HOMOLOGY IN TERMINOLOGY FOR CRINOID CALCIFIED HINDGUT STRUCTURES (PHYLUM ECHINODERMATA)

Terminology for calcified crinoid hindguts is confused, inconsistent, and usage includes both non-homologous structures with the same name and homologous structures with different names. Here, we propose that homology should determine the terminology.

The calcified hindgut is a structure elevated above the tegmen surface that elevates the anus away from the remainder of the crown. In most living crinoids and many fossil crinoids, the hindgut is a simple conical extension from the tegmen integument. However, in many Paleozoic crinoids, the hindgut is encased within a simple to elaborate, plated structure. In addition to a sanitation function, some of the more elaborate structures have been proposed to have had a respiratory or reproductive function.

For both living and fossil crinoids, these calcified hindgut structures are variously called the anal sac, ventral sac, anal tube, anal cone, anal pyramid, proboscis, etc. Commonly, within a single publication more than one name is used. To stabilize terminology, we propose a more precise restriction of the existing and most commonly used terms, with consideration of homology. Providing a rationale for consistent usage of present terminology is preferred to new morphological terms. The term anal tube is restricted to camerate crinoids that have a subtegminal mouth. Anal sac is for structures of crinoids with the mouth on the tegmen, i.e., cladids, flexibles, articulates, and probably disparids. A caveat for disparids concerns whether the origin of this anal sac is distinct from that of cladid crinoids. This relationship remains to be resolved, and the morphology of aethocrinids and protocrinids is insufficiently known to determine the mouth position. Other terms such as conical, cylindrical, tubular, pyramidal, etc. should be used only to describe the geometric shape on an anal tube or an anal sac.