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Neopilina_5.jpg (231516 bytes)


(Above) A simplified schematic view of a long-hypothesized gastropod considered to be representative of the more primitive ancestral form. Main nerve trunks are shown in red (above), with a brain-like central ganglion complex encircling the esophagous and a chemosensitive (taste/smell) organ, the osphradium, which is located near the inward water flow to the gills. Note, especially the paired internal organs. In the more advanced forms of gastropod, one of each paired organ tends to become vestigial

In 1957, Neopilina galatheae was discovered (Class:Tryblidia or "Monoplacophora). It shows many of the key features illustrated above; e.g., complete absence of torsion and a straight-line rather than rotated gut. As an extant species, it was unusual in confirming the hypothesized ancestral form. Originally, the ancestral form had been inferred from anatomical developments seen as greater niche specializations occurred evolutionarily. The organization of the classification sytem reflects to a large extent these progressive changes.

Most other living snails --but not all-- show some degree of torsion and twisting of the body mass, as their hallmark. The effects of torsion are to twist the nerve cords and to rotate the anus toward the head end. Torsion serves a protective purpose by allowing the animal to withdraw its vital mass into the shell, and it also permits a more efficient use of the mantle cavity for the circulations of waste and influent nutrient streams. Cilia, lining the mantle cavity tissue propel water in and out.

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