The composition of body fluids of marine invertebrates, including the ascidians are similar to seawater. Such animals need not expend much energy in regulating the osmolarity of their body fluids. In a few vertebrates too plasma is found isosmotic to their environment. In hagfish (Myxine), for example, the concentrations of ca2+, Mg2+ and SO2-4 are maintained significantly lower than they are in the seawater, whereas Na+ and Cl- are maintained higher in the body fluids than the seawater. Like the hagfish, the cartiligenous fishes such as sharks, rays and skates have plasma that is isosmotic to the seawater.
However, in these fishes the concentration of inorganic electrolyte is maintained far lower than the seawater. The excess inorganic electrolytes such as NaCl are excreted via the kidneys and also by means of a special excretory organ, the rectal gland, located at the end of the alimentary canal.