Summary — Revising the Animal Kingdom by Classification Criteria

This chapter has surveyed the Animal Kingdom through ten non-chordate phyla — Porifera, Coelenterata (Cnidaria), Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata and Hemichordata — and the phylum Chordata, within which seven classes were studied: Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia. Running through every one of these groups are the same six basis-of-classification criteria introduced at the start of the chapter, and a fast way to revise the whole chapter is to recall each phylum/class against this same checklist: level of organisation (cellular in Porifera; tissue in Coelenterata/Ctenophora; organ from Platyhelminthes; organ-system from Aschelminthes onward), symmetry (asymmetrical in most Porifera; radial in Coelenterata, Ctenophora and adult Echinodermata; bilateral in every other group, including the echinoderm larva), diploblastic/triploblastic organisation (diploblastic only in Coelenterata and Ctenophora; triploblastic in every other group from Platyhelminthes onward), coelom (acoelomate in Platyhelminthes; pseudocoelomate in Aschelminthes; coelomate from Annelida onward, including all chordates), segmentation (absent until Annelida, then present in Annelida, Arthropoda and, internally, in the vertebrate axial skeleton and musculature) and, cutting across the whole kingdom, the notochord (absent in every phylum through Hemichordata, present at some life stage only in Chordata).

Within Chordata, the same logic of successive, more advanced features distinguishes the seven classes: jawlessness in Cyclostomata versus jaws in every other class; a cartilaginous skeleton and placoid scales in Chondrichthyes versus a bony skeleton, bony scales and an operculum in Osteichthyes; a moist, glandular, scale-less skin and an aquatic larval stage in Amphibia versus a dry, scaled skin and a shelled, land-laid egg in Reptilia; and, finally, the two homoiothermous, four-chambered-heart classes, Aves (feathers, wings, oviparous) and Mammalia (hair, mammary glands, mostly viviparous). …