An Introduction to the Invertebrates, Second Edition - tiera.ru

66 PLATYHELMINTHES AND ACOELOMORPHAFig. 6.1 Platyhelminths: the structure of (a) a catenulid, (b) a planarian (longitudinal section) showing branching gut (both speciesabout10 mm in length); (c) transverse section of planarian in the pharyngeal region; (d) side view of head and proboscis.flat, being limited by the diffusion of oxygen, and since they aresoft-bodied with the length greater than the width, they are knownas flatworms.6.2 What groups of worms constitutethe Platyhelminthes?Free-living platyhelminths with ciliated epidermal cells. Theyare the Turbellaria, a heterogeneous group including theCatenulida (Figure 6.1a), small mostly freshwater worms withmany simple characters, and the Rhabditophora, a large andvaried group including planarians (Figure 6.1b,c), in freshwater or in damp earth, and marine polyclads.