Nematodes are nonsegmented roundworms that are abundant in
freshwater lakes. Nematodes often comprise 15% of the total
biomass on lake bottoms. This reflects the incredible abundance
of these organisms, since most only grow to a maximum length of
a centimeter. Life stages of these species are often as complex
as they are abundant. Some are free living their entire lives
while others are only free living as adults or juveniles. At
other stages some are parasitic on invertebrates, vertebrates,
or even plants. Most freshwater free-living nematodes are about
1 mm in length though parasitic forms are often even smaller.
Their body wall is covered by a cuticle that is four layers
thick. As the worm grows it moults, sloughing off the outer
layer. At the same time another layer is created on the inside.
The pseudocoel is small in the free-living forms but tends to be
much larger in the parasitic forms. All freshwater nematodes
bear a spinneret at the tip of their hind end that secretes a
sticky mucous which anchors t