Zoologists dream of connecting up the evolutionary pathways among the ragbag of phyla that have been placed together in the super-phylum Deuterostomia. This super-phylum includes the chordates — back-boned animals such as ourselves — and the hemichordates ('half chordates'), which are mostly small, bottomliving marine organisms. Hemichordates arguably possess two chordate features: gill slits and a hollow, dorsal nerve cord. But in other ways, the two main hemiehor date classes — the microscopic, tube-dwelling 'pterobranchs', which feed on suspended particles, and the much larger, mudswallowing 'acorn' worms (the enteropneusts) — differ considerably from each other in lifestyle and anatomy.
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