Basis of Classification

4.1 Introduction — Why Classify Animals?

You have already learnt that there is a wide variety of organisms present in our biosphere. Animals alone account for nearly a million identified species — from microscopic Hydra (a few millimetres) to the gigantic blue whale (over 30 metres). When new species keep being discovered every year, the urgent need for an organised classification system becomes clear.

Classification not only helps in identifying organisms but also gives an idea of evolutionary relationships among different groups. Even though all animals share certain features (eukaryotic, multicellular, heterotrophic), there exist striking differences in their structure and form. Therefore, biologists use a few fundamental features as the basis of classification — these features are common to many phyla.

4.2 Basis of Classification

The basic fundamental features of classification used for the animal kingdom include:

• Levels of organisation
• Symmetry
• Diploblastic / Triploblastic organisation
• Coelom
• Segmentation
• Notochord

4.2.1 Levels of Organisation

Although all animals are multicellular, all of them do not exhibit the same pattern of organisation of cells. There are four levels seen across the animal kingdom:

• In sponges (Porifera), the cells are arranged as loose cell aggregates, i.e., they exhibit the cellular level of organisation. There is some division of labour among cells.
• In Coelenterates (Cnidaria) and Ctenophores, the arrangement of cells is more complex. Here the cells performing the same function are arranged into tissues. Hence, this is called tissue level of organisation.
• A still higher level of organisation, i.e., organ level is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialised for a particular function.
• In animals like Annelids, Arthropods, Molluscs, Echinoderms and Chordates, organs have associated to form functional systems, each system concerned with a specific physiological function. This pattern is called organ system level of organisation. Organ systems in different groups of animals exhibit various patterns of complexities. For example, the digestive system in Platyhelminthes has only a single opening (incomplete), whereas in higher animals it has both mouth and anus (complete). Similarly, the circulatory system may be of two types — open type (heart pumps blood into open spaces called sinuses, e.g., arthropods, molluscs) or closed type (blood is circulated through closed blood vessels, e.g., annelids, chordates).

4.2.2 Symmetry

Animals can be categorised on the basis of their symmetry:

• Asymmetrical: Any plane that passes through the centre does not divide them into equal halves. E.g., sponges.
• Radial symmetry: When any plane passing through the central axis of the body divides the organism into two identical halves. Found in coelenterates, ctenophores and echinoderms (adult).
• Bilateral symmetry: Only a single plane (the vertical/sagittal plane) divides the body into two identical right and left halves. Found in animals like annelids, arthropods, etc.

4.2.3 Diploblastic and Triploblastic Organisation

Animals in which the cells are arranged in two embryonic layers — an external ectoderm and an internal endoderm — are called diploblastic animals, e.g., coelenterates. An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm.

Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals (platyhelminthes to chordates).

4.2.4 Coelom

Presence or absence of a cavity between the body wall and the gut wall is very important in classification. The body cavity, which is lined by mesoderm, is called coelom. Animals possessing coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.

In some animals, the body cavity is not lined by mesoderm; instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates, e.g., aschelminthes.

The animals in which the body cavity is absent are called acoelomates, e.g., platyhelminthes.

4.2.5 Segmentation

In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs. For example, in earthworm, the body shows this pattern called metameric segmentation and the phenomenon is known as metamerism.

4.2.6 Notochord

Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. Animals with notochord are called chordates and those animals which do not form this structure are called non-chordates, e.g., porifera to echinoderms.

Worked Examples