NCERT Solutions · Class 9 Science Patterns in Life: Diversity and Classification

The features that separate two animals are the same ones that place them in different groups. Take a garden ant and a house lizard (use any pair you can actually observe — the reasoning is what is being marked).

Backbone — the biggest split. The lizard has a vertebral column; the ant has none. This one feature separates them at the top: lizard → chordata (vertebrata), ant → non-chordata (invertebrata).

Skeleton. The ant's hard covering is on the outside — an exoskeleton, which Fig. $\displaystyle 12.16$ gives as the mark of Arthropoda. The lizard's skeleton is internal.

Legs. The ant has jointed legs — the defining arthropod feature. The lizard's four limbs are built on an internal bony frame.

Body segmentation. The ant's body is divided into segments specialised for different functions; the lizard's body is not built that way.

Body covering. Dry scaly skin on the lizard; a hard chitinous covering on the ant, which also cuts its water loss.

What does not separate them: both are multicellular, heterotrophic, eukaryotic and show locomotion and rapid response to stimuli. Those features only confirm both are Animalia.

Placing them: ant → Animalia → non-chordata → Arthropoda; lizard → Animalia → chordata → vertebrata → reptiles.

The method behind it: start with broad, easily visible features and move to detailed ones. Each feature you add carries the animal one step down Kingdom → Phylum → Class → Order → Family → Genus → Species.

A classification criterion must be usable on everything being classified. Cell type (prokaryote or eukaryote), presence and composition of a cell wall, and unicellular or multicellular can be checked in a bacterium, an amoeba, a mushroom, a mango tree and a tiger alike. 'Has xylem and phloem?' is meaningless for all but plants — you cannot use it to sort Monera, Protista, Fungi or Animalia at all.

The five kingdom system is built on cellular criteria for exactly this reason — cell type, cell structure, level of organisation and mode of nutrition are what Fig. $\displaystyle 12.5$ branches on.

Classification moves from broad to specific. Cellular organisation is the broad feature that opens the hierarchy; vascular tissue is a detail used much further down, to separate the classes within Kingdom Plantae (thallophytes and bryophytes lack it; pteridophytes, gymnosperms and angiosperms have it).

History proves the point. It was a cellular feature that forced a change: an amoeba has a true membrane-bound nucleus and a bacterium does not, so bacteria were pulled out of Protista into Monera. No tissue-level feature could have made that distinction.

Cellular features also track ancestry more reliably. Cell structure and DNA reflect deep relationships, whereas a tissue can be gained or lost as an adaptation — and Aristotle's system already showed how far a classification built on superficial features can go wrong.