Tubes, sacs, cones, pyramids, and proboscises: Toward a homology-based understanding and terminology for plated, erect hindgut structures among the Crinoidea

摘要

Terminology of erect hindgut structures should be limited to anal sac and anal tube. Erect hindgut structures (plated or unplated) on crinoids with a suprategminal mouth (cladids, flexibles, disparids, and articulates) should be termed an anal sac. Anal tube should only be used on camerate crinoids that have a subtegminal mouth. Discovery of an unplated anal sac on a Mississippian cladid crinoid prompts the speculation that an unplated sac may have been common, because the anal sac is not known in many crinoids.