P Nemertea Ribbon Worms Nonparasitic life cycle Mostly

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. 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.