1. 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!

2. Ph. Nemertea (Ribbon Worms)

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

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

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

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

8. P: Mesozoa

10. SEX
ASEX

11. 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.

12. 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.