Animal classification based on fundamental features

Correct answer: B. Complexity of cell structure and mode of nutrition

Correct answer: B. Whittaker used meaningful biological criteria, especially cell structure and complexity, body or thallus organisation, mode of nutrition, reproduction, and evolutionary relationships. Cell structure separates prokaryotes from eukaryotes, while mode of nutrition helps distinguish organisms that make food, ingest food, or absorb nutrients. These criteria provide a much stronger scientific basis than superficial appearance. Option A, colour, can vary and is not a fundamental classification criterion. Option C, geographical distribution, tells where organisms live but does not define their major biological group. Option D, size alone, is also unreliable because unrelated organisms can be large or small. Memory cue: remember “cell structure + nutrition” as the core pair for Whittaker.

Correct answer: B. Animals with red blood and animals without red blood

Correct answer: B. Aristotle divided animals into Enaima, or animals with red blood, and Anaima, or animals without red blood, according to the terminology commonly used in school biology. This was an early, simple classification based on an observable feature. A is incorrect because vertebrate and invertebrate grouping is a later and more formal classification based on the presence or absence of a vertebral column. C is incorrect because habitat was not the two-part division asked for in Aristotle’s scheme. D is incorrect because ability to fly also does not represent his principal two-group division. Be careful: “animals without red blood” does not mean that every such animal literally has no blood or no circulatory fluid; it describes Aristotle’s old observational category. Memory cue: Aristotle’s animal division = red blood versus no red blood, not backbone versus no backbone.

Correct answer: A. Absence of a cell wall and holozoic nutrition

Correct answer: A. Animal cells do not have a cell wall, and animals generally obtain food by ingestion, followed by digestion and absorption; this is called holozoic nutrition. Plants generally have a cellulose cell wall and, in school-level classification, are mainly autotrophic because they photosynthesise. B is therefore a characteristic associated mainly with Plantae, not Animalia. C is not a distinguishing feature because both animals and plants are generally multicellular. D is also shared by both kingdoms because both have eukaryotic cells with a true nucleus. The best method is to eliminate shared features and compare cell structure and nutrition. Memory cue: animals ingest food and lack a cell wall; plants generally make food and have a cell wall.

Correct answer: B. Holozoic

The correct answer is B, holozoic nutrition. Most animals obtain food by taking in solid or liquid material, digesting it inside the body, absorbing useful nutrients and eliminating undigested waste. This complete pattern is called holozoic nutrition. Option A, autotrophic nutrition, means making food from inorganic materials, as plants do; animals generally cannot do this. Option C, saprophytic nutrition, involves obtaining nutrients from dead and decaying matter and is characteristic of fungi. Option D, parasitic nutrition, occurs in some animals that live on or inside hosts, but it is not the primary mode for Animalia as a whole. Memory cue: ingestion followed by internal digestion means holozoic nutrition.