Class Monoplacophora
This class was established to contain a group of primitive pregastropods known solely from fossil records and believed to have been extinct for over three hundred and fifty years. However, several specimens were discovered in nineteen fifty-two off the coast of Costa Rica and were assigned to this class. Resembeling its fossil ancestors, the Neopolina genus is unique among mollusks because it demonstrates internal segmentation. It is, like the pelecypods, bilaterally symmetrical, with eight pairs of foot-retractor muscles, five or six pairs of niphridia (kidneylike structures used in excretion) and other elements indicating a segmented body plan. It has a ladderlike nervous system consisting of longitudinal nerve cords connected to a nerve ring, similar to the more advanced chitons but with little or no ganglionation.
Some experts believe monoplacophora, or gastroverms, to be an evolutionary link between mollusks and other, more obvoiously segmented invertebrates like annelid worms. Others feel that monoplacophora is simply an unusual, possibly secondarily segmented group of mollusks of which Neopilina is a surviving member.