Q.If you are given a specimen, what are the steps that you would follow to classify it?

To classify an unknown specimen, examine it for the same fundamental features that separate the major animal groups, taking one character at a time:

• Check the level of organisation of its cells — cellular, tissue, organ, or organ-system.
• Determine its body symmetry — asymmetrical, radial, or bilateral.
• Find how many germ layers it has — two (diploblastic) or three (triploblastic).
• Examine the body cavity — acoelomate, pseudocoelomate, or a true coelom lined by mesoderm.
• Look for segmentation of the body.
• Test for the presence or absence of a notochord, which sorts it into chordate or non-chordate.

Also compare the patterns of its digestive, circulatory and reproductive systems. The particular combination of all these characters fixes the phylum, and finer characters then place it in a class.

A specimen is classified not by how it looks but by the deep structural characters it shares with related animals. No single feature decides everything; each broad group is marked by a particular combination of characters, so the sensible approach is to test the specimen against each character in turn and let the combination point to its place.

The steps to follow are:

• Level of organisation. See how far the body's cells are organised. Loose aggregates of cells indicate the cellular level; cells grouped into tissues indicate the tissue level; tissues built into organs indicate the organ level; and organs associated into functional systems indicate the organ-system level.
• Symmetry. Test whether, and in how many planes, the body divides into matching halves. If no plane gives equal halves it is asymmetrical; if any plane through the central axis gives identical halves it is radially symmetrical; if only one plane gives matching left and right halves it is bilaterally symmetrical.
• Germ layers. Determine whether the body arises from two layers (ectoderm and endoderm, with mesoglea between — diploblastic) or three layers, with a true mesoderm in between (triploblastic).
• Coelom. Examine the body cavity between the gut and the body wall. No cavity means acoelomate; a cavity not lined by mesoderm means pseudocoelomate; a true cavity lined by mesoderm means coelomate.
• Segmentation. Check whether the body is built of repeated segments, both outside and inside — that is, whether it shows metameric segmentation.
• Notochord. Look for a notochord. Its presence marks the animal as a chordate; its absence marks it as a non-chordate.

Once these characters have placed the specimen in a phylum, its finer distinguishing features are used to settle its class — for instance, among chordates, whether jaws are present, how many chambers the heart has, and whether it is cold-blooded or warm-blooded.

Method: The taxonomic-hierarchy (top-down) approach

Rather than listing organisation, symmetry, germ layers, coelom, segmentation and notochord as an unordered checklist, follow the classification the way a taxonomist actually narrows a specimen down, rank by rank, from Kingdom to Species.

• Kingdom: confirm it is an animal (heterotrophic, multicellular, no cell wall) — this alone rules out huge unrelated categories of life.
• Phylum: this is exactly where the fundamental characters do their main work, but used as a branching KEY rather than a flat list. Ask body organisation first (does it have organs? organ systems?) — this single question eliminates whole phyla (Porifera, Coelenterata) immediately. Then ask symmetry, then germ layers, then coelom type, then segmentation, then notochord — each answer prunes the set of remaining possible phyla until only one is left.
• Class: within the surviving phylum, apply finer distinguishing marks (for example, within Chordata: jawed or jawless? bony or cartilaginous skeleton? number of heart chambers? warm- or cold-blooded?).
• Order, Family, Genus, Species: apply progressively finer shared-character comparisons, each level narrowing a smaller set of close relatives, until the specimen matches exactly one species.

Framed this way, the "checklist" of fundamental characters is not applied all at once — it functions as a dichotomous key used specifically at the Phylum step, exactly like a field guide's identification key: each character is a fork in the road, and the specimen's answer sends it down one branch while discarding entire other phyla at that fork.

Conclusion: Working top-down through the fixed taxonomic ranks, using the fundamental characters as a branching key at the phylum step, reaches the same final placement as checking each character independently — but makes explicit why the process narrows steadily down to one definite position rather than merely listing traits.