Q.Trace, phylum by phylum, the progressive appearance of

Step 1. Porifera and Coelenterata/Ctenophora. Porifera lacks true germ layers altogether (cellular level); Coelenterata and Ctenophora are diploblastic, built from only two germ layers (ectoderm and endoderm) with an undifferentiated mesoglea between them — none of these three phyla is triploblastic, none has a coelom, and none is segmented.

Step 2. Platyhelminthes. This is the first phylum in the sequence to be triploblastic, having a true third germ layer, the mesoderm. However, in Platyhelminthes this mesoderm does not enclose a body cavity at all — the animal is acoelomate, the space between gut and body wall being filled with solid parenchyma — and the body remains unsegmented.

Step 3. Aschelminthes. Also triploblastic, Aschelminthes shows the next incremental step: a body cavity is now present, but it is not completely lined by mesoderm, making it a pseudocoelom rather than a true coelom. The body remains unsegmented.

Step 4. Annelida. This is the phylum where both remaining features appear together for the first time: the body cavity is now completely lined by mesoderm on all sides, making Annelida the first truly coelomate phylum in the sequence; and the body is, for the first time, divided into a linear series of repeating segments — true metameric segmentation. …

Move through the non-chordate phyla in classification order, stating at each step whether triploblasty, coelom and segmentation are …

• Assuming coelom and segmentation appear at different points — both actually appear together for the first time at Annelida, which is a detail worth stating explicitly. …