Aplacophora is a class of mollusks comprised of about 320 species. They were considered echinoderms until around 1987 when they were moved to Mollusca. Species ingest sediment while inhabiting burrows. A shell is lacking but the epidermis secretes a cuticle in which are embedded scales (seen above) that point posteriorly. The posterior end (right) contains the ctnedia (gills, and can be seen in the photo as the fluffy material at end) and the anus. The mouth located at the anterior (left) can also been seen (flat region on right side of anterior end). The internal structures anterior are the gut and posterior are a combination of the gut and gonad. This individual (Subclass Chaetoderma) is a about 3mm long.
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So it took them till 1987 to notice that aplacophorans had radulae and ctenidia and not madreporites, buchal tentacles or cuverian tubules (diagnostic of holothurians, the only possible echinoderm type this beastie could belong to)?!?
Strange...
It is actually not surprising. Several characteristically molluscan organ systems are absent: lack of a distinct cephalization, no sensory eyes or tentacles, no excretory organs or gonoducts, and some species lack a radula. Very few people actually (maybe less than 5) work on the taxonomy and only within the last 30 years.
I can understand the confusion over the molluscan characters (or lack thereof), but i'm not buying the echinoderm thing! Maybe sipunculun or parasitic something or other, but not echinoderm.... But then again, hindsight is 20/20 right? I would be interested in seeing that 1987 paper, is it a Scheltema paper?
Yes, the aplacophorans were once categorized as holothurians, but they were recognized as mollusks more than a century earlier, in the late 1800's, not 1987. This seems to be a common mistake, perhaps originating from an incorrect listing in Wikipedia.
Graff (1875) is often given credit as being the first to recognize aplacophorans as belonging to mollusks. Many other authors in the late 1870's correctly placed aplacophorans within the mollusks. Pruvot (Comptes Rendus cxi, p. 689-692) studied the embryology of an aplacophoran, and confirmed the similarity of its development to another molluscan taxon, the amphineurans.
Thanks for the clarification Tom. I was really worried that it could have taken up to 1987 for researchers to notice the molluscan affinities.
These kind of corrections raise a question: should one change posted text to rectify a mis-statement, or leave it to the comments?