Echinoderms cannot control the osmolarity (concentration of dissolved solutes) of their body fluids, so the osmolarity is the same as the medium they are in. this explains why they

This explains why they water vascular systems.

Echinodermata are a group of marine organisms that are osmoconformers. This means that their body fluids are similar to their surrounding water in osmolarity, in other words their internal environment is osmotic to their external environment. So, Echinodermata don’t expend energy to expel water or salt from the body. Echinodermata have unique water vascular system that helps them in gas exchange, feeding, sensory reception and locomotion.  

Disadvantage of osmoconformers is that they are sensitive to changes in the osmolarity of their environment.

yes it is weird but true