Phylum Nemertea (Rhynchocoela)

Nemertean Bauplan
“Functionally” acoelomates
With true coelomic cavities (rhynchocoel, blood vessels) but with
relatively solid bodies
A number of Bauplan “improvements” solve surface/volume ratio
Circulatory system, protonephridia remove waste from blood, rather
than the mesenchyme tissues (flatworms).
a) No relying on diffusion for internal transport and exchange b)
complete gut, so anterior end is specialized on feeding and
ingestion, c) development of layered muscles after the reduced
reliability on diffusion for transport though mesenchyme
Result: Relatively large animals (up to 30 m), with complex
feeding and digestive activities