Frog

7.6 Frog — Rana tigrina

Frogs belong to class Amphibia of Phylum Chordata. Frogs are seen to live both on land and in freshwater (ponds, ditches). They are referred to as amphibians as they can live both in aquatic as well as on land. They are cold-blooded (poikilotherms), i.e., they do not have a body temperature regulation mechanism. Their body temperature varies with the temperature of the environment.

Frogs can change the colour to hide them from predators (camouflage). This is called mimicry. They are not seen during peak summer and winter. During this period they take shelter in deep burrows to protect them from extreme heat and cold. This is known as aestivation and hibernation, respectively.

7.6.1 Morphology

The skin is smooth, slippery due to the presence of mucus. Skin is always maintained in moist condition. The colour of the dorsal side of the body is generally olive green with dark irregular spots. On the ventral side the skin is uniformly pale yellow. The frog never drinks water but absorbs it through the skin. The body of the frog is divisible into head and trunk; no neck or tail. A pair of nostrils is present above the mouth. Eyes are bulged and covered by a nictitating membrane that protects them while in water. On either side of the eyes a membranous tympanum (ear drum) receives sound signals.

The forelimbs and hind limbs help in swimming, walking, leaping and burrowing. The hind limbs end in five digits and forelimbs in four digits. Feet have webbed digits that help in swimming. Frogs exhibit sexual dimorphism. Male frogs can be distinguished by the presence of sound producing vocal sacs and also a copulatory pad on the first digit of the fore limbs which are absent in the female frogs.

7.6.2 Digestive System

The digestive system consists of alimentary canal and digestive glands. The alimentary canal is short because frogs are carnivores and hence the length of intestine is reduced. The mouth opens into the buccal cavity that leads to the oesophagus through pharynx. Oesophagus is a short tube that opens into the stomach which in turn continues as the intestine, rectum and finally opens outside by the cloacal aperture. Cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms or ova.

Liver secretes bile that is stored in the gall bladder. Pancreas, a digestive gland produces pancreatic juice containing digestive enzymes. Food is captured by the bilobed tongue. Partially digested food called chyme is passed from stomach to the first part of the intestine, the duodenum. The duodenum receives bile from gall bladder and pancreatic juices from the pancreas through a common bile duct. The bile emulsifies fat and pancreatic juices digest carbohydrates and proteins. Final digestion takes place in the intestine. Digested food is absorbed by the numerous finger-like folds in the inner wall of intestine called villi and microvilli. The undigested solid waste moves into the rectum and passes out through cloaca.

7.6.3 Respiratory System

Frogs respire on land and in the water by two different methods. In water, skin acts as aquatic respiratory organ (cutaneous respiration). Dissolved oxygen in the water is exchanged through the skin by diffusion. On land, the buccal cavity, skin and lungs act as the respiratory organs.

The lungs are a pair of elongated, pink-coloured sac-like structures present in the upper part of the trunk region (thorax). Air enters through the nostrils into the buccal cavity and then to lungs. During aestivation and hibernation gaseous exchange takes place through the skin.

7.6.4 Circulatory System

Frog has a well-developed closed circulatory system with heart, blood vessels and blood. Since frogs also have lymph and lymph channels, the lymphatic system is also present. The heart is a muscular structure situated in the upper part of the body cavity. It has three chambers, two atria and one ventricle and is covered by a membrane called pericardium. A triangular structure called sinus venosus joins the right atrium. It receives blood through the major veins called vena cava. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart. The blood from the heart is carried to all parts of the body by the arteries (arterial system). The veins collect blood from different parts of body to the heart and form the venous system. Special venous connections between liver and intestine as well as the kidney and lower parts of the body are present in frogs. The former is called hepatic portal system and the latter is called renal portal system.

The blood is composed of plasma and cells. The cells are RBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytes and platelets. RBC's are nucleated and contain red coloured pigment namely haemoglobin. The lymph is different from blood. It lacks few proteins and RBCs. The blood carries nutrients, gases and water to the respective sites during the circulation.

7.6.5 Excretory System

The excretory system consists of a pair of kidneys, ureters, cloaca and urinary bladder. These are compact, dark red and bean-like structures situated a little posteriorly in the body cavity on both sides of vertebral column. Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons. Two ureters emerge from the kidneys in the male frogs. The ureters act as urinogenital duct which opens into the cloaca. In females the ureters and oviduct open separately in the cloaca. The urinary bladder is also present which opens in the cloaca. The frog excretes urea and thus is an ureotelic animal. The excretory wastes are carried by blood into the kidney where it is separated and excreted.

7.6.6 Nervous and Endocrine System

The system for control and coordination is highly evolved in the frog. It includes both neural system and endocrine glands. The chemical coordination of various organs of the body is achieved by hormones which are secreted by the endocrine glands. The prominent endocrine glands found in frog are pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.

The nervous system is organised into a central nervous system (CNS) — brain and spinal cord, a peripheral nervous system (cranial and spinal nerves) and an autonomic nervous system (sympathetic and parasympathetic). There are ten pairs of cranial nerves arising from the brain. Brain is enclosed in a bony structure called brain box (cranium). The brain is divided into fore-brain, mid-brain and hind-brain. Forebrain includes olfactory lobes, paired cerebral hemispheres and unpaired diencephalon. The midbrain is characterised by a pair of optic lobes. Hindbrain consists of cerebellum and medulla oblongata. The medulla oblongata passes out through the foramen magnum and continues into spinal cord, which is enclosed in the vertebral column.

Frog has different types of sense organs, namely organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with internal ears). Out of these, eyes and internal ears are well-organised structures and the rest are cellular aggregations around nerve endings.

7.6.7 Reproductive System

The male reproductive organs consist of a pair of yellowish ovoid testes, which are found adhered to the upper part of kidneys by a double fold of peritoneum called mesorchium. Vasa efferentia are 10-12 in number that arise from testes. They enter the kidneys on their side and open into Bidder's canal. Finally it communicates with the urinogenital duct that comes out of the kidneys and opens into the cloaca. The cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms.

The female reproductive organs include a pair of ovaries. The ovaries are situated near kidneys and there is no functional connection with kidneys. A pair of oviduct arising from the ovaries opens into the cloaca separately. A mature female can lay 2500 to 3000 ova at a time. Fertilisation is external and takes place in water. Development involves a larval stage called tadpole. The tadpole undergoes metamorphosis that forms the adult.

7.6.8 Economic Importance of Frogs

Frogs are beneficial:

• They eat insects and protect the crops; thus they help in maintaining ecological balance.
• In some countries, the muscular legs of frogs are used as food by man.
• Frogs are widely used as an experimental specimen in biological research and dissection laboratories.

7.7 Worked Examples