Final answer:
Sea cucumbers have an internal madreporite, unlike most echinoderms which have an external one, an adaptation potentially related to their horizontal benthic lifestyle.
Step-by-step explanation:
What is weird about the madreporite on sea cucumbers? In most echinoderms, the madreporite is part of their water vascular system and is typically a visible structure on the surface from where water enters. However, sea cucumbers are unique in that they often have a madreporite that is internal rather than external, which is atypical for echinoderms. This internal madreporite is connected to a structure known as the stone canal, which is part of the sea cucumber's circulatory system. This adaptation may relate to their lifestyle as benthic organisms, lying horizontally on the sea floor, which differs from the typical echinoderm body plan of radiating symmetry with an upwards-facing oral surface.