Oligochaete worms

Oligochaete Annelida spp.Annelida performing a reproductive form 'paratomical fission' where a new head is formed in the middle of the organism and then break away from the parent worm leaving it with a new tail and the other Annelida will swim away.

Oligochaete Chaetochaster diastrophus
Commonly found in ponds and aquariums. The ones in the photos belong to the fresh water habitats. They are harmless. Fish will readily eat them. They can grow to about 4 mm. Their bodies are very transparent which exposes their well developed digestive and nervous system.

Photos by Dusko Bojic

Flatworm

Phylum Platyhelminthes
This worm can grow to 15 mm and in most cases is harmful. The one common in aquariums and ponds is the Turbellarian which feed on microorganisms. There is over 25.000 species of this worm and many are free-living like this one on the photo, but many are also parasitic.
Fish will eat them.

Photo by Dusko Bojic.