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Previously considered to be a class of the Platyhelminthes , the “Turbellaria” is a paraphyletic group of flatworms that includes all members of the Phylum Acoelomorpha (Acoela and Nemertodermatida) and some members of the Phylum Platyhelminthes (excluding the exclusively parasitic members of the Rhabditophora: Trematoda and Cestoda). Regardless of their phylogenetic relationships, the turbellarians share a number of similarities in their morphology: solid, soft bodies, hermaphroditic reproductive systems, sacklike gut, multiciliated epidermal cells, and fiber-form muscles. The Acoelomorpha is a diverse group of over 330 known species, found predominately in marine habitats . Ranging in size from less than 500 μm to over 10 mm in length, acoelomorphs live in intertidal, subtidal, and pelagic habitats, with the majority of species occurring interstitially in fine sediment. Acoelomorphs have a prominent statocyst at the anterior end of the body that contains either a single statolith (Acoela), or two statoliths (Nemertodermatida). As a result of the dearth of studies on acoelomorphs of the Gulf of Mexico (GMx), only 3 positively identified species are presently known from this region (Fox and Powell 1986, Meyers, Powell, and Fossing 1987, 1988, Sterrer 1998). However, acoelomorphs are numerous in the NW Atlantic and Caribbean, and the GMx is likely to have an equal abundance and diversity of species. In the one study from the GMx in which acoelomorphs were examined in detail, Meyers, Powell, and Fossing (1987) reported the presence of at least 10 unidentified species from a sand bar in Corpus Christi Bay, Texas. The Catenulida are small aquatic platyhelminths that have a saclike gut, a simple pharynx, and a dorsally positioned male gonopore. The marine catenulids are composed of 2 families (Retronectidae and Tyrrheniellidae), containing 13 species of interstitial worms. The Retronectidae often have a statocyst (with one or 2 statoliths), are extremely fragile, and move with slow ciliary gliding with frequent reversal of ciliary beat. Two species of Retronectidae , Paracatenula kalliope Sterrer and Rieger, 1974, and P. polyhmnia Sterrer and Rieger, 1974, are known from Big Pine Key, Florida (Sterrer and Rieger 1974), from anaero413 22 Turbellarians (Acoelomorpha and Free-Living Platyhelminthes) of the Gulf of Mexico Matthew D. Hooge and Leslie J. Newman  Turbellaria. After Pratt 1916. 414 ~ Turbellarians (Acoelomorpha and Free-Living Platyhelminthes) directly on the shells of oysters (Hoploplana inquilina, Latocestus whartoni (Pearse, 1938), Stylochus oculiferus (Girard, 1853), Prosthiostomum lobatum Pearse, 1938, and Pseudoceros texanus Hyman, 1955. Over 400 species of polyclads have been documented from around the world, and their biodiversity seems to be directly related to sampling effort (Newman and Cannon 2003, 2005). Hyman’s (1954) review of polyclads from the GMx was limited, for the most part, by the lack of illustrations of living animals and lack of sampling data for areas other than the intertidal and shallow subtidal of Florida and Texas. Since that time, little additional sampling has been conducted; nevertheless, a few additional polyclad records have been added, as have all the records for other taxa of turbellarians occurring in the GMx. Based upon this limited sampling, there may be 5 species (15%) of turbellarians endemic to the GMx (see the taxonomic summary). The majority of species collected from the GMx have extensive geographic ranges that typically extend into the Caribbean or NW Atlantic. The tabular checklist displays turbellarians of the GMx in alphabetical order within their respective taxonomic groups. Only taxa that have been conclusively identified to species level are included. For most species, knowledge regarding their biology, habitat, and depth preference is limited. Geographic limits of listed species are taken from the references listed in the following column, and generally refer to the intertidal and shallow subtidal regions that have been sampled. Depths are given for specific collection records for the GMx, and in some cases, are estimated based on qualitative descriptions. Species records reflect only sampled regions and may therefore not be useful for biogeographical studies. Abbreviations Abbreviations used in this chapter include ben = benthic ; bns = bay and nearshore; bsl = beach and shoreline; cep = coastal surface and epipelagic; epi = epibiotic; est = estuarine; hsb = hard substrate; ins = interstitial; plk = planktonic; sft = soft substrates; sym = symbiotic; unk = unknown. Acknowledgments This material is based upon work supported by the National Science Foundation under Grant No. 0118804. We bic sediment immediately underlying oxidized sediment, which is the preferred habitat for retronectids. Both species lack a mouth and pharynx, and as such, their feeding habits are unknown. Additional, unidentified retronectids were reported from Corpus Christi Bay, Texas (Meyers , Powell, and Fossing 1987). The Rhabditophora is a diverse class...

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