Ontogenetic Changes Reflected in the Morphology of the Molluscan Shell

Summary

The course of ontogeny finds its reflection in the shape and structure of the molluscan shell. The shell becomes mineralized after it has first been secreted. The type of embryonic development is imprinted on the shape of the first mineralized shell and its course can be reconstructed from this shape in modern and fossil cephalopods. The place in the environment causes a reaction in morphology. Gastropod limpets clinging to hard substrates thus have developed in many convergent lines starting out from ancestors that withdrew into their shell. When the later portion of life deviates from that of the ancestors, this may be reflected onto earlier portions of the ontogeny or not. The uncoiled Caecidae may have helically coiled, planspiral, or decoiled larval shells and secondarily coiled caecids may or may not revers this tendency. Larval life in the plankton follows very different pathways of adaptation than adult life in the benthos, as is documented with some mesogastropods. Both adaptations have to be fitted into the same life cycle; but when the life cycle takes a short cut, a major change may be the result, as in the pteropods. It is not easy to differentiate between changes that can be reversed and others that are final. Sometimes pathological cases illustrate a possible way in which phylogeny proceeded, as is demonstrated in an atavistic bivalve embryo.