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Functional morphology analyzes the relationships between form and function in organisms. However, a comprehensive analysis of any organic structure requires an integrated approach to morphology. For this purpose constructional morphology was developed, where function, phylogeny and construction together explain form. This thesis investigates functional and constructional aspects of gastropods and bivalves; two groups of molluscs which are among the most common shell-bearing invertebrates. Their shell protects the soft parts and different morphologic specializations enhance this function. Morphology and mode of life are often closely coupled.

Comparison of the distantly related cardiid bivalves Cardium costatum and Budmania spp. reveals similar shell modifications. Both have prominent keels functional in anchoring the shell within the sediment. The straight keels in C. costatum indicate an additional strengthening function, whereas the keels in Budmania spp. often are deformed and do not. Other shell modifications include secondary resorption of shell material and hollow keel interiors, reducing shell weight. These similarities are explained by parallel evolution and a common cardiid Bauplan.

Morphological shell characters in cerithiform gastropods have evolved independently in different taxonomic groups and multiple times within the same group. Shell characters are adaptive within five functional areas: defence from shell-peeling predators, burrowing and infaunal life, clamping, stabilization, and righting of the shell. Most characters are made feasible by determinate growth and a count-down programme.

In most environments predators that crush, peel and bore shells are present. As bivalves and gastropods grow by marginal accretion, and are able to replace lost shell material, traces of unsuccessful predation are preserved as scars. Experiments on the snail Nucella lamellosa show that repaired shells are just as strong as shells without damage. However, new scars follow old scar lines in 43% of tested specimens. This might be due to a higher organic content in this area.