. Bulletins of American paleontology. North American Mitrate Stylophorans: Parsley 11 cophore. The feeding mechanics of the mitrate aula- cophore, through the intersegmental arcuate openings made by the overlapping cover plates, is discussed be- low. Earlier suggestions by Parsley (1980) that the cov- er plates may have rotated outwards to expose the food groove do not, in light of newer data on the nature of cover plate suturing, seem to be viable as a feeding mechanism. In comutes, the cover plates open to ex- pose the food groove. This is especially manifest in irregular or asymmetrical for
. Bulletins of American paleontology. North American Mitrate Stylophorans: Parsley 11 cophore. The feeding mechanics of the mitrate aula- cophore, through the intersegmental arcuate openings made by the overlapping cover plates, is discussed be- low. Earlier suggestions by Parsley (1980) that the cov- er plates may have rotated outwards to expose the food groove do not, in light of newer data on the nature of cover plate suturing, seem to be viable as a feeding mechanism. In comutes, the cover plates open to ex- pose the food groove. This is especially manifest in irregular or asymmetrical forms, which appear to have been sessile (Parsley, 1988). The stylophoran aulacophore and the homoiostelean stele are very similar. In both, the proximal part of the appendage is tetramerous. In some of the dendrocys- titid homiosteleans, the tetrameres secondarily break up into multiplated rings and smaller annulations of platelets (see Caster, 1968, pp. S594- S598). Distally, the stele grades into a dimerous distal stele, commonly through an intermediate area, the mesistele, that necks down and twists. In some genera, large intercalates make this area trimerous. The function of the mesistele seems to have been one of stiffening, to impart more leverage to the flexible dististele. This leverage pre- sumably made locomotion more efficient. The mesistele-stylocone/styloid and distal stele-dis- tal aulacophore, while they are commonly superficially similar, are basically different structures, and their shared homeomorphy is apparently due to functional convergence. The adthecal part of the aulacophore structure, the proximal aulacophore, has a very large internal lumen which is continuous with the interior of the theca. The proximal aulacophore extends through, not out from the surface of the marginals as, for example, in most blastozoans, and might best be viewed as a proboscis with the mouth opening at the distal end. The only other bilateral echinoderms with a feeding proboscis t
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