flatworms  Platyhelminthes
acoelomate
mesenchyme or parenchyma
digestive tract--enteron
incomplete
phagocytosis
extracellular and intracellular digestion
ciliated epithelium  muscle fibers
ladder type nervous system
"brain" cerebral ganglia
subepithelial nerve net
eye  spots and anterior collection of sensory structures
Cnidarians
two body forms--polyp and medusa
gastrea theory of invertebrate originsHaeckel

egg and sperm to fertilized unicellular zygote to multicellular ball of cells (morula) morula becomes hollow to produce blastula, blastula invaginates to produce a two layer thick structure called the gastrula