Hook Patterns on the Acanthocephalan Proboscis

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Abstract
In the most generalized forms, the hooks are in quincunxial arrangement on the proboscis in perfect radial symmetry without conspicuous differentiation except for progressive axial change in form and size. Deviations from this static geometric arrangement involve both teratological and normal variations, of which the latter are especially important. These include (1) dorso-ventral asymmetry affecting one or more entire longitudinal rows, (2) focal asymmetry, affecting segments of individual rows or groups of rows, (3) zonal asymmetry, reflecting particularly division into anterior and posterior zones of differing hook types and regional specializations in other zones, (4) longitudinal condensation wherein adjacent circles become coalesced. Examples are cited and drawings are included for each type of modification. Significance of longitudinal condensation in reference to taxo-nomic descriptions and systems of hook designation is particularly stressed. Spiral arrangement of hooks, at times involving spiral bifurcations, is discussed as a modification of the basic plan of quincunxial arrangement, not as a fundamental type of hook pattern. From point of view of hereditary mechanism, the form and pattern of arrangement of the hooks on the proboscis represent a mosaic, little influenced by growth or by functional relations and probably intimately predetermined in the zygote.

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