ABSTRACT
The water vascular system with its protrusible hydraulic tube-feet probably arose as a respiratory system which, because of its arrangement with respect to the mouth, allowed it to fulfil a food collecting role too, as in the crinoids. Echinoderms have unusual anatomical arrangements linked with peculiar physiological and behavioural ones which have considerable effects upon food perception and feeding. Large scale movements or aggregations of asteroids and echinoids have been related to food availability, whereas the presence of suspension feeders is more strongly related to current conditions which bring the food to the awaiting masses. The whole issue of substrate selection and substrate/food perception in irregular echinoids warrants attention. The turbidity of inshore waters and the irregularity of much of the sea-bed limit the effectiveness of sight as a major sensory modality for many benthic marine invertebrates. Adult echinoderms are generally sluggish and creep slowly over or through the substrate.
