Homeostasis is the ability of the cell to maintain the state of balance despite of changing external environmental conditions. Osmotic pressure is the hydrostatic pressure of water across the cell membrane. Tonicity is the measurement of this pressure. If the concentration of solutes on both sides of the membrane is equal then water will not move across the membrane. Osmotic pressure maintains the homeostasis of the cell. In hypertonicity osmotic pressure of the solution outside the cell is higher than the inside the cell. The water inside the cell will exit the cells in order to equalize the osmotic pressure on both sides, causing the cells to shrink.
Arthropoda and molluscs have open circulatory systems - meaning haemolymph (Their equivalent to blood) directly bathes the tissue, and isn't enclosed in blood vessels, as seen in a closed blood system, as in humans for example.
