Generally speaking people consider the animals of this world to be divided into two groups, the 'vertebrates' (those with backbones) and the 'invertebrates' (those without backbones). Although this is correct as far as it goes it does not give a accurate picture of the importance of the two groups nor of their relative sizes and complexities. The invertebrates make up easily more than 99% of all recorded animal species and in terms of individuals would account for more than 99.99% of all animals.
Scientists have divided the living world into a series of categories based on the degree of similarity between different organisms and what we think might be their evolutionary relationship this is known as taxonomy.
In this way the animal kingdom is divided into 36 or so different 'Phyla', of which the Chordata is one. It in turn contains the subphylum vertebrata in which there are less than 60,000 species nearly half of which are fish. Considering that there are probably more than 2 million, and maybe as many as 15 million different species of animal in this world you can easily see that much less than five percent of the animal kingdom is in possession of a backbone.
The invertebrates then are of incredible importance to the world and hence to us. The activities of the simplest organisms allow for the existence of more complicated organisms which in turn prepare the way for still more complicated organisms, hence the old adage that 'life makes room for life'.
To some people's way of thinking all invertebrates are bugs, to most others bugs are invertebrates with legs, i.e. Insects, Spiders, Millipedes, Centipedes, and Woodlice and related Crustaceans, to some other people only insects are bugs, and to a small group entomologists a bug is defined officially as a member of the insect order Hemiptera. You can see from this just how confusing common word usage can be, and maybe understand why scientist use the more accurate taxonomic names.
Anything you want it to be. Because the term is not in use in any scientific way it is a valuable term to describe small critters of all sorts.
This list represents just one of several taxonomic classifications of the animal kingdom, it is one which I find useful. I have listed the Phyla in approximate order of evolutionary complexity starting with the simplest, and given an approximate value for the number of species it contains, these values represent minimum values and it is worth remembering that there are an unknown number of undiscovered species in every Phyla. In the future I hope to add a short description of each Phyla. The phyla should be read from left to right across the table if you wish to follow the order of increasing complexity.
|Phylum||Aprox. No. Sp.||*||Phylum||Aprox. No. Sp.|
|Protozoa||50 000||*||Porifera (sponges)||10 000|
|Cnidaria (Anenomes)||10 000||*||Platyhelminthes (flatworms)||13 000|
|Nemertina (very long worms)||600||*||Mesozoa||50|
|Gnathostomulida||80||*||Nematoda (round worms)||10 000|
|Acanthocephala||800||*||Mollusca (slugs,snails,squid)||80 000|
|Annelida ( segmented worms)||5 500||*||Pogonophora||100|
|Tardigradia (Water Bears)||400||*||Onychophora||70|
|Arthopoda (insects, spiders, crabs)||1 000 000||*||Entoprocta||60|
|Bryozoa||4 000||*||Echinodermata (starfish,sea urchins)||6 000|