Key out a mystery animal.

Walk Figure 4.4 as a live key: answer organisation, symmetry, coelom, segmentation, notochord for a mystery animal and the tree lights the phylum.

Key out 'bilateral, triploblastic, pseudocoelomate' and reach Aschelminthes; then find the single toggle that turns an annelid into a mollusc.

Aschelminthes called acoelomate; hemichordates placed in Chordata.

• Classification leans heavily on the body cavity: is there a space between the gut and the body wall, and what lines it?
• A coelom is a body cavity lined by mesoderm. Coelomates: annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
• In a pseudocoelom the cavity is not lined by mesoderm; the mesoderm lies as scattered pouches between ectoderm and endoderm. Aschelminthes are pseudocoelomates.
• Acoelomates have no body cavity at all; Platyhelminthes are the example.
• Metameric segmentation (metamerism): the body is divided externally and internally into segments, with at least some organs repeated in series, as in the earthworm.
• The notochord is a rod-like structure of mesodermal origin formed on the dorsal side of the embryo. Animals that form it are chordates; those that do not (Porifera to Echinodermata) are non-chordates.
• Figure 4.4 sorts the phyla step by step: cellular level (Porifera) versus tissue or higher; radial versus bilateral symmetry; then no coelom, pseudocoelom or true coelom; then segmented or not; then notochord or not.

Teaching this idea? Put the simulation on your own page or LMS with this code.