Basis of animal classification

Why is a jellyfish in one group, a cockroach in another and a frog in a third? Animals are sorted by a few basic features of their body plan. The phyla, in order, can be remembered as "Principal Calls To Punish Amar Akbar Anthony for Misbehaving In History Class": Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata.

Level of organisation. Sponges are made of cells working loosely together (cellular level). Coelenterates have tissues. Flatworms have organs. From roundworms and annelids up to chordates, organs form organ systems. The gut may be incomplete, with one opening for both food and waste (a blind sac), or complete, with a mouth and an anus (a tube within a tube). Blood may flow freely in open spaces (open circulation) or stay inside vessels (closed).

Symmetry. Cut a piece of ginger any way you like and the halves never match: that is asymmetry (most sponges). If cuts through the centre in many planes give matching halves, it is radial symmetry (coelenterates, ctenophores, and adult echinoderms like the starfish). If only one plane does it, like cutting a fly down the middle, it is bilateral symmetry (annelids, arthropods and most others).

Germ layers. The embryo forms layers that build the body. Diploblastic animals have two: an outer ectoderm and an inner endoderm, with a jelly-like mesoglea between them (coelenterates). Triploblastic animals have a third, middle layer, the mesoderm (flatworms up to chordates).

Coelom (body cavity, the space where the organs sit). A true coelom is lined by mesoderm (annelids to chordates: coelomates). In roundworms the mesoderm lies in scattered pouches and does not line the cavity: a pseudocoelom. Flatworms have no cavity at all; mesoderm fills the space: acoelomates.

Segmentation. In the earthworm the body is divided outside and inside into repeated segments, with organs repeated in each. This is metameric segmentation (metamerism); you can see the rings on an earthworm's body. Notochord. A rod-like support made from mesoderm along the back (dorsal side) of the embryo. Animals with one are chordates; Porifera to Echinodermata (and Hemichordata) are non-chordates.