User:KDS4444/Archimollusc

An archimollusc, also sometimes called a schematic mollusc or a hypothetical ancestral mollusc (HAM), is a theoretical construct of a creature which is intended to display the physical characteristics common to most members of the molluscan order. Malacologists disagree on whether or not such a creature ever existed, and some have argued that the entire premise of such an animal may even be a hindrance to the study of molluscan evolution. However, the tenets of the theory of evolution seem to suggest that molluscs, like other groups of animals, share some form of common ancestor and that the archimollusc is at least not an unlikely form for it to have taken. Theorists have also suggested that the archimollusc construct should be understood only as a bauplan rather than a depiction of any actual historical creature.

Few characteristics are common to virtually all of these depictions: many have a pair of gills or ctenidia (though sometimes only one is shown), a pair of anterior tentacles, a flat ventral foot, several paired ganglia, at least one set of untorted nerve cords, a radular organ in the mouth, a digestive track with a stomach, and a posterior anus. Several other organs are implied and would have been biologically necessary but are not always depicted, including some form of heart (often shown, when shown at all, in the form of a single ventricle and one or more auricles), an organ or pair of organs for blood-filtration (kidneys or nephridia), and a gonad.

1853
The first published attempt to illustrate an archimollusc was performed by the biologist Thomas Henry Huxley in 1853. His side elevation of what he calls a molluscan "archetype" shows an animal with a straight digestive track connecting a forward-facing mouth to a stomach and posterior anus, a posterior "contractile sac" which performed the functions of a kidney, several ganglia which are mostly anterior, a set of posterior wing-like gills, an anterior tentacle and eye/ optic nerve (although Huxley shows two gills in his picture, the depiction of a single eye and anterior tentacle do not imply that he supposed these to have occurred singly), a "pyloric cecum" (the equivalent of a digestive cecum, digestive gland, or style sac), a ring-like epipodium encircling the upper part of the body (which may have been meant as the equivalent of a mantle), a small posterior heart with accompanying auricles and long anterior (only) aorta, and a flat ventral foot divided into three lobes or sections. It has a buccal mass with what appear to be a radula and odontophore. Surprisingly, it has no shell nor other hard structural parts. It also lacks any reproductive organs, though Huxley was very familiar with molluscan biology and knew that a gonad would have been present somewhere in the animal.

Since Huxley made clear that he was not trying to illustrate any actual animal that may have existed but wished rather only to bring together in one picture what he believed to be many of the most likely qualities that any common ancestor might have had, his diagram should be understood as a creature of theory. Nevertheless, by having made such a drawing, he set in motion a belief that illustrations of such an animal were possible to create and were useful for understanding molluscan biology and evolution, and (Huxley was an ardent Darwinian.

1883
The image of this creature was drawn again in 1883 and reproduced in 1912 for the ninth edition of the Encyclopædia Britannica by Ray Lankester who was responsible for composing the edition's article on molluscs. Lankester's animal, which he called a "schematic mollusc", bore a more than superficial similarity to that of Huxley. It, too, has no shell, though this time a "shell sac" is depicted which would have wrapped around the shell and provided the mechanisms for its growth. Unlike Huxley's animal, Lankester's lacks any breathing apparatus. It has a gonadal opening, but no gonad. It also lacks tentacles and eyes.

1896
The next development, by Nichols and Lydekker in 1896, shows an animal similar to that of Lankester. Some differences are evident, however: this animal has a definite and complete shell covering, and has a distinct feather-like anterior gill structure above the animal's head within an anterior mantle cavity. It also has a posterior mantle cavity. It retains the arrangement/ placement of the heart, the digestive gland, the nerves, and Huxley's three-lobed foot, and though it no longer includes the gonad opening of Lankester (which did not appear to connect to the gonad), it is in all other respects a veritable copy of its predecessor including the absence of any kind of gonad.

1901
By 1901 the form of the ancestral mollusc had changed somewhat. Here it is shown with shell which is partially covered with a recurved portion of the mantle, exposing the shell on the top. This is similar to the diagram made by Lankester. The digestive gland/ liver is located below the stomach, the gonad is located below the heart/ pericardium, the foot contains only one lobe, and an anterior tentacle and eye are implied by the drawing. Other angles of the same animal show two ctenidia located at the rear, one on each side, within the posterior mantle cavity. The digestive tract has no loops, and no part of the nervous system is shown.

1935
By 1935 the form of the schematic mollusc had made some significant shifts. The gill remains at the animal's posterior end, the digestive gland is now much larger and is located above the stomach rather than under it, and it now has a gonadal/ genital coelom. The shell has disappeared, though the shape of the viscera implies it theoretically exists and is present above the animal. Though eyes remain absent, a cerebral tentacle is present. The three lobes of the foot visible in Huxley's model are gone, and a single foot remains.

1992
In 1992 a distinction began to be made between a "hypothetical ancestral mollusc" and a "typical molluscan bauplan". The former was characterized as a worm-like creature with no distinct head and a hardened cuticle but no shell. It's narrow foot moved the animal via cilia, was carnivorous with a simple tube-like gut, had no eyes or tentacles, and may have had ctenidia but not for respiration. The latter had a distinct head and visceral hump, a hardened shell, shell plates, or scales, was herbivorous with a long looped intestine, had a broad, sucking foot, an anterior set of tentacles and eyes, and ctenidia located within a large mantle cavity that were used for respiration. The distinction being made was between what some authors identified as an actual likely molluscan ancestor and a theoretical construct never supposed to have actually existed.

2003 meta-analysis
In 2003 a paper was published which conducted a tongue-and-cheek evolutionary analysis of these various diagrams and their associated explanations. The purpose of this paper was to critique the idea of the usefulness of the concept of a hypothetical ancestral mollusc, and to demonstrate its fallacious nature by placing it in the context of its own of "evolution".

2008
In 2008 the creature depicted in a "typical molluscan body plan" lacks tentacles and eyes, has no nephridia, and no nerves or ganglia. Its heart has no pericardium or specific ventricle or auricles, and it lacks a gonad. It has a shell, radula, mantle, and digestive gland located below its stomach. It has a posterior mantle cavity containing ctenidia.