however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure. Marine teleosts also use their gills to excrete osmolytes (e.g. Na⁺, Cl−). The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids. Seawater contains more osmolytes than the fish's internal fluids, so marine fishes naturally lose water through their gills via osmosis. To regain the water, marine fishes drink large amounts of sea water while simultaneously expending energy to excrete salt through the Na+/K+-ATPase ionocytes (formerly known as mitochondrion-rich cells and chloride cells). Conversely, fresh water contains less osmolytes than the fish's internal fluids. Therefore, freshwater fishes must utilize their gill ionocytes to attain ions from their environment to maintain optimal blood osmolarity. Lampreys and hagfish do not have gill slits as such. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally and a separate tube which has no respiratory tissue (the pharyngocutaneous duct) develops beneath the pharynx proper, expelling ingested debris by closing a valve at its anterior end. Lungfish larvae also have external gills, as does the primitive ray-finned fish Polypterus, though the latter has a structure different from amphibians. Amphibians Tadpoles of amphibians have from three to five gill slits that do not contain actual gills. Usually no spiracle or true operculum is present, though many species have operculum-like structures. Instead of internal gills, they develop three feathery external gills that grow from the outer surface of the gill arches. Sometimes, adults retain these, but they usually disappear at metamorphosis. Examples of salamanders that retain their external gills upon reaching adulthood are the olm and the mudpuppy. Still, some extinct tetrapod groups did retain true gills. A study on Archegosaurus demonstrates that it had internal gills like true fish. Invertebrates Crustaceans, molluscs, and some aquatic insects have tufted gills or plate-like structures on the surfaces of their bodies. Gills of various types and designs, simple or more elaborate, have evolved independently in the past, even among the same class of animals.