where does water enter the vascular system of a starfish
Water vascular system In sea stars, water enters the system through a sieve-like structure on the upper surface of the animal, called the madreporite. This overlies a small sac, or ampulla, connected to a duct termed the stone canal, which is, as its name implies, commonly lined with calcareous material. The stone canal runs to a circular ring canal, from which radial canals run outwards along the ambulacral grooves. Each arm of a sea star has one such groove on its underside, while, in sea urchins, they run along the outside of the body.[11]:933â€“939
Turgor pressure It is also called hydrostatic pressure, and more intricately defined as the pressure measured by a fluid, measured at a certain point within itself when at equilibrium.[2] Generally, turgor pressure is caused by the osmotic flow of water and occurs in plants, fungi, and bacteria. The phenomenon is also observed in protists that have cell walls.[3] This system is not seen in animal cells, seeing how the absence of a cell wall would cause the cell to lyse when under too much pressure.[4] The pressure exerted by the osmotic flow of water is called turgidity. It is caused by the osmotic flow of water through a selectively permeable membrane. Osmotic flow of water through a semipermeable membrane is when the water travels from an area with a low-solute concentration, to one with a higher-solute concentration. In plants, this entails the water moving from the low concentration solute outside the cell, into the cell's vacuole.[5]