Important distinguishing features of invertebrates are mentioned below:
“Invertebrate” is not a natural group of classification and the animals included in this group, do not show a single common character except for the absence of vertebral column and notochord except few (Protochordates).
Even though there are some major characteristics or patterns that had been kept in mind by the taxonomists to classify the invertebrates.
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1. Arrangement of cells :
Invertebrates vary in this pattern from single celled body (Protozoa) to multicellular body (Metazoa). In metazoans cells are arranged in layers (porifera) or tissues.
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2. Germ layers :
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Protozoans, Poriferans, Coelentrates and Ctenophorans are all diploblastic (two germ layers) and the rest of invertebrates are triploblastic (three germ layers).
3. Body organization :
Invertebrates show a variety of levels of organization. It is protoplasmic in Protozoans, cellular in Poriferans, tissue grade in Coelentrates and Ctenophorans.
Rest of the higher invertebrates show organ and organ-system grade organization.
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4. Symmetry:
Both radial and bilateral symmetry are found in invertebrates.
5. Segmentation:
Except annelids and arthropods segmentation is absent in all the invertebrates.
6. Digestive tract:
It is incomplete (without anus) in lower invertebrates and complete, but protostomic in higher invertebrates except arthropods and echinoderms.
7. Excretory organ:
Various types of excretory organs, e.g., flame cells, pseudo coelomic, lophophorate coelomic, reduced schizocoelomic etc are found in invertebrates.
8. Coelom:
It is falsely present in lower invertebrates but true coelom is found in higher invertebrates
9. Respiratory organs:
Except Molluscs, Annelids, Arthropods, and Echinoderms specific respiratory organs are absent in rest of the invertebrates.
Further, each phylum of Invertebrates has its own distinguishing features that are characteristics for that phylum only.