Final answer:
Sea lilies and feather stars, both crinoids, differ in mobility and habitat, with the former being sessile and the latter free-moving. Unlike other echinoderms, crinoids do not have madreporites because water enters their system through other means.
Step-by-step explanation:
Sea lilies and feather stars are examples of Crinoidea, belonging to the larger group of echinoderms. These marine invertebrates exhibit distinct features; sea lilies are sessile and anchored to the substrate by a stalk, whereas feather stars are free-moving and use their cirri for locomotion. Though both are suspension feeders, utilizing their feather-like arms for capturing food, they differ in their ability to move and habitat preferences, with feather stars often found in shallow tropical waters while sea lilies are more common in deep waters.
Crinoids, such as sea lilies and feather stars, do not possess madreporites. A madreporite is a structure through which water enters into the water vascular system of many other echinoderms, including sea stars and sand dollars. However, in crinoids, water enters their system through their mouth or grooves in their arms, negating the need for a madreporite.