17 September

PhylumPlatyhelminthes (flatworms)

• acoelomate
  
• mesenchyme or parenchyma
  
• digestive tract--enteron
  
• incomplete digestive tract, food particles are engulfed in the digestive tract (by phagocytosis)
  
• extracellular and intracellular digestion
  
• ciliated epithelium  muscle fibers run both ventrally and anterior-posteriorly
  
• ladder type nervous system , "brain" cerebral ganglia
  subepithelial nerve net
• eye  spots and anterior collection of sensory structures
• creatures are often hermaphroditic (that is, one individual produces both sex products, although not usually self-fertilized)
  

Most zoologist believe the flatworms are the basal unit of metazoan phylogeny and that an acoel-like creature gave rise to both the cnidaria and the rest of the invertebrates

Phylum Cnidaria
two body forms--polyp and medusa
gastrea theory of invertebrate origins (photo)

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