesoderm cell of the sponges.
d = similar cell reproducing by transverse fission.
e = normal cells, with their collars contracted.
f = substance of test.
g = individual reproducing by multiple fission, producing
microzoospores, comparable to the spermatozoa of sponges.]
[Illustration: FIG. 3.