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Introduction to Invertebrates Phyla:
The invertebrates include those which are without backbone as opposed to vertebrates in which a series of vertebrae constitute a backbone, but this division of the animal kingdom into invertebrates and vertebrates is largely a matter of convenience.
The invertebrates constitute about 90 per cent of the known animals which number over a million. Vast and heterogeneous groups have been placed in the invertebrates.
There is not even one positive character which is common to all invertebrates, and the differences between the groups are very large, each group of invertebrates has certain structural peculiarities, a special terminology, and a distinct classification.
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However, the life of invertebrates is as fascinating, revealing and complicated a subject as that of vertebrates. Without a thorough and careful study of invertebrates, it is hardly possible to peep into the secrets of life of animals on the whole.
Present Invertebrate Phyla:
Presently there are 30 invertebrate phyla, which are characterised by a unity of basic structural pattern in each of them. This means that in each phylum, though the members may differ in external features, the anatomical features are constructed on the same ground plan in many respects.
The common anatomical ground plan exhibits a unique relationship among the groups of structural units which compose it. Other significant features of inter-relationships among the members of same phylum are functional. Another important feature, by which the members of the individual phylum are related with one another, is the common ancestry.
Evolutionary studies have confirmed that all the members of an individual phylum have been derived directly or indirectly from a common primitive ancestral type. Thus, the 30 phyla display 30 patterns, each manifesting a characteristic, anatomical and functional integrity and common ancestry.
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The following table lists the 30 invertebrate phyla with approximate number of species in each phylum:
Major and Minor Invertebrate Phyla:
Customarily the invertebrate phyla have been divided into major and minor phyla.
The concept of major and minor phyla is based on two factors:
(i) The number of species and individuals;
(ii) Their participation in ecological communities. On the basis of the first factor, 11 phyla appear to be clearly major (as is evident from the species number in Table 10.1), these are Protozoa, Porifera, Coelenterata, Platyhelminthes, Rotifera, Nematoda, Mollusca, Annelida, Arthropoda, Ectoprocta and Echinodermata.
On the basis of second factor, if the phyla are represented in great majority of ecological communities, they would be regarded as major phyla. Whereas, the minor phyla form only a fraction of animal communities.
On this basis, the two phyla, Rotifera and Ectoprocta, cannot be considered as major phyla. Although they are greater in number of species, but they are included in minor phyla due to their limited participation in animal communities. Thus, keeping in view the utility of the above two factors, we can regard only nine as major phyla and the rest as minor phyla.
Lower and Higher Invertebrates Phyla:
The invertebrate phyla are usually referred to as lower and higher invertebrates. The lower invertebrates are simple in body organisation and generally smaller in size. These are thought to have originated in the main lines of evolution, near the base of the phylogenetic tree of the Animal Kingdom.
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The lower invertebrates include various phyla such as Protozoa, Porifera, Coelenterata(Cnidaria), Platyhelminthes and Nematoda. On the other hand, the higher invertebrates are generally larger in size and possess a complex body organisation.
These occupy higher position in the phylogenetic tree of the Animal Kingdom. The higher invertebrates also include various phyla such as Mollusca, Annelida, Arthropoda and Echinodermata.
