Frog — External Features and General Body Plan

Having surveyed the four basic tissue types that build every animal organ, this chapter now turns to how those tissues are assembled into the complete organ systems of a real animal, using the frog — specifically Rana tigrina, the common Indian bull frog, along with closely related species found across the ponds, ditches and paddy fields of West Bengal — as the case-study organism named in the WBCHSE syllabus for this purpose. The frog is an especially instructive example precisely because it is amphibious: it leads a genuinely dual life, active in water for part of the year and on land for the rest, and its anatomy at every level, from its skin to its heart to its breeding behaviour, reflects the compromises that dual existence demands.

Externally, the adult frog's body is divided into just two obvious regions, a head and a trunk, with no distinct neck separating them and, unlike its own tadpole larva, no tail at all in the adult stage. The head bears a wide mouth, a pair of protruding eyes each covered by a nictitating membrane (a thin, transparent third eyelid that can be drawn across the eye for protection, particularly useful underwater), a pair of external nostrils used for both smell and as an air inlet during pulmonary respiration, and, in the male, a pair of vocal sacs used to amplify the mating call. Immediately behind each eye lies a rounded tympanic membrane, the external hallmark of the frog's hearing organ. The trunk carries two pairs of limbs of very different size and function: short, weaker forelimbs used mainly to break the frog's fall on landing after a jump, and much longer, more powerful hindlimbs, webbed between the toes, that serve both for swimming (as paddles) and for the frog's characteristic leaping locomotion on land. …