Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1

Necessity of Classification and Hickman's Major Phyla.

More than one and a half million animal species are already described, in just nine major phyla. Nobody memorises that many names, so the animal world has to be sorted. Classification is that sorting: know one animal well and you already know something about its whole group.

1. Why classify?

Classification has five theoretical uses. One: know a single animal of a group and you get an idea of the other animals in it. Two: you can learn about many members of the animal world with little effort and in a short time. Three: it gives information on the mutual relationships of animals, their phylogeny. Four: it helps to work out the evolutionary course of animals. Five: it is essential for identifying new species.

There are five applied uses too. To control species harmful to public health, agriculture and forests, you need the exact identity of the species, and classification gives it. It helps to select animals of economic importance. It helps in the conservation of wildlife. An accurate picture of geological events needs the help of the classification of living things. And it makes it easier to develop improved breeds of animals and birds by artificial breeding.

Now put the ten needs into two boxes. In an exam answer on the need for classification, the easiest slip is to blur the two lists.

2. Major and minor phyla

In Hickman's scheme the animal phyla are grouped into major and minor phyla. The grouping is not by evolutionary relationship or by characters. It rests on three things: the number of species, ecological importance, and how distinct the group is as a phylum.

A major phylum has more than five thousand species, members with an important ecological role, and a clearly separate identity as a phylum. Eight nonchordate phyla and one chordate phylum make 9 major phyla.

A minor phylum has few species, almost no ecological importance, and an unclear or disputed position. Examples are Placozoa and Ctenophora. Of Hickman's 33 phyla, 9 are major and the rest are minor.

The nine major phyla are Porifera, Cnidaria, Platyhelminthes, Nematoda, Mollusca, Annelida, Arthropoda, Echinodermata and Chordata. Pick these nine out of twelve chips.

3. The kingdom on the book's chart

On Hickman's chart, Kingdom Animalia first splits into three branches. Mesozoa has few cells and no tissues. Parazoa has many cells but no tissues or organ systems; it holds Porifera and Placozoa. In Eumetazoa, tissues and organs form.

Eumetazoa has two grades. Radiata is diploblastic and radially symmetrical, with no organ systems; it holds Cnidaria and Ctenophora. Bilateria is triploblastic, mostly bilaterally symmetrical, with well-formed organ systems.

Eight yes-or-no questions are enough to separate the nine major phyla. Pick an animal and walk the key; the ladder shows at which rung each phylum leaves.

Bilateria then splits into three groups by the kind of coelom. Acoelomates: 3 phyla, major one Platyhelminthes. Pseudocoelomates: 9 phyla, major one Nematoda. Eucoelomates: 16 phyla, major ones Mollusca, Annelida, Arthropoda, Echinodermata and Chordata. Now place the chips on the right rung of the chart.

4. How many species in each phylum

The nine major phyla together hold 1,508,963 described species worldwide (Zhang 2013). Arthropoda has the most, 1,257,040. Next comes Mollusca with 84,977. Echinodermata has the fewest, 7,550.

Bangladesh has 5,152 described species of these phyla (Ahmed 2008-2009). Here too Arthropoda leads with 2,483. Chordata follows with 1,611. Porifera has the fewest, 29.

See the counts as bars. Tap one for its share of the total; take Arthropoda out and the gaps between the rest show up.

Now rank the nine phyla by their world count, from most species to fewest.

5. Either side of the notochord

Animals fall into two groups, Nonchordata and Chordata, by whether a notochord is absent or present along the dorsal midline, in the embryo or throughout life. In the table, phyla 1 to 8 are Nonchordata and phylum 9 is Chordata.

A nonchordate never has a notochord. Its nerve cord is ventral, solid and ganglionated. Pharyngeal gill slits and a post-anal tail are absent.

A chordate has a notochord. Its nerve cord is dorsal and hollow. Pharyngeal gill slits and a post-anal tail are present. Tap any row of the grid to compare the two sides.

6. Check yourself

Start with the first question.

Take the second one a little slower.

The last question.

The whole note on eight cards.

Source: Biology 2nd Paper (Zoology), Classes 11-12 (NCTB), Chapter 1 “Animal Diversity and Classification”, the “Necessity of Classification and Hickman's Major Phyla” part.