a.
Presence or Absence of Tissues
Tissues are groups of similar cells that together carry out a specific function in the body(e.g. muscle tissue, nerve tissue, etc.) All animals except for sponges(phylum Porifera) produce two or three primary tissue layers(sometimes called primary germ layers). Animals with radial symmetry, like cnidarians, produce two germ layers(the ectoderm and endoderm) making them diploblastic. Animals with bilateral symmetry produce a third layer between these two layers(the mesoderm) making them triploblastic. Germ layers eventually give rise to all of an animal's tissues and organs.