Generally most of the osmotic balances are controlled by  ionic exchange system but surprisingly there is no unique/specific  mechanism dealing on  this issue in echinoderms. Firstly each class of echinoderms have their own way of control system  for example in Sea urchin Heat shock proteins are key regulators while in  of Sea cucumbers potassium channel and in case brittle star it was sodium and calcium  like this they have diverse kind of control system. For your reference here by am attaching a paper,  i hope that can help you to understand further things in depth. Good luck 

Sea urchins may temporally tolerate high salinity conditions as well as they can stay alive outside of water for relative long periods. However, they may not live in high salinity conditions if this is not their common habitat. Sea urchin gametes are much more sensitive to physico-chemical conditions of water, so the adult sea urchin may resist those conditions while their gametes dont.

here is the paper on related topic which might be interesting to you:

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0049798