The dominant forces determining the motion of interplanetary particulates are gravitation, solar radiation pressure and Lorentz force. The latter two becoming significant for micron- and submicron- sized particles. In situ measurements by spaceprobes, microcrater distributions and remote observations both in the IR and visible wavelength range have established the mass frequency and spatial distribution of dust particles in interplanetary space. Consequences of the Poynting-Robertson effect and mutual collisions on these distributions and the contributions of various sources (interstellar dust, asteroids and comets) are discussed. It is shown that the contribution from a distributed source of large particles in the inner solar system is most important. Collisions between these meteor sized particles (m > 10^-5 g) produce large amounts of zodiacal light particles (10^-5 g to 10^-10 g) and
β-meteoroids (m < 10^-10 g) which leave the solar system on hyperbolic orbits. At the present time the Poynting-Robertson effect transports into the inner solar system less than 10% of the zodiacal light particles which are produced by collisions from bigger particles.
A new species of Prochristianella Dollfus, 1946 is described from the spiral intestine of the wedgenose skate, Dipturus whitleyi (Iredale) (Rajiformes: Rajidae), off the north-western coast of Tasmania (Australia). Prochristianella mattisi sp. n. is characterised by an acraspedote scolex, two oval bothria, elongate, bent bulbs, a retractor muscle inserting at the base of each bulb and the presence of gland-cells within the bulbs and prebulbar organs. The tentacular armature is typical heteroacanthous, heteromorphous, with a characteristic basal oncotaxy and a metabasal armature with hooks first increasing and then decreasing in size along each principle row. It can be differentiated from other species of Prochristianella by a combination of morphological characters, such as the metabasal tentacular armature with eight hooks per principle row, a unique basal armature without enlarged hooks on the basal swelling and genital pores slightly posterior to the mid-line of the segment. The description of P. mattisi sp. n. increases the number of known species within Prochristianella to 20, eight of which occur in Australian waters. A key for the identification to species within Prochristianella is provided.