The illustration depicts respiratory system of fish:

1. Mouth - A muscular pump pulls water in through the mouth and pushes it back across the gills.
2. Gill Filaments - Water is pumped past thousands of threadlike gill filaments, which are rich with capillaries. Filaments absorb oxygen from water and release carbon dioxide.
3. Operculum - Water carrying carbon dioxide is pumped out behind the operculum, or gill cover.