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Q251.
Assertion (A): The class name Reptilia refers to their creeping or crawling mode of locomotion.
Reason (R): Reptiles are mostly terrestrial animals.
[Animal-Kingdom] [class-xi]
Correct option: B [ ]
Remark: Both A and R are true but R is not the correct explanation of A. Explanation: Both statements are true. The reason is a related fact but does not directly explain the etymology of Reptilia.
Q252.
Assertion (A): Most reptiles have a three-chambered heart.
Reason (R): Crocodiles, a type of reptile, have a four-chambered heart.
[Animal-Kingdom] [class-xi]
Correct option: B [ ]
Remark: Both A and R are true but R is not the correct explanation of A. Explanation: Both are key characteristics of reptiles. However, having modified forelimbs is a separate adaptation for flight, not an explanation for the presence of feathers.
Q253.
Assertion (A): Reptiles are poikilotherms.
Reason (R): They are able to maintain a constant body temperature.
[Animal-Kingdom] [class-xi]
Correct option: C [ ]
Remark: A is true but R is false. Explanation: Reptiles are poikilotherms (true). The reason is false; poikilotherms cannot maintain a constant body temperature.
Q254.
Assertion (A): The most characteristic feature of Aves is the presence of feathers.
Reason (R): The forelimbs in birds are modified into wings.
[Animal-Kingdom] [class-xi]
Correct option: B [ ]
Remark: Both A and R are true but R is not the correct explanation of A. Explanation: Both statements are true. However, the presence of modified forelimbs (wings) does not explain the origin of feathers.
Q255.
Assertion (A): The long bones of birds are hollow with air cavities (pneumatic).
Reason (R): A pneumatic endoskeleton helps in making the body light for flight.
[Animal-Kingdom] [class-xi]
Correct option: A [ ]
Remark: Both A and R are true and R is the correct explanation of A. Explanation: The hollow, air-filled (pneumatic) nature of bird bones is a crucial adaptation to reduce body weight, which is essential for flight.
Q256.
Assertion (A): The most unique mammalian characteristic is the presence of mammary glands.
Reason (R): Mammary glands are used to nourish the young ones.
[Animal-Kingdom] [class-xi]
Correct option: A [ ]
Remark: Both A and R are true and R is the correct explanation of A. Explanation: Mammary glands are considered the most unique mammalian trait because their function—producing milk to nourish young—is exclusive to this class.
Q257.
Assertion (A): All mammals are viviparous.
Reason (R): The platypus (Ornithorhynchus) is an oviparous mammal.
[Animal-Kingdom] [class-xi]
Correct option: D [ ]
Remark: Assertion (A) is false but R is true. Explanation: Assertion is false because there are oviparous (egg-laying) mammals. The reason is true; the platypus is a well-known example of an exception to the general rule of viviparity in mammals.
Q258.
Assertion (A): The skin of mammals is unique in possessing hair.
Reason (R): Mammals are homoiothermous and have a four-chambered heart.
[Animal-Kingdom] [class-xi]
Correct option: C [ ]
Remark: A is true but R is false. Explanation: Assertion is correct. The reason is false; the most unique mammalian characteristic is the presence of mammary glands (and hair is a typical mammalian feature), but homoiothermy and four-chambered heart do not explain hair presence.
Q259.
Assertion (A): Phylum Annelida is the first phylum to exhibit segmentation.
Reason (R): The body of annelids is distinctly marked out into segments or metameres.
[Animal-Kingdom] [class-xi]
Correct option: A [ ]
Remark: Both A and R are true and R is the correct explanation of A. Explanation: Annelids are considered the first phylum to show true segmentation; the body is distinctly metameric.
Q260.
Assertion (A): Coelenterates have a central gastro-vascular cavity.
Reason (R): It has two openings, a mouth and an anus.
[Animal-Kingdom] [class-xi]
Correct option: C [ ]
Remark: A is true but R is false. Explanation: Coelenterates possess a gastrovascular cavity; it is incomplete and has a single opening (mouth) serving also as the anus, not two openings.