Jan writes:
Those are probably naids (Naididae). Many of them have eyespots and
reproduce by forming chains of zooids. Most are harmless to fishes and
invertebrates. They are abundant in the diatom-felt that covers submerged
wood in blackwater streams.
The older aquarium literature (< 1960) often contains information on hydras,
flatworms, and segmented worms, probably because old-time hobbyists were
more likely to collect their own fish food and encounter them. Two books
(previously mentioned on this list) that cover these groups are: "Field
Guide to Lower Aquarium Animals" by Edward T. Boardman (Cranbrook Institute
of Science, 1939) and "Aquarium High Lights" by William T. Innes (Inness
Publishing Company, 1951). The first views invertbrates as pets, the second
as pests, but both contain a lot of good information and are available from
used book dealers.
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