| Abstract |
Nemertea is a clade of unsegmented, worm-shaped Spiralia comprising about 1.300 described species (Fig. 8.1A–F; Kajihara et al. 2008). The vast majority inhabits marine benthic habitats, but several species are limnic, terrestrial, or marine pelagic. Most species have been described as predators although a number of parasitic, commensalic, and probably even scavengers are known (Gibson 1972). Prey is captured by means of an eversible proboscis that may be armed with one to numerous calcareous stylets in some clades. The proboscis apparatus comprises the proboscis and the rhynchocoel. It represents the apomorphic character that has led to the alternative name Rhynchocoela. The rhynchocoel is a dorsally located, fluid-filled secondary body cavity surrounded by muscle layers housing the proboscis. It opens to the tip of the head via a tube-shaped rhynchodeum (Fig. 8.2A, B). Additional characters that unequivocally qualify Nemertea as monophyletic are the ring-shaped brain surrounding the proboscis insertion instead of the esophagus, a pair of laterally located longitudinal medullary cords, and an endothelialized blood-vascular system. Apart from that nemertean anatomy is marked by characters that are arguably plesiomorphic for Spiralia (Turbeville 2002). These include a largely compact arrangement of the tissue; a medullary cord type organization of the nervous system; a body wall muscle tube comprising minimally two, an outer circular and an inner longitudinal, muscle layers; and one to several paired lateral protonephridia that are not arranged in a segmental fashion. Characters that place Nemertea closer to Trochozoa are a regionalized through-gut with mouth, foregut, midgut, and anus and the presence of glial type cells in the nervous system (Turbeville and Ruppert 1985; Turbeville 1991). |
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