Fill in the following blank with a suitable word:
$Gills$ are the breathing organs in . . . . . . .

How are the lungs designed in human beings to maximize the area for exchange of gases?

Name the substance whose build-up in the muscles during vigorous physical exercise may cause cramps.

What is the name of the extensions of the epidermal cells of a root which help in respiration?

In fermentation, the incomplete oxidation of glucose is achieved under anaerobic conditions by a set of reactions where pyruvic acid is converted to $CO_2$ and ethanol. The enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase, catalyse these reactions. Answer the following questions:
$(i)$ Write the chemical equation for alcoholic fermentation.
(ii) $A$ test tube containing molasses solution and yeast is kept in a warm place overnight. What gas is collected from the mixture?
(iii) How does ethyl alcohol fermentation occur?
(iv) What is pyruvic acid?

From where do aquatic animals and terrestrial animals obtain oxygen for breathing and respiration?