The first molluscs ‐ small animals

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Abstract
Recent data on early Cambrian fossils from various sources suggest that the earliest molluscs up to the conchiferan level of organization were animals of very small (1–2 mm) size. These data are in contrast to current theories on the origin and evolution of molluscan organ systems, most of which postulate an increase of size at the beginning of molluscan evolution. The evolutionary and functional implications and constraints of an original small body size on molluscan locomotion, respiration, excretion and reproduction are discussed. Based on recent fine‐structure work the evolutionary history of the Mollusca is reconsidered. In addition, some alternative ideas are presented to explain the origin of the mantle cavity (originally also used as a fertilization chamber?), ctenidia (originally for ventilation rather than for respiration?), coelom (originally a: gono‐nephrocoel?), and pecularities of the alimentary tract.