Question

After the rain, tadpoles were found swimming in a pond but not the frogs. This is due to
A. Lack of pituitary hormones
B. Lack of sex hormones
C. Lack of thyroxine
D. Lack of nitrogen in the water

Answer 1

Frogs embryos are typically surrounded by several layers of gelatinous material. When several eggs are clumped together, they are collectively known as frogspawn. The jelly provides support and protection while allowing the passage of oxygen, carbon dioxide and ammonia. It absorbs moisture and swells on contact with water. After fertilization, the innermost portion liquifies to allow free movement of the developing embryo.

Complete step by step answer: At the end of the tadpole stage, a frog undergoes metamorphosis in which its body makes a sudden transition into an adult form. This metamorphosis typically lasts only for 24 hours and is initiated by the production of the hormone ‘thyroxine’. This causes different tissues to develop in different ways. The principal changes that take place involve the development of the lungs and the disappearance of the gills and gill pouch, making the front legs visible. The lower jaw transforms into the big mandible of the carnivorous adult, and the long, spiral gut of the herbivorous tadpole is replaced by the typical short gut of a predator. The nervous system becomes adapted for hearing and stereoscopic vision, and new methods of locomotion and feeding. The eyes are repositioned higher up on the head and the eyelids and associated glands are formed. The eardrum, middle ear and the inner ear are developed. The skin becomes thicker and tougher. The lateral line system is lost, and skin glands are developed. The final stage is the disappearance of the tail. Frogs are at their most vulnerable to predators when they are undergoing metamorphosis. After metamorphosis, young adults may disperse into terrestrial habitats or continue to live in water. Almost all frog species are carnivorous as adults, preying on invertebrates, including arthropods, worms, snails, and slugs.
Hence, the correct answer is option C.

Note: The larvae that emerge from the eggs, known as tadpoles, typically have oval bodies and long, vertically flattened tails. Free-living larvae are fully aquatic, but at least one species has semi-terrestrial tadpoles that live among wet rocks. Tadpoles lack eyelids and have cartilaginous skeletons, lateral line systems, gills for respiration (external gills at first, internal gills later), and vertically flattened tails they use for swimming.