In this family the mycelium is very sparsely developed or is
wanting. The entire plant consists principally or entirely of a
single zoosporangium whose zoospores have generally one cilium. The
resting-spores arise either directly from the zoosporangium, which,
instead of forming zoospores, surrounds itself by a thick cell-wall; or
they are formed by the conjugation of two cells (in which case they are
spoken of as oospores). Microscopic Fungi, parasitic on water plants
(especially Algæ) or small aquatic animals, seldom on land plants.
Order 1. =Olpidiaceæ.= Without mycelium. Swarmspores and
resting-spores.
In the _Olpidieæ_, the swarmspores, probably, most frequently
form themselves into a plasmodium (naked mass of protoplasm)
which may become a single zoosporangium or a resting sporangium.
_Olpidium trifolii_ occurs in _Trifolium repens_.--In the
_Synchytrieæ_ the plasmodium emerging from the swarmspores
breaks up either at once, or after a period of rest, into
smaller plasmodia, each of which will become a zoosporangium.
_Synchytrium anemones_ is found on _Anemone nemorosa_; _S.
mercurialis_ on _Mercurialis perennis_; _S. aureum_ on many
plants, particularly _Lysimachia nummularia_.
[Illustration: FIG. 86.--_Chytridium lagenula._ Zoosporangium _a_
before, _b_ after the liberation of the swarmspores.]
[Illustration: FIG. 87.--_Obelidium mucronatum_: _m_ mycelium;
_s_ swarmspores.]
Order 2. =Rhizidiaceæ.= Mycelium present. Zoospores and
resting-spores.
_Chytridium_ (Fig. 86). _Obelidium_ (Fig. 87) is bicellular; the
one cell is the mycelium, the other the zoosporangium; found
on insects. The species of _Cladochytrium_ are intercellular
parasites on marsh plants. _Physoderma._
Order 3. =Zygochytriaceæ.= Mycelium present. Zoospores and oospores.
The latter are the product of the conjugation of two cells (Fig. 88).
_Polyphagus euglenæ_ on _Euglena viridis_. _Urophlyctis pulposa_
on species of _Chenopodium_.