Q.(a) What are the four common characteristics found in most animals ? Explain. OR

Most animals can be compared and classified using four basic organisational features: level of body organisation, symmetry, number of germ layers, and type of body cavity (coelom).

Answering part (a): Zoologists use several fundamental features of body plan to group animals into phyla, and four of the most important, common to the vast majority of animals, are as follows. (1) Levels of organisation: animals show increasing complexity of body organisation, from the cellular level of organisation (where cells are loosely aggregated but do not form true tissues, as in sponges), to the tissue level of organisation (where similar cells combine into tissues performing a common function, as in cnidarians), to the organ level of organisation (where different tissues combine to form organs, as in flatworms), to the most complex organ-system level of organisation (where organs work together as systems for major body functions, as in higher animals). (2) Symmetry: this refers to the arrangement of body parts in relation to a central axis. Animals may be asymmetrical (no definite symmetry, e.g., most sponges), radially symmetrical (body parts arranged around a central axis so that any plane passing through the axis divides the body into similar halves, e.g., coelenterates, echinoderm adults), or bilaterally symmetrical (only one plane, through the central axis, divides the body into identical right and left halves, e.g., most other animals). (3) Germ layers (diploblastic/triploblastic organisation): during embryonic development, animals may form only two germ layers, an outer ectoderm and inner endoderm with an undifferentiated layer (mesoglea) between them (diploblastic, e.g., coelenterates), or three germ layers — ectoderm, mesoderm, and endoderm (triploblastic, e.g., most other animal phyla), with the mesoderm giving …