Body Structure of Invertebrate Animals | Zoology
Animals probably evolved from marine protists, although no group of protists has been identified from an at-best sketchy fossil record for early animals. Cells in primitive animals (sponges in particular) shows similarities to collared choanoflagellates, as well as pseudopod- producing amoeboid cells. Multicellular animal fossils and burrows (presumably made by multicellular animals) first appear nearly 700 million years ago, during [...]