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Ph. Nemertea (Ribbon Worms) Characteristics of the Phylum Nemertea • Marine, freshwater, or terrestrial. • Triploblastic, [probably] coelomate, bilaterally symmetrical un-segmented worms. • Complete digestive tract with anus. • Protonephridia. • Unique proboscis apparatus. • Closed circulatory system. Diversity of Proboscis. Nemerteans are voracious predators with well-developed chemical senses scyphozoan medusa fish eggs Eplectonema feeding on barnacles. Closed circulatory system, thought to be homologous to coelomic cavities. Malacobdella grossa Gastrodermis Gastrodermis Reproduction • Asexual and Sexual Reproduction. • Most species are dioecious, but a few are protandric or simultaneous hermaphrodites. Some produce planktotrophic larvae Diversity of Nemerteans: About 900 Species Lineus longissimus reported to reach 30 meters in length, and suspected to be 60 meters in length when fully stretch-out! A few species are commensal and/or parasitic? • Malacobdella species commensal within the mantle cavity of bivalve molluscs. • Do not feed on the host. There is a relationship between prevalence of infection and number of worms per clam, increasing with size. A few species are commensal and/or parasitic? • Carcinonemertes species are ecto-parasites (egg predators) on branchyuran crabs. • Different species are specific to different body region of the host crab (predominantly on the gills of female crabs), but all feed on eggs. Up to 100,000 worms per crab! Initially worms infect the gills of male and female crabs. They form capsules about themselves by secreting mucus to cement adjacent gill lamellae together. When the female crabs become gravid and ovigerous, the worms migrate to the egg mass. Here they become mature and deposit their own ova. The larvae worms either remain amongst the host eggs and feed, or leave the host. Some Pelagic Species. Nectonemertes sp. Phallonemertes murrayi FRESHWATER NEMERTEA: THE PROBOSCIS WORMS, PROSTOMA SP. COLLECTED FROM WISCONSIN. PROSTOMA SP. NOTE THE SENSORY GROOVE AND 3 PAIRS OF EYES X Big Picture • This is a poorly studied phylum that historically was placed among the acoelomates, later pseudocoelomates, and now true coelomates. • In the near future it will probably change our understanding of invertebrate evolution and phylogeny.
