Showing papers on "Shell balance published in 1986"

Variation in Land-snail Shell form and Size and its Causes: a Review

Abstract: Variation in land-snail shell form has been extensively documented, but its causes are poorly understood. For no character are there general rules relating shell form to environ- mental characteristics, although certain correlations are common. Size variation generally has a large genetic component. Larger snails are often associated with moister conditions; the effect may be inductive (direct) or selective, but the mechanism is not documented. Snails may attain smaller adult sizes at higher population densities, apparently through the effects of pheromones on growth rate. Relative aperture area tends to be smaller under drier conditions, probably because of selection for smaller whorl cross-sectional area to reduce water loss. Larger snails tend to have higher whorl expansion rates. This pattern is variously interpreted as relating to the maintenance of constant attachment area/weight, whether of foot surface area when the snail is active or when attached to a substrate or of aperture perimeter when attached. Apertural denticles are generally thought to represent adaptations to reduce predation. Relative shell height of snail species relates to the angle of the substrate on which activity occurs; this could be related to the mechanics of shell balance. For unknown reasons, helicid species in the Med- iterranean area frequently have forms with keeled and with rounded shell peripheries. Snails living on calcareous substrates sometimes have thicker shells; the effect is not necessarily direct. Surprisingly, only a weak relationship exists between shell thickness and moisture conditions. Shell coiling sometimes occurs in the opposite direction between sympatric species, probably as a result of selection for reproductive isolation. A recurring problem in the explanation of shell form is the interpretation of covarying shell characters. Identification of nonadaptive variation which results from developmental dependence on another character is dependent on the study of the selective and direct-environmental causes of variation in land snail shell form. (Snail; gastropod; shell; form; shape; size; denticles; variation.)

Steady Laminar Flow of Power‐Law Fluids in the Inlet Region of Rectangular Ducts

Abstract: A steady laminar flow of an incompressible non‐Newtonian fluid (inelastic power law fluid, n<1) in the inlet region of rectangular ducts was studied by the theoretical analysis using finite difference methods and the experiment on pressure drops. Theoretical predictions were compared with experimental data and analytical results available and the agreement was shown to be satisfactory. Therefore, the axial pressure distributions and the velocity profiles in the inlet region were discussed on the basis of typical computations. It was shown that the velocity in the duct center and the pressure drop of a power law fluid were smaller than those of a Newtonian fluid; the inlet length of a power law fluid was larger than that of a Newtonian fluid and increased with the decreasing power law index. Also, the additional pressure loss in the inlet region was smaller in a power law fluid than in a Newtonian fluid and decreased with the decreasing power law index. In addition, when the effect of duct shape was consid...