Hemichordata is a group of exclusively marine animals, consisting of two subgroups, the sessile and small colonial pterobranchs and the solitary, vermiform enteropneusts (acorn worms) (Fig. 2.1; van der Horst 1939; Hyman 1959; Benito and Pardos 1997). With about 130 described species, Hemichordata comprises a relatively small taxon of benthic animals (http:// www. marinespecies. org/ index. php; Cameron 2005). They are distributed worldwide and inhabit shallow coastal areas but are also found in the deep sea. For a long time, pterobranchs have been known only from deep waters, whereas enteropneusts were thought to burrow mainly in shallow waters. However, within the last five decades, about a dozen of different enteropneusts have been documented in the deep sea (Osborn et al. 2012). In contrast, pterobranchs have been found in intertidal zones of tropical waters only recently (Lester 1985) and might have been overlooked previously due to their minute size and superficial similarities in their gross morphology with other tube-dwelling animals, such as polychaetes and bryozoans.