true tissues (several cell types work together at a single function) are a synapomorphy of the Eumetazoa
early eumetazoans had only 2 tissue layers (ectoderm & endoderm) surrounding a non-cellular middle layer (diploblastic)
higher metazoans (Bilateria) have 3 tissue layers: ectoderm, endoderm, and extensive middle layer with complex organs (triploblastic)
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: 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)
a true coelom is a fluid-filled internal body cavity lined by epithelium
fluid-filled internal body cavities take several forms and arose at least twice independently in higher invertebrates
8) COELOM ORIGIN (schizocoely vs enterocoely) True coeloms arise developmentally via two different methods: schizocoely (splitting of mesenchyme), enterocoely (outpocketing of epithelium)
9) TYPE OF SKELETON (all provide support & muscle antagonism).
Exoskeleton- lies outside body, produced by ectoderm, often molted.
Endoskeleton- lies inside body, produced by middle body layer; may be rigid (like vertebrates) or fluid-filled (=hydrostatic skeleton)