Q.Give one example each for an animal possessing placoid scales and that with cycloid scales.

Imagine walking into a vast library with more than a million books and no shelves, no catalogue, and no order at all. Finding anything, or even understanding what the collection contains, would be hopeless. The animal kingdom presents the same challenge: biologists have already described more than a million animal species. On that scale, a systematic way of grouping animals is not a mere convenience but a necessity.

Animal classification is the orderly grouping of animals so we can make sense of their enormous variety. It lets us do three things: make sense of the sheer diversity, study a few representative types instead of every single species, and give each newly discovered animal a definite, systematic position alongside its relatives. Crucially, the grouping is not arbitrary. Although animals differ enormously in outward shape and form, they share certain deep, fundamental features, and classification is built on these shared features rather than on superficial appearances.

The fundamental characters used as the basis of classification are:

• The level of organisation of the cells (cellular, tissue, organ, or organ-system).
• The body symmetry — whether, and in how many planes, the body divides into matching halves (asymmetry, radial, or bilateral).
• The number of germ layers in the embryo (diploblastic or triploblastic).
• The nature of the coelom, or body cavity — absent, false, or a true mesoderm-lined cavity (acoelomate, pseudocoelomate, or coelomate).
• The presence of segmentation of the body.
• The presence or absence of a notochord (chordate or non-chordate).

The patterns of the digestive, circulatory, and reproductive systems are compared as well. …