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Shell balance

About: Shell balance is a(n) research topic. Over the lifetime, 154 publication(s) have been published within this topic receiving 3691 citation(s).


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BookDOI
01 Jan 1987

289 citations

Journal ArticleDOI
TL;DR: Variation in land-snail shell form has been extensively documented, but its causes are poorly understood and 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.
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.)

263 citations

Journal ArticleDOI
Abstract: Basic fluid mechanical concepts are reformulated in order to account for some structural aspects of fluid flow. A continuous spin field is assigned to the rotation or spin of molecular subunits. The interaction of internal spin with fluid flow is described by antisymmetric stress while couple stress accounts for viscous transport of internal angular momentum. With constitutive relations appropriate to a linear, isotropic fluid we obtain generalized Navier‐Stokes equations for the velocity and spin fields. Physical arguments are advanced in support of several alternative boundary conditions for the spin field. From this mathematical apparatus we obtain formulas that explicitly exhibit the effects of molecular structure upon fluid flow. The interactions of polar fluids with electric fields are described by a body‐torque density. The special case of a rapidly rotating electric field is examined in detail and the induction of fluid flow discussed. The effect of a rotating electric field upon an ionic solution is analyzed in terms of microscopically orbiting ions. This model demonstrates how antisymmetric stress and body torque can arise in ``structureless'' fluids.

232 citations

Journal ArticleDOI
Abstract: The linear boundary-layer analysis by Stewartson & Roberts (1963) and by Roberts & Stewartson (1965) for the motion of a viscous fluid inside the spheroidal cavity of a precessing rigid body is extended to include effects due to the nonlinear terms in the boundary-layer equation. The most significant consequence is a differential rotation super-imposed on the constant vorticity flow given by the linear theory. In addition it is shown that a tidal bulge of the cavity forces a fluid motion similar to that caused by the precessional torque. The relevance of both effects for the liquid core of the earth is briefly discussed.

222 citations

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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20181
20173
20168
20155
20144
20136