This is a specimen of Freshwater Fish Gill. The Fish Gill allows it to take in oxygen much like the lungs of land based animals do. The main difference is that gills are desgined to work with oxygen dissolved in water rather than free oxygen in the atmosphere.

This slide was purchased on Ebay and is part of 100 prepared slides " set A" I believe you'll find these images of Freshwater Fish Gill to be superior to those found elsewhere!