Introduction

Surveying the Invertebrate Phyla

Animals without a vertebral column (backbone) are grouped together as invertebrates, and they make up the overwhelming majority of the animal kingdom by both species count and diversity of body plan. Rather than forming one natural group, invertebrates are organised into several distinct phyla, each defined by a shared body architecture that sets it apart from the others.

Zoologists distinguish these phyla using a handful of recurring structural criteria, applied consistently across the animal kingdom:

• Levels of organisation — whether the body is built at the cellular level (a loose aggregation of cells, as in sponges), the tissue level (cells organised into definite tissues, as in cnidarians), or the organ/organ-system level (tissues grouped into organs and organ-systems working together, as in most higher phyla).
• Symmetry — whether the body can be divided into similar halves. Some invertebrates show radial symmetry (body parts arranged around a central axis, so any plane through that axis gives mirror-image halves — sponges and cnidarians), while most others show bilateral symmetry (only a single plane through the length of the body gives mirror-image left and right halves).
• Germ layers — whether the embryo develops from two layers (diploblastic, as in cnidarians) or three (triploblastic, as in flatworms and every phylum after them), since a third, middle layer (mesoderm) is what makes true organs possible.
• Coelom — whether the body cavity between the gut and the body wall is absent (acoelomate, as in flatworms), a pseudocoelom not fully lined by mesoderm (pseudocoelomate, as in roundworms), or a true coelom fully lined by mesoderm (coelomate, as in annelids, arthropods, molluscs and echinoderms).
• Segmentation — whether the body is divided into a linear series of repeating segments (metameres), most clearly seen in annelids and arthropods.

This chapter surveys the major invertebrate phyla in roughly increasing order of structural complexity — Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca and Echinodermata — using exactly these criteria to explain what sets each phylum apart from the ones before it.