Animals are often distinguished into two major categories, the “chordates” and the “non-chordates”.
This division is based on the presence or absence of a singular character the notochord: Notochord is a stiff rod-like structure where serves as an endoskeleton to support elongated body posture of chordatto.
Animals, lacking a notochord are called “Non-chordates” and those lacking a vertebral column are called “Invertebrates”.
Image Source: dev.biologists.org
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Nonchordates vs. invertebrates :
The terms ‘Nonchordates’ and ‘Invertebrates’ are often used for each other as synonyms, but strictly speaking they are not synonymous because Invertebrates include both nonchordates (lacking vertebral column and notochord) and protochordates (lacking vertebral column but possessing notochord).
Further all the nonchordate is invertebrates but all the invertebrates are not nonchordates. This means that the Invertebrates minus Protochordates are equal to Nonchordates.
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Lower and higher invertebrates :
On the basis of body size and body organization invertebrates can be divided into two groups-Lower Invertebrates and Higher Invertebrates.
Being small in size and simple body organization Protozoa, Porifera, Coelentrata, Platyhelminthes and Nematoda fall in the category of lower invertebrates.
On the other hand the higher invertebrates are generally large in size and-complex in body organization. They take up higher positions in the phylogenetic tree of the Animal kingdom. Annelida, Arthropoda, Mollusca and Echinodermata are higher invertebrates.
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Contrast between lower and higher Invertebrates
Lower Invertebrates:
1. Generally body size is small.
2. Body organization is simple.
3. Radial, biradial or no symmetry.
4. No germ layers or 2 and 3 germ layers are present.
5. No coelom or pseudocoelom.
6. Generally mouth and anus are not separate.-
7. Muscular gut is absent.
8. Blood vascular system is not well developed.
Higher Invertebrates:
1. Body size is larger
2. Body organization is complex.
3. Bilateral symmetry.
4. 3 germ layers are present.
5. True coelom.
6. Mouth and anus is separate.
7. Muscular gut is well developed.
8. A well developed blood vascular system is present.
Major and minor phyla of invertebrates :
It is customary to divide the invertebrate phyla into major and minor phyla on the basis of two factors:
(i) Number of species and individuals.
(ii) Their participation in ecological communities.
Utilizing a combination of the above two factors we consider only 9 as major phyla and 21 as minor phyla of invertebrates.