THE 1ST MOLLUSKS - SMALL ANIMALS

Recent data on early Cambrian fossils from various sources suggest tha t the earliest molluscs up to the conchiferan level of organization we re animals of very small (1-2 mm) size. These data are in contrast to current theories on the origin and evolution of molluscan organ system s, most of which postulate an increase of size at the beginning of mol luscan evolution. The evolutionary and functional implications and con straints of an original small body size on molluscan locomotion, respi ration, excretion and reproduction are discussed. Based on recent fine -structure work the evolutionary history of the Mollusca is reconsider ed. In addition, some alternative ideas are presented to explain the o rigin of the mantle cavity (originally also used as a fertilization ch amber?), ctenidia (originally for ventilation rather than for respirat ion?), coelom (originally a gono-nephrocoel?), and pecularities of the alimentary tract.