When studying living creatures, it is important to know the details of their lives. Water is the liquid that fills an echinoderms vascular system.
Water
A pitcher, or other container filled with liquid.
when a substance is in liquid phase
All vertebrates have closed circulatory systems, meaning that the blood never travels outside of a system of veins, arteries and capillaries.
Cytoplasm.
it completely fills its container, takes the shape of its container
Stretch receptors in the bladder wall are activated when the bladder fills with urine or liquid. These receptors signal the brain to indicate the fullness of the bladder, triggering the urge to urinate.
It is the liquid that fills the eyeball from the lens to the retina.
"Vitreous" . . . this is the jelly-like clear liquid that fills the eyeball.
Empty space.
A liquid has a definite volume but an indefinite shape. It takes the shape of its container.
Gas completely fills its container, liquid stays as a unit and fills the container with respect to gravity, and solids do not fill their containers