P. Nemertea Ribbon Worms Non-parasitic life cycle Mostly marine Digestive tract is complete*** Have a long proboscis which lays within the rhynchocoel, and closed circulatory system****. Most are less than 20 cm long however Lineus longissimus is reported to measure 60 m in length!

Ph. Nemertea (Ribbon Worms)

Nemerteans are voracious predators with well-developed chemical senses scyphozoan medusa fish eggs

FRESHWATER NEMERTEA: THE PROBOSCIS WORMS, PROSTOMA SP FRESHWATER NEMERTEA: THE PROBOSCIS WORMS, PROSTOMA SP. COLLECTED FROM WISCONSIN.

PROSTOMA SP. NOTE THE SENSORY GROOVE AND 3 PAIRS OF EYES

P: Mesozoa Members of the phylum are minute, ciliated animals only 0.5-7 mm in length. Highly specialized parasites or symbionts living in marine invertebrates. Most are composed of only 20-30 cells arranged in two layers.

P: Mesozoa

SEX ASEX

P: Mesozoa The two layers are not homologous to germ layers of other metazoans. It is still unclear as to whether they are degenerate platyhelminthes (flatworms) or truly-primitive, basal metazonas. Mesozoa were once thought to be evolutionary intermediate forms between Protozoanas and Metazoans, but now they are thought to be degenerate or simplified metazoa.

The Big Picture Flatworms are a divers group of bilaterally symmetrical invertebrates with a head region. A large proportion of these are parasitic and having complex life cycles. The diversity of life cycle strategies must overcome the small probability of finding and infecting a host were the parasite can grow and reproduce.