P. NemerteaRibbon 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

fish eggs

scyphozoan medusa

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

ASEX

SEX

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.