true tissues (several cell types work together at a single function) are a defining trait (synapomorphy) of the Eumetazoa
basal eumetazoans are diploblastic: only 2 tissue layers (from ectoderm & endoderm) surrounding a non-cellular middle layer
higher eumetazoans (Bilateria) are triploblastic: 3 tissue layers (from ecto-, endo-, & meso-derm), large middle layer with complex organs
5) TYPE & ORIGIN OF GUT (blind vs complete) A blind gut (mouth, no anus) was primitive in Eumetazoa; a complete gut (mouth & anus) evolved twice: once each in Protostomia & Deuterostomia
6) TYPE OF CILIATION (mono- vs multiciliated) Monociliated cells are primitive; multiciliated cells arose twice (Protostomia & some derived Deuterostomia)
7) COELOM TYPE (acoelomate vs pseudocoelomate vs coelomate)
fluid-filled internal body cavities take several forms and arose at least twice independently in higher invertebrates
a true coelom is a fluid-filled internal body cavity lined by epithelium
8) COELOM ORIGIN (schizocoely vs enterocoely) True coeloms arise developmentally via two very different methods: schizocoely (splitting of mesenchyme), enterocoely (outpocketing of epithelium)
9) TYPE OF SKELETON (provide support & muscle antagonism)
Endoskeleton- lies inside body, produced by middle body layer; may be rigid (like vertebrates) or fluid-filled (=hydrostatic skeleton)
Exoskeleton- lies outside body, produced by epidermis, often molted