Hydrodictyon reticulatum or water net cells aggregate to form colonies or mats.colonies or mats are

• a single-layered membrane or sac-like network of several thousand cells
• free-floating or secondarily attached
• reaching over 50cm long
• meshes clearly visible in large colonies

Cells are

• cylindrical to oblong-oval, up to 15mm long
• 2-4 adjacent cells connected by their edges and forming 5-6-sided meshes