Kingdom Animalia — Overview

NCERT grounding

NCERT Class 11 Biology, Chapter 2 (Biological Classification), devotes section 2.5 to Kingdom Animalia. The text states that the kingdom is characterised by heterotrophic eukaryotic organisms that are multicellular and whose cells lack cell walls; they depend, directly or indirectly, on plants for food, digest food in an internal cavity, store reserves as glycogen or fat, follow holozoic nutrition, show a definite growth pattern and are mostly capable of locomotion. The chapter then states that the salient features of the various phyla are described separately in the Animal Kingdom chapter.

"This kingdom is characterised by heterotrophic eukaryotic organisms that are multicellular and their cells lack cell walls… Their mode of nutrition is holozoic — by ingestion of food."
— NCERT Class 11 Biology, §2.5 Kingdom Animalia

The NIOS Biology lesson on Kingdoms Plantae and Animalia reinforces the same four overview pillars — animals are multicellular eukaryotes with ingestive (heterotrophic) nutrition, the power of locomotion, and increased sensitivity through a nervous system. For this Chapter-2 subtopic we stay at the level of these defining characters and the kingdom's place in the five-kingdom table; the phyla themselves are a separate chapter.

Defining features of Animalia

Whittaker's 1969 five-kingdom system grouped organisms by cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships. Animalia occupies the position reserved for the eukaryotic, multicellular, wall-less heterotrophs. Each of the kingdom's defining characters can be read straight off this set of criteria, and each is independently testable in NEET. We take them one at a time.

No cell wall — the unique character

Among the eukaryotic kingdoms, only Animalia lacks a cell wall. Plantae have a cellulose wall and Fungi a chitin wall; the protistan wall is present in some members only. The absence of a rigid wall is the single feature that the five-kingdom table assigns uniquely to Animalia, and it underlies much of animal biology. A naked plasma membrane allows cells to change shape, to engulf food particles by phagocytosis, and to form the contractile and contractile-supported tissues that make locomotion possible.

Holozoic, heterotrophic nutrition

Animals cannot fix carbon; they are heterotrophs that depend directly or indirectly on plants for food. Their characteristic mode is holozoic — solid food is taken in, broken down inside an internal digestive cavity, the soluble products absorbed and the waste expelled. This is the ingestive route, and it stands apart from the absorptive heterotrophy of fungi, which secrete enzymes onto a substrate and absorb the dissolved nutrients without an internal cavity. NCERT notes that animals may also be saprophytic in some forms, but ingestion is the defining mode for the kingdom.

Nervous control, locomotion and definite growth

Higher animals show an elaborate sensory and neuromotor mechanism, and most members are capable of locomotion — both points NCERT records as overview features. Coupled with the absence of a cell wall, the nervous system and contractile tissues let animals respond rapidly and move. Growth is also distinctive: animals follow a definite growth pattern and mature into adults of definite shape and size, in contrast with the open, indefinite growth of plants. Reproduction is largely sexual, by copulation of a male and a female, followed by embryological development.

Position in Whittaker's scheme

R.H. Whittaker (1969) proposed five kingdoms — Monera, Protista, Fungi, Plantae and Animalia. Animalia sits at the end of this sequence because it represents the most elaborate body organisation, climbing from the tissue grade to the organ and organ-system grades. The earlier two-kingdom system of Linnaeus had only Plantae and Animalia and could not separate prokaryotes from eukaryotes, or autotrophs from heterotrophs. Whittaker's added criteria — cell structure, body organisation, mode of nutrition, reproduction and phylogeny — placed Animalia precisely as the wall-less, holozoic, multicellular eukaryotes.

Animalia vs Plantae vs Fungi

Animalia, Plantae and Fungi are all eukaryotic and have a nuclear membrane, so they cannot be told apart by cell type. The discriminating characters are the cell wall, the mode of nutrition, and the storage product. NEET sets these three side by side often enough that they are worth holding in a single comparison.

Note the one overlap that traps students: fungi, like animals, store glycogen, not starch. So glycogen alone does not separate animals from fungi. The reliable separators are the cell wall (absent in animals, chitinous in fungi) and the mode of nutrition (holozoic ingestion in animals, absorptive saprophytism/parasitism in fungi).

Phyla — covered in Animal Kingdom

Chapter 2 deliberately stops at the overview. The salient features of the various animal phyla — Porifera, Cnidaria, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata and Chordata, with their criteria of body symmetry, coelom, germ layers and notochord — are taken up in the dedicated Animal Kingdom chapter. For this subtopic it is enough to know that Animalia is internally classified into phyla using those structural criteria; the detail is studied separately.

Worked examples

Common confusion & NEET traps