Structural Organisation in Animals — Class 11 Biology

This chapter introduces the idea that in multicellular animals groups of similar cells form tissues, tissues form organs, and organs form organ systems, illustrating division of labour, and then studies the morphology and anatomy of the frog as a representative vertebrate. The common Indian frog, Rana tigrina, belongs to class Amphibia of phylum Chordata; it is a cold-blooded (poikilothermic) animal that can change colour for camouflage and undergoes aestivation and hibernation. Externally the body is divided into head and trunk, with moist slippery skin through which water is absorbed, bulged eyes with a nictitating membrane, a tympanum, and limbs with webbed digits; males show vocal sacs and a copulatory pad. Internally the frog has well-developed digestive, respiratory, circulatory, excretory, nervous and reproductive systems. It is carnivorous with a short alimentary canal, respires through skin, buccal cavity and lungs, has a three-chambered heart with a closed single circulation and nucleated red blood cells, excretes urea (it is ureotelic), and has a brain divided into fore-, mid- and hind-brain. Fertilisation is external and development passes through a tadpole stage that metamorphoses into the adult. Frogs are ecologically and agriculturally useful.