STRUCTURAL ORGANISATION IN ANIMALS

In multicellular animals, similar cells and intercellular substances perform a specific function as a tissue. Tissues form organs, and organs interacting for a common function form organ systems. Complex animals have four fundamental tissue types: epithelial, connective, muscular and neural.

Epithelial tissues form sheets that cover body surfaces and line cavities. Cells are compactly packed with little intercellular material. Simple squamous epithelium permits exchange in alveoli and blood vessels; cuboidal epithelium lines kidney tubules and ducts; columnar epithelium lines stomach and intestine. Ciliated epithelium moves particles or gametes in bronchioles and oviducts. Compound epithelium protects areas subject to wear and tear, such as skin.

Connective tissues have cells embedded in abundant matrix. Areolar tissue fills spaces and supports organs; adipose tissue stores fat; cartilage provides pliable support; bone has a calcified matrix; blood transports materials; lymph returns tissue fluid and carries absorbed fats.

Neural tissue contains neurons specialised to receive stimuli and conduct nerve impulses, along with supporting neuroglial cells. A neuron has cell body, dendrites and an axon; it is the structural and functional unit of the nervous system.

Frogs belong to class Amphibia and live on land and in fresh water. The common Indian frog is Rana tigrina. Frogs are poikilothermic: body temperature varies with environmental temperature. Their body is divided into head and trunk; skin is smooth, moist and lacks scales.

Forelimbs end in four digits and hindlimbs end in five webbed digits. Hindlimbs are larger and more muscular, enabling swimming, leaping, walking and burrowing. Male frogs have vocal sacs and copulatory pads on the first forelimb digit, producing sexual dimorphism.

The frog alimentary canal is short because frogs are carnivorous. The route is mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca. The bilobed tongue captures prey. Liver secretes bile, stored in gall bladder; pancreas provides pancreatic juice. Bile emulsifies fats, while pancreatic enzymes digest carbohydrates and proteins. Intestinal villi and microvilli absorb digested food.

In water, frogs respire through moist skin by diffusion: cutaneous respiration. On land, skin, buccal cavity and lungs contribute; lung breathing is pulmonary respiration. During aestivation and hibernation, cutaneous respiration becomes important.

The frog heart is three-chambered: two atria and one ventricle. It has incomplete double circulation. Blood contains nucleated red blood cells. The hepatic portal system links digestive tract to liver, and the renal portal system carries blood from hindlimbs to kidneys.

Frogs have paired kidneys, ureters, urinary bladder and cloaca. They excrete urea and are ureotelic. In males, ureters also act as urinogenital ducts. The nervous system consists of central (brain and spinal cord), peripheral (cranial and spinal nerves), and autonomic (sympathetic and parasympathetic) parts. Frogs have ten pairs of cranial nerves.

Male frogs have a pair of testes attached to the kidneys. Vasa efferentia carry sperms into kidneys; urinogenital ducts open into the cloaca. Females have ovaries near kidneys and paired oviducts that open separately into cloaca. A mature female lays about 2500-3000 ova at a time.

Fertilisation is external and occurs in water. Development includes a larval tadpole stage; metamorphosis produces the adult frog. Frogs support ecological balance by controlling insects and serving as links in food chains and food webs.

1. Differentiate the four basic animal tissues.
2. Compare striated, smooth and cardiac muscles.
3. State the external features and sexual dimorphism of frog.
4. Trace food through the frog alimentary canal.
5. Compare cutaneous and pulmonary respiration in frog.
6. Why is frog called ureotelic and poikilothermic?
7. Describe fertilisation and development in frog.