C HAPTER . Basis of Classification . Classification of Animals 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 to the outside of the body that serves as both mouth and anus, and is hence called incomplete. A complete digestive system has two openings, mouth and anus. Similarly, the circulatory system may be of two types: (i) open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it and (ii) closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).
. . Symmetry Animals can be categorised on the basis of their symmetry. Sponges are mostly asymmetrical, i.e., any plane that passes through the centre does not divide them into equal halves.
When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry . Coelenterates, ctenophores and echinoderms have this kind of body plan (Figure .1a). Animals like annelids, arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry (Figure .1b). .
. 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 (Figure .2a). Figure .
Showing germinal layers : (a) Diploblastic (b) Triploblastic Ectoderm Mesoglea Endoderm Mesoderm Figure . (b) Bilateral symmetry Figure . (a) Radial symmetry . .
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 (Figure .3a). 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 (Figure .3b). The animals in which the body cavity is absent are called acoelomates, e.g., platyhelminthes (Figure .3c). Figure . Diagrammatic sectional view of : (a) Coelomate (b) Pseudocoelomate (c) Acoelomate 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, Figure .2b).
. . 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 .
. . 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.