It is advanced than protonephridia (meta = beyond). Both open to the outside but metanephridia also open internally to the body fluids and are multicellular and both are structurally different.
Most annelids and many invertebrates have metanephridial excretory system. In earthworm each segment has a pair of metanephridia. It begins with a ciliated funnel, the nephrostone that opens from the body cavity of a segment into a coiled tubule. As beating cilia move the fluid through the tubule, a network of capillaries surrounding the tubule reabsorbs and carries away ions. Each tubule leads to an enlarged bladder that empties to the outside through neprhidiopore. The excretory system of molluscs includes protonephridia is larval stage and meta nephridia in adults.
Temperature regulation in fishes:
Temperature of surrounding water determines the body temperature of most fishes. Fishes that live in cold water have anti-freeze material in their blood. Poly alcohols (e.g. sorbital, glycerol) or water soluble peptides and glycopeptides lower the freezing point of blood plasma and other body fluids.
These fishes also have proteins or proteins sugar compounds that stunt the growth of ice crystals that begin to form. These fishes stay flexible and swim freely in a super cooled. Some active fishes maintain a core temperature significantly above the temperature of the water.
Temperature regulation in Amphibians and Reptiles:
Amphibians and Reptiles have air as surrounding medium and face daily and seasonal temperature changes. Mostly they are ectotherms. Most amphibians produce little heat metabolically and rapidly lose it. Behavioural adaptations keep their body temperature within homeostatic range most of the time. Their moist skin can act as a natural evaporative cooling system i.e. heat loss. It limits the habitats and activities of amphibians to warm, moist areas. Some amphibians as bull frogs can vary the amount of mucus they secrete from their body surface to regulate evaporative cooling.
Reptiles have dry skin which reduces the heat loss through evaporative cooling of the skin. They also have an expandable rib cage which allows for more powerful and efficient ventilation. Reptiles are endothermic. They have low metabolic rate and warm themselves by behavioural adaptations. Diving reptiles (sea turtles, sea snakes) conserve body heat by routing blood through circulatory shunts into the centre of the body.
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