Article: The structure, function, and evolution of tube feet and ambulacral pores in irregular echinoids

The variation in the tube foot morphology of twenty species of Recent irregular echinoids, with representatives from all four principle groups, is described and correlated with the structure of the associated ambulacral pores. Tube feet are often highly modified and sensory, respiratory, suckered, feeding, and funnel-building tube feet are all described. Three types of pore are recognized—isopores, anisopores, and unipores— and these are further subdivided on the basis of pore size and shape, size and position of the neural canal, development of an attachment area, and the extent of the periporal area. Tube foot morphology is interpreted in functional terms and related to the animal's mode of life. Functionally significant features of pores include their shape and size, the number of pores, their divergence, and the attachment and periporal areas. The structure and arrangement of pores can often be used to identify the morphology and function of the associated tube foot. The evolution and diversification of irregular echinoids is seen as a direct consequence of changes in feeding method. Primitive irregulars had abandoned grazing or scavenging for selective particle ingestion and rapidly diversified as bulk sediment swallowers in less organic-rich substrates. Like Recent holectypoids and cassidu-loids, the early groups appear to have used suckered tube feet for feeding and were thus restricted to coarse sediments. By the Cretaceous, mucous adhesive tube feet had developed in the disasteroids enabling them to feed on finer substrates and spatangoids and holasteroids probably evolved from an ancestor with penicillate tube feet. Clypeasteroids evolved from cassiduloids initially to gain access to finer sediments. Pourtalesiids have evolved a feeding mechanism that does not involve tube feet.