Top 11 Types of System in Toads | Amphibians

The following points highlight the top eleven types of system in toads. The types are: 1. Muscular System 2. Digestive System 3. Respiratory System 4. Circulatory System 5. Arterial System 6. Venous System 7. Lymphatic System 8. Nervous System 9. Endocrine System 10. Excretory System 11. Reproductive System.

Type # 1. Muscular System:

The muscular system is composed of mus­cles which are used primarily for the move­ment of the body.

There are three types of muscles:

(a) Striated muscles,

(b) Un-striated muscles and

(c) Cardiac muscles.

The striated muscles can be contracted at will and are attached to the skeleton. So they are also known as the skeletal muscles. These muscles form the main mass of the external muscula­ture. There are many skeletal muscles in the body of toad and some important ones are shown in Fig. 7.4. Table 25 shows the different kinds of muscles in Bufo.

The muscles possess the power of contraction and relaxation. All the muscles are mostly arranged in opposing groups in such a fashion that when one set contracts, the opposing set remains in a relaxed state. This co-ordination is controlled by the nervous system.

The skeletal muscles in Bufo are grouped into the following general types depending on the mode of action:

(a) Flexor muscle:

This type of muscle bends one part on another.

Example: Biceps flexes forearm towards upper arm.

(b) Extensor muscle:

This muscle extends or straightens a part.

Examples:

Triceps extends forearm on upper arm.

(c) Abductor muscle:

Such a type of mus­cle draws a part away from the axis of the body or of a limb.

Example:

Deltoid muscle draws arm forward.

(d) Adductor muscle:

This type of muscle draws a part toward the axis of the body or of a limb.

Example:

Latissimus dorsi muscle draws arm up and back.

(e) Depressor muscle:

This muscle lowers a part.

Example:

Depressor mandibular muscle moves lower jaw down for opening the mouth.

(f) Levator muscle:

This muscle raises or elevates a part.

Example:

Masseter muscle raises lower jaw to close the mouth.

(g) Rotator muscle:

This muscle rotates a part.

Example:

Pyriformis muscle raises and rotates the femur.

Type # 2. Digestive System:

The digestive system consists of alimen­tary canal and its associated digestive glands. This system is primarily concerned with the process of nutrition. The alimentary canal is a long tube which starts from the mouth and ends in the cloaca. This canal has a basic histological picture throughout its length and becomes regionally modified to perform certain specific functions.

The alimentary canal is made up of four layers. The outermost layer is thin and called serous coat. Next to the serous coat lies the muscular layer which is composed of outer longitudinal muscles and the inner circular muscles. The innermost layer lining the lumen is the mucous coat.

The mucous layer is com­posed of epithelial cells and glands at certain regions. The region between the muscular coat and the mucous coat is filled up with connective tissue matrix with blood and lym­phatic vessels. This layer is known as sub-mu­cous coat.

The mouth is a wide aperture and is bounded by upper and lower jaws. Both the jaws are toothless. The mouth leads into a spa­cious buccal cavity. The roof of the buccal cavity contains a pair of internal nostrils, impressions of the eye balls, a pair of eustachi­an apertures.

The floor of the buccal cavity bears a large fleshy tongue which is attached to the hyoid arch. The tongue is very peculiar, because it is fixed anteriorly but free behind. The tongue is kept sticky by the secretion of inter maxillary glands. It can quickly be pro­truded to capture insects (Fig. 7.9).

Behind the tongue there is a longitudinal slit, called glottis, which leads into the laryngotracheal chamber. There are mucous glands in the buc­cal cavity which do not produce any enzyme. There is a system of cilia in the buccal cavity which possibly keep the oral fluid in circulation.

The buccal cavity narrows towards the pharynx which leads into a wide tube known as oesophagus or gullet. The oesophagus opens into the stomach. The stomach is a thick- walled spacious sac and is slightly curved (Fig. 7.10 A). The broad anterior part of the stomach is known as cardiac end and the other end is called pyloric end.

The pyloric part of the stomach opens into intestine. This opening is guarded by circular sphincter muscle, called pyloric valve which regulates the exit of food from the stomach. The mucous coat of the stomach opens into intestine. This opening is guarded by circular spinster muscle, called pyloric valve which regulates the exit of food from the stomach.

The mucous coat of the stomach contains tubular gastric glands which secrete digestive juices and unicellular parietal glands (or oxyntic cells) which secrete hydrochloric acid.

In addition to the longitudi­nal and circular muscle layers in the muscular coat, oblique muscles are present in the stom­ach. The part of the intestine next to stomach is known as small intestine which is again divided into an anterior short duodenum and posteriorly into an ileum.

The duodenum receives duct from liver and pancreas. The ileum is extensively coiled and is held in posi­tion by mesentery. The mucous coat of the ileum gives off numerous finger-like elevations, known as villi which increase the inner sur­face areas. In addition, it contains intestinal glands which secrete intestinal juices.

The portion of alimentary canal next to ileum is usually called large intestine which is divided into rectum and cloaca. The cloaca is a common chamber where urine, gametes and faecal matters are temporarily stored. The cloaca opens to the exterior through the vent (or cloacal aperture).

Besides gastric and intestinal glands, the liver and the pancreas are the important diges­tive glands. The liver is the largest gland and consists of two main lobes, left and right. The lobes are connected with one another by a bridge. The left lobe of the liver is the larger one and is subdivided into two lobes. The secretory as well as excretory products of the liver are called bile.

The bile comes out by hep­atic ducts and is stored in the gall-bladder. The cystic duct from the gall-bladder and the hep­atic duct from the liver unite to form the com­mon bile duct. This common bile duct passes through pancreas and receives numerous minute pancreatic ducts.

The common bile duct is usually called hepato pancreatic duct which ultimately opens into the duodenum to pour the hepato pancreatic juices into the duo­denal cavity. Fig. 7.10 B shows the relationship of the liver and pancreas with the duodenal part of the intestine.

The pancreas is an irregu­lar and yellowish gland. It is both an exocrine as well as an endocrine gland. The exocrine part secretes the pancreatic juice. The endocrine part, islets of Langerhans, produces the hormone, known as insulin.

Physiology of digestion:

Toad is a carnivorous animal which feeds on small animals, preferably the insects. The food consists primarily of carbohydrates, pro­teins, fats, vitamins, mineral salts and water. The vitamins, mineral salts and water are absorbed as such but the carbohydrates, pro­teins and fats are complex organic compounds which cannot be absorbed unless these are broken down into simpler forms.

Digestion is effected by the digestive juices. These juices contain specific enzymes which act on a spe­cific type of food by causing hydrolysis (i.e., chemically, addition of a molecule of water with a molecule of the substance upon which enzymes act).

Three groups of enzymes are present:

(a) Amylolytic or diastatic enzymes, such as the amylase of the pancreatic juice,

(b) Lipolytic enzyme, the lipase of the pancre­atic juice and

(c) Proteolytic enzymes, viz., the pepsin of gastric juice, the trypsin of pancreat­ic juice, erepsin of the intestinal juice.

Thoroughly mixed with mucous in the buccal cavity, the food comes into the stomach. The parietal glands or the oxyntic cells in the mucous coat of stomach secrete hydrochloric acid. In the acidic medium, pepsin acts on proteins and converts them into peptones.

After remaining for some time in the stomach, the half-digested acid chyme passes into the duodenum where it comes in contact with the bile. The bile being alkaline, neutralizes the acid. The trypsin then converts the peptones into the soluble amino acids which are readi­ly absorbable. The erepsin of the intestinal juices quickens the process of conversion.

The lipase converts fats into the soluble fatty acids and glycerol. The amylase of pancreatic juice and the maltase of the intestinal juice converts fats into the soluble fatty acids and glycerol. The amylase of pancreatic juice and the mal­tase of the intestinal juice convert starch into glucose.

The soluble food is absorbed and assimilated into the body and the indigestible products are egested as faeces through vent. The surplus glucose is converted into glycogen and stored in liver and skeletal muscles. The fatty acids are transformed into fat and stored in the body as the fat bodies. The amino acids are not stored, the excess being transformed into urea and excreted through urine.

Spleen:

The spleen is a ductless glandular body of dark-red colour. It is spherical in shape and remains morphologically connected with the mesentery near the junction of the ileum and the rectum. The spleen acts as a storehouse of blood and also destroys old and worn out erythrocytes. It is believed that the leukocytes are manufactured in the spleen.

Type # 3. Respiratory System:

Respiration is essentially a physicochemical process in which oxygen is taken in and carbon-dioxide and water are given out. During this process, the stored food within the cells becomes slowly oxidised. During the process of oxidation energy is liberated in the form of heat which is necessary for the vital activities. The carbon-dioxide and water vapour are given out during the process.

Modes of respiration:

Toad is truly an amphibious animal in strict sense. As a result, two modes of respira­tion are observed. Terrestrial respiration involves the utilisation of oxygen present in atmospheric air and in aquatic respiration the oxygen dissolved in water is used.

Terrestrial mode of respiration:

Toad is capable to breathe air by the lungs and skin. Respiration with the help of lung is called pulmonary respiration and that with the skin is called integumentary or cutaneous respiration.

Pulmonary respiration:

The organs involved in pulmonary respiration are:

(a) External nares or nostrils,

(b) Internal nares or nostrils,

(c) Buccal cavity,

(d) Glottis,

(e) Laryngotra­cheal chamber,

(f) Bronchi and

(g) Lungs.

The buccal cavity communicates into the laryngo­tracheal chamber is a stout box-like structure with two elastic vocal cords stretching across the cavity (Fig. 7.11 A). The wall of the laryn­gotracheal chamber is supported by arytenoid and cricoid cartilages.

Sound is produced by the vibration of the vocal cords which also control the intensity and pitch of the sound. The vocal sac in male helps to intensify the sound by acting as the resonator. The laryngotracheal chamber gives off two extremely short bronchi.

Each bronchus opens into a thin-walled spongy lung. Internally, the lungs have innumerable simple sacs (Fig. 7.11B), known as alveoli (or air-sacs). Each alveolus is a highly vascular structure and the alveolar epithelia are actual­ly the centres of exchanges of gases.

Physical mechanism of pulmonary respi­ration:

The physical mechanism of pulmonary res­piration involves three successive stages: aspi­ration, expiration and inspiration (Fig. 7.12).

Aspiration:

During the process toad closes- its mouth but the external nostrils are kept open. The floor of the buccal cavity is then lowered. As a result of lowering of the floor, partial vacuum is created and thus fresh atmospheric air rushes into the buccal cavity through the nostrils. The glottis remains closed, so the air cannot enter the lungs.

Expiration:

Aspiration is quickly followed by expiration. The trunk muscles contract upon the lungs and cause expulsion of air from the lungs to the buccal cavity through glottis. This expelled air is rich in carbon-dioxide. As a result of inhaled and expelled air, the buccal cavity becomes highly distended. Mixture of these two types of air now goes out through open nostrils.

Inspiration:

Expiration is immediately fol­lowed by inspiration. The external nostrils are then tightly closed and the floor of the buccal cavity is raised forcibly as a result of which mixed air from the buccal cavity is pushed into the lungs through glottis. The hyoid apparatus plays an important role in lowering and raising the floor of the buccal cavity.

Buccopharyngeal respiration:

The mucous membrane lining the buccopharyngeal cavity is always kept moist and is highly vascular. Through this membrane exchange of gases occurs normally.

Cutaneous respiration:

The skin of toad is moist and richly supplied with the blood vessels. It acts as an additional respiratory organ. The cutaneous breathing is a continuous process and is very important spe­cially during hibernation.

Aquatic respiration:

The early phase of the life cycle of toad is spent in water. The tadpoles are the larval forms which respire by means of external and internal gills in addition to skin. The external gills are the epithelial extensions and contain blood capillaries. The gills can absorb oxygen dissolved in water and give out carbon dioxide.

Exchange of gases:

The oxygen is conveyed to different tissues by the blood. The oxygen combines with the haemoglobin to form as unstable compound oxyhaemoglobin. In this state, oxygen reaches the cells and the oxy­haemoglobin on reaching the places release oxygen and regains its former state.

This circle is repeated. The carbon-dioxide is transported by the blood plasma in the form of bicarbonates and finally expelled through the respira­tory surface.

Type # 4. Circulatory System:

The circulatory system includes two sys­tems: the blood vascular system (cardio vas­cular system) and the lymphatic system. Circulatory system is the internal transport mechanism by which nutritive materials, hor­mones, waste products, carbon-dioxide and oxygen are conveyed to the different parts of the body. The cardiovascular system of toad is well-developed. This system is composed of three main components: blood, heart and blood vessels.

Blood:

The blood is the main circulatory fluid. It consists of a straw-coloured fluid, called blood plasma and different blood corpuscles sus­pended in the plasma. The plasma is a watery liquid and contains many mineral salts, food- wastes and hormones. The corpuscles are of three types.

These are:

(a) Red blood cells (RBC or erythrocytes),

(b) White blood cells (WBC or leucocytes) and

(c) Thrombocytes (or blood platelets).

The erythrocytes are oval, biconvex and nucleated cells ranging from 15 to 20 micra in size. They contain the red coloured iron-containing protein, the haemoglobin. The affinity of haemoglobin for oxygen in toad is lower than that in mammals. The leucocytes are colourless, nucleated and amoeboid cells.

They are phagocytes and destroy bacteria and thus protect the animal from the invading microbes. The thrombocytes or blood platelets are small spindle-shaped nucleated cells and play an important role in the process of coagulation of blood. When there is shedding of blood, platelets release a particular type of enzyme which helps in coagulation.

Heart:

The heart is the central pumping organ in the cardiovascular system (Fig. 7.13). In toad, it is a pear-shaped muscular structure situated in the anterior part of the body cavity. It remains enclosed by a transparent protective membrane called pericardium. The space between heart and the pericardium is called pericardial cavity.

The heart of toad is com­posed of the following parts:

(a) Receiving parts. Two auricles and a sinus venosus are the receiving parts of the heart.

(b) Forwarding parts. The ventricle and the conus arteriosus are the forwarding parts of the heart.

(c) Intercommunicating apertures and valves between the different parts of the heart.

The auricles are two in number and are of unequal size. Both the auricles are sharply marked off from the ventricle by a narrow con­striction, called coronary sulcus. The left auri­cle is smaller than the right auricle. The two auricles are completely separated by inter-auricular septum.

The sinus venosus is a triangular thin- walled chamber formed by the union of three major veins, two Percivals and one postcaval (Fig. 7.13A). It is situated on the dorsal side of the right auricle. It communicates with the right auricle through a sinuauricular aperture which is guarded by sinuauricular valves.

Deoxygenated blood, collected in the sinus venosus from the precaval and postcaval veins, enters into the right auricle but the back flow is prevented by the sinuauricular valves. The left auricle receives oxygenated blood through a small opening of the common pul­monary vein. This aperture is also guarded by valves.

The right and left auricles communicate to the ventricle by a common auriculoventricular aperture. This aperture is guarded by membranous valves, known as auriculoventricular valves. The free ends of the valves are attached with the wall of the ventricle by fine thread-like chordae tendineae. The auriculoventricular valves give one-way traffic of blood from the auricles to ventricle.

The ventricle is a highly muscular cham­ber. It is more or less conical in shape. Its cavity is greatly reduced by a large number of interlacing muscles, called Columnae carnae or Trabeculae carnae. The conus arteriosus is a stout tubular body situated on the ventral side of the heart (Fig. 7.13B). It is continued anteriorly into the truncus arteriosus.

The truncus arterio­sus is not to be considered as the part of heart. It is the basal stem of the three main arteries. It remains uncovered by the pericardium and lacks cardiac muscles. A set of pocket-like semi­lunar valves demarcate the truncus from the conus.

A similar set of three semilunar valves guard the opening between the conus and the ventricle. These valves prevent back flow of blood from conus to the ventricle. A twisted longitudinal spiral valve divides the cavity of the conus into two channels.

The left channel is designated the cavum pulmocutaneum and the right one is the cavum aorticum (Fig. 7.13C). The deoxygenated blood passes through the cavum pulmocutaneum and the oxygenated blood travels through the cavum aorticum by the manipulation of the spiral valve.

Mechanism of circulation through the heart:

Periodic contraction (systole) and relaxation (diastole) is an innate property of heart. During diastole, the sinus venosus receives deoxy­genated blood from the two Percivals and one postcaval veins. The left auricle becomes also simultaneously filled up with the oxygenated blood from lungs carried through the common pulmonary vein.

Just with the onset of systole, the sinus venosus contracts and the deoxy­genated blood is rushed into the right auricle through sinuauricular aperture. So both the auricles now become filled up with blood, the left one is filled with oxygenated blood and the right auricle is filled with deoxygenated blood. After the auricles being filled up with blood the auricular systole starts.

Two auricles contract simultaneously and drive the contents into the ventricle through auriculoventricular aperture. Consequently two types of blood enter into the cavity of the ventricle where admixture of two types of blood takes place.

Due to the spongy nature of the ventricular cavity, a major quan­tity of the deoxygenated blood is kept in the right side, the left side contains mostly the oxy­genated blood while the middle part contains mixed type of blood.

Simons (1959) has shown that little mixing takes place in between two types of blood when they pass from the atria to the ventricle. The experiments of Foxon (1955) by injecting X-ray opaque materials have shown that no separation of the oxygenated blood actually occurs in the ventricular cavity. As the skin sub-serves respiratory function, the right auricle also receives oxygenated blood.

The spongy ventricular walls presumable help in metabolic exchanges and have nothing to do in the separation of the two types of blood. Next the ventricle starts contraction, the back flow of blood into the auricles is prevented by the auriculoventricular valves. The blood from the ventricular cavity finds its way through the conus.

As the conus arises from right side, a large quantity of deoxygenated blood from the ventricle enters first. This blood is now con­veyed through the cavum pulmocutaneum by the spiral valve. From the cavum pulmocuta­neum blood goes to pulmocutaneous arteries for oxygenation.

The deoxygenated blood is aerated in lungs and brought back to left auri­cle through common pulmonary vein and thus completes the pulmonary circuit.

With the enhancement of contraction force of the ventri­cle, the mixed blood from the middle region of the ventricle is pushed into the systemic arches through the cavum aorticum.

Lastly, the pres­sure exerted by the carotid labyrinth is over­come and mostly the oxygenated blood from the left side of the ventricle is forcefully pumped into the carotid arches. It is to be noted that the spiral valve directs the entry of blood into different arches.

Blood vessels:

The arteries and veins are the blood ves­sels through which the blood circulates. These vessels are different in structure and function. The arteries take the blood away from the heart to different parts of the body and the veins return the same towards the heart.

The arteries supply blood to the different parts and break up into finer branches, the arterioles, which finally end into a network of capillaries. These capillaries again reunite to form small venules. The venules unite to form the veins. Histologically, a typical blood vessel is com­posed of three distinct layers.

The outermost covering is made up of fibrous connective tissue known as tunica adventitia. The middle layer is composed largely of involuntary muscles, called tunica media. The innermost layer is made up of a endothelium and elastic fibres and is called tunica interna. (Fig. 7.14) the differences between the arteries, veins and capillaries are given in Table 26.

Type # 5. Arterial System:

The arteries and their branches constitute the arterial system (Fig. 7.15).

The truncus arteriosus divides into two main branches, each of which again splits up into three arches:

(a) The anterior carotid arch,

(b) The middle systemic arch and

(c) The posterior most pulmocutaneous arch.

Embryonic arrangement of the arterial arches:

In the embryonic state, six pairs of arterial arches are present. Of these paired arches, the first and second pairs disappear in an adult. The third pair are converted into the carotid arches. The fourth pair are transformed into the systemic arches. The fifth pair disappear and the sixth pair persist as the pulmocuta­neous arches in adult.

Carotid arches:

There are two carotid arch­es in toad. Each carotid arch proceeds forward and outward. It soon divides into an outer branch, called internal carotid artery supplying blood to the brain, the meninges and into an inner branch, called external carotid artery which supplies blood to the outer side of the head, thoracic musculature, tongue and the buccal cavity.

Just at the point of bifurcation and towards the base of internal carotid artery, there is a small swelling, known as carotid labyrinth (or carotid gland). This gland is derived from the remnants of gill and connec­ting blood vessel between the first afferent and efferent bronchial arteries. The inner cavity of the carotid labyrinth contains a network of small vessels and forms a spongy structure.

Though this structure is a receptor, it is physio­logically connected with the regulation of the blood pressure. It controls blood pressure in the internal carotid artery. The internal carotid artery, after its origin, takes a backward course and comes very close to the systemic arch and is being tied with it by fibrous carotid ligament.

Systemic arches:

There are two systemic arches in toad. Each systemic arch takes the median position and sweeps outward to sur­round the oesophagus. It then comes to the dorsal side and joins with its fellow of the opposite end to form the dorsal aorta.

The following branches arise from each systemic arch:

(a) A laryngeal artery which is very short and supplies the laryngotracheal cham­ber.

(b) An occipito vertebral artery which gives branches to the pharynx, back of the head, vertebral column and spinal cord.

(c) A stout subclavian artery supplies blood to the shoulder and forelimb. All these branches exhibit bilateral symmetry but the left sys­temic arch gives off an additional branch, called oesophageal artery, which supplies blood to the oesophagus. This branch is not present on the right side. The dorsal aorta occupies the mid-dorsal position and is situat­ed ventral to the vertebral column and ends posteriorly into two iliac arteries.

The dorsal aorta gives off the following branches antero­posteriorly:

(i) A stout coeliacomesenteric artery which emerges out just from the origin of the dorsal aorta. The artery immediately- breaks up into a coeliac branch to supply blood to the stomach, liver, gallbladder and pancreas and a mesenteric artery supplying blood to the mesenteries, intestine, cloaca and spleen.

(ii) Four or five pairs of renal arter­ies supply the kidneys. From the anterior renal arteries, additional branches arise for the gonads. These small branches are called gen­ital arteries. The renal arteries and genital arteries are collectively known as urinogenital arteries.

(iii) The iliac arteries are the last branches of the dorsal aorta. From each iliac artery an epigastricovesicalis artery is given off to supply the urinary bladder and the ven­tral body wall. The iliac artery then enters into the hind limb and divides into femoral and sciatic arteries.

Pulmocutaneous aches:

These are the short­est and the hindermost arches which carry deoxygenated blood to the lungs and skin. Each main arch enters into lung as pulmonary artery and a very slender branch from it goes to the skin as cutaneous artery.

Type # 6. Venous System:

The veins and their branches constitute the venous system (Fig. 7.16).

The venous system of toad can be described under 3 headings:

(a) Systemic,

(b) Portal and

(c) Pulmonary.

Systemic veins:

Three large veins or venae cavae which open into the sinus venosus rep­resent the systemic veins. The systemic veins carry deoxygenated blood from almost all parts of the body excepting lungs. The anteri­or two venae cavae are known as left and right Percivals and the single posterior one is called postcaval. Each precaval vein is formed by the union of three branches.

These are:

(i) External jugular vein,

(ii) Innominate vein and

(iii) Subclavian vein.

The external jugular vein is formed by two veins, a lingual carrying blood from the tongue and a faciomandibular from the snout and jaws. The innominate vein is also formed by the union of two veins, an internal jugular bringing blood from the head and a subscapular from the back of shoulder.

The subclavian vein is similarly formed by two veins, a brachial vein bringing blood from the forelimb and a musculocutaneous vein from the muscles and skin. As the skin acts as an accessory respiratory organ, the musculocuta­neous vein brings oxygenated blood.

The post­caval vein is formed by four or five pairs of renal veins which receive blood from the kid­neys. Blood from reproductive organs is also poured into renal veins by the genital veins. The postcaval vein then ascends to enter into the sinus venosus.

Portal veins:

A portal vein has its origin in capillaries and it ends in capillaries. The blood from the portal vein returns to the heart through an intermediate organ. The hepatic and renal portal systems are two portal sys­tems in toad.

The capillaries from the gut unite to form a hepatic portal vein, which again breaks up into capillaries in liver. The capilla­ries from the posterior part of the body unite to form two renal portal veins which in turn break up into capillaries in the kidneys on their way to the heart.

Hepatic portal system:

This system compri­ses of hepatic portal vein and the anterior abdominal vein or epigastric vein. Hepatic portal vein carries blood from stomach, intes­tine, pancreas and spleen. The main vessel receives the anterior abdominal vein under the liver and enters into the substance of the liver.

The anterior abdominal vein is formed by the union of two pelvic veins in the mid-ventral line. The pelvic vein arises as an offshoot of the femoral vein. On its anterior, the abdomi­nal vein receives small branches from the uri­nary bladder and the ventral body wall.

Renal portal system:

The blood from the hind limbs is carried by the femoral and sciatic veins. Each femoral vein on entering the body cavity gives off a pelvic vein. The main trunk of the femoral vein receives the sciatic vein above the level of the pelvic vein to form the renal portal vein.

The renal portal vein proceeds by the side of the corresponding kidney and enters into it to break up into capillaries. Each renal portal vein receives two or three dorsolumbar veins carrying blood from the body wall.

Type # 7. Lymphatic System:

While circulating through the body, the blood does not come directly into the cells. During the transit of blood through capillaries plasma exudes from the capillary wall into the intercellular spaces as the lymph. It is a colourless fluid with few leucocytes. The lymph from the intercellular spaces is collec­ted by lymphatic vessels which in turn lead into lymph sacs.

Some of the lymph vessels become contractile and are known as lymph hearts which pump back the lymph into the veins. A pair of lymph-hearts opening into femoral vein is present near the urostyle and another pair are situated below the scapulae which open into subscapular veins. Some openings of small dimension in the peri­toneum communicate with the lymphatic vessels or lymphatics.

Mechanism of circulation:

The transport of materials inside the body of Bufo is performed by the circulatory system which includes heart, arteries, capillaries, veins and lymphatic’s together with fluid components—the blood and lymph. The plan of circulation is outlined in this page.

Type # 8. Nervous System:

The nervous system of toad includes:

(a) The central nervous system,

(b) The periphe­ral nervous system and

(c) The autonomic nervous system.

The nervous system controls and co-ordinates the various activities of the body.

Central nervous system:

The central nervous system includes the brain and the spinal cord. It is essentially a hollow tube whose anterior portion becomes the brain and the posterior part narrows down to form the spinal cord.

The cavity of the brain and the spinal cord is filled up with the cerebrospinal fluid. The brain and the spinal cord are made up of nerve cells (neurons) and nerve fibres. A collection of the extended processes of the neurons enclosed by a connective tissue sheath constitute the nerves.

Structure of a neuron:

A neuron has a very large cell-body with a number of processes. The nucleus of the cell-body is very conspicuous and the cytoplasm contains numerous deeply stainable granules, called Nissl granules. The processes are usually branched, short and are called dendrites. One of the cell-processes is long and un-branched. It is known as axon which forms the axis cylinder of the nerve fibre.

The solid part of the central nervous system is composed of grey matter and white matter. The aggregation of nerve cells with their nuclei gives a greyish appearance and is recognised as the grey matter, while the collection of nerve fibres giving a white colour is known as the white matter. The relative arrangement of grey matter and white matter differs in different regions.

In brain, the grey matter is situated on the outer side and the white matter lies towards the lumen. But the arrangement is just reverse in the spinal cord, i.e., the grey matter is situat­ed on the inner side and the white matter on the outer side.

The whole of the central nervous system is enclosed by two protective coverings or meninges. The outer one is the Dura matter which is thicker and fibrous in nature. The inner one is thin, highly vascular and called Pia matter.

Brain:

The brain is located inside the cranium. It is primarily differentiated into three parts by the development of two primary constric­tions: the forebrain (or prosencephalon), the midbrain (or mesencephalon) and the hind-brain (or rhombencephalon): The prosen­cephalon constitutes the anteriormost part of the brain and becomes further subdivided into an anterior telencephalon and a posteri­or diencephalon (or thalamencephalon).

The telencephalon gives off a pair of small olfac­tory lobes. These lobes are responsible for the sense of smell. The rest of the telencephalon consists of two elongated ovoid lobes (Fig. 7.17 A), called cerebrum (or cerebral hemi­spheres). The roof of the cerebrum is very thin and the floor is very thick and called cor­pus striatum. Two corpora striata are con­nected together by an anterior commissure.

Above this commissure, there is an ill-devel­oped pallial commissure. The cerebrum is the seat of consciousness, intelligence and it regulates voluntary motion. The surface of the cerebral hemispheres is smooth. The diencephalon is a very depressed part behind the cerebrum. It gives off dorsally a small projection, the epiphysis.

Attached to the epi­physis, there is-a small pineal body of unknown function. In front of the epiphysis there is a vascular membrane projecting into the lateral ventricle through the foramen of Monro. This projection is known as anterior choroid plexus.

The ventral side of the diencephalon bears an X-shaped optic chiasma. The optic chiasma is formed by the optic nerves (Fig. 7.17B). Just a little behind the optic chiasma hangs a projection, the hypophysis (or infundibulum). The pituitary gland is attached at the tip of the infundibu­lum. The lateral sides of the diencephalon become thick to form the optic thalami.

The midbrain remains undivided and grows out dorsally as a pair of hollow ovoid bodies, known as corpora bigemina (or optic lobes). Two longitudinal bands are present on the ventral sides which connect the forebrain with the hindbrain. These are designated as crura cerebri. The rhombencephalon is the posterior most sector of the brain and becomes divi­ded into metencephalon and myelencephalon.

The metencephalon is represented by a thin transverse band-like cerebellum. The floor and the sides of the myelencephalon become very thick to form the medulla oblongata. The cere­bellum coordinates the movement of the body.

The medulla oblongata controls and regulates some of the vital processes, viz., the regula­tion of heart beat, metabolism and respira­tion. The roof of the myelencephalon is formed by a thin vascular membrane, called posterior choroid plexus.

Cavities in brain:

The brain is not a solid structure but contains well-formed internal cavities, called ventricles. The cavities in the cerebral hemispheres constitute the lateral ventricles (or the first and the second ventri­cles). The cavity in the diencephalon consti­tutes the third ventricle and that of medulla oblongata is the fourth ventricle.

The lateral ventricles remain in communication with the third ventricle by a small opening, called foramen of Monro. The third ventricle communicates with the fourth ventricle by a narrow passage (Fig. 7.17C), called aqueduct of Sylvius (or iter). The fourth ventricle leads into the cavity of the spinal cord. The cavity in the optic lobes is known as optocoel and that in the olfactory lobes as rhinocoel.

Spinal cord:

The spinal cord is a hollow tube. It extends posteriorly from the medulla oblonga­ta and remains encased within the neural canal of the vertebral column. It extends even within the urostyle as a slender filament, the filum terminale.

The spinal cord has a mid- dorsal groove, called dorsal fissure and a sim­ilar ventral fissure is present on the mid-ventral line (see Fig. 7.20). The cavity of the spinal cord is known as neurocoel which is also con­tinuous with the ventricles of the brain.

Peripheral nervous system:

The peripheral nervous system includes the cranial and the spinal nerves arising from the cerebrospinal axis. A nerve is a bundle of nerve fibres, but in some cases, an aggregation of nerve cells causes swelling on the nerve fibre. This swelling is called ganglion.

The nerve fibres are of two types:

(a) Afferent or Sensory fibres. These fibres convey the infor­mation from the receptor organs to the central nervous system and

(b) Efferent or Motor fibres. These fibres carry impulses from the central nervous system to the effector organs. When the nerves are exclusively made up of sensory nerve fibres, these are called sensory nerves and those composed only of motor nerve fibres are known as motor nerves. Mixed nerves are also present which are com­posed of sensory as well as motor nerve fibres.

Cranial nerves:

Ten pairs of nerves (and the 0 nerve) originate from the brain (Fig. 7.18, 7.19). These paired nerves are designated as cranial nerves. Table 27 will give an idea about the origin and distribution of the cranial nerves in toad.

Spinal nerves:

There are ten pairs of spinal nerves in toad (Fig. 7.20). All the spinal nerves are of mixed nature. Each spinal nerve has a dorsal sensory root and a ventral motor root. The dor­sal root possesses a ganglion near its origin.

The dorsal and ventral roots unite and form a common trunk which comes out through intervertebral foramen and gives off three branches (Fig 7.20): (1) a large ventral branch which innervates the skin and muscles on the ventral surface of the body, (2) a small dorsal branch which supplies the skin and muscles on the dorsal side of the body and (3) a very small ramus communicants which joins with the nearest sympathetic ganglion.

The first spinal nerve is known as hypoglossal. It sup­plies the muscles of the tongue. The second and third spinal nerves form the brachial plexus. The fourth, fifth and sixth spinal nerves supply the integument and the muscles of the trunk region. The rest of the spinal nerves form the sciatic plexus which gives origin to the large sciatic nerve and supplies the hind limb. The last spinal nerve is insignificant.

Autonomic nervous system:

The autonomic nervous system consists of two sympathetic trunks one on either side of the dorsal aorta. The sympathetic trunks start near the point of formation of iliac arteries and each trunk contains ten small ganglia connec­ting with the corresponding spinal nerve by ramus communicans.

Anteriorly, the sympa­thetic trunk divides to encircle the subclavian artery. It then enters into the cranium, con­nects by a twig with the vagus ganglion and terminates after connecting with the Gasserian ganglion. The sympathetic trunks give out branches to innervate the cardiac muscles, blood vessels and alimentary canal.

These branches may sometimes unite to form ganglionated plexuses, such as the cardiac plexus and solar plexus. The autonomic nervous system regulates the involuntary activities of the body like heart beats and peristalsis of the alimentary canal. It is an autonomic nervous system in the sense that its activities are some­what independent of the central nervous sys­tem.

Type # 9. Endocrine System:

The endocrine system includes many duct­less glands which secrete the hormones direct­ly into the blood stream.

The endocrine glands of toad are as follows:

(a) Pituitary gland:

This gland has two lobes, an anterior lobe and a posterior lobe. The anterior lobe of the pituitary gland secretes a growth-stimulating hormone, a thyroid-stimulating hormone, a gonad-stimulating hormone and many other hormones for the regulation of the activities of the other endocrine glands.

(b) Thyroid glands:

There are pair of small-oval thyroid glands situated on the floor of the buccal cavity one on each side of the hyoid apparatus. The thyroid glands produce a hormone called thyroxin.

(c) Parathyroid glands:

These are oval yellowish glands situated near the thyroid glands.

(d) Thymus glands:

These are small yellowish tympanum.

(e) Adrenal glands:

These glands are represented by narrow orange coloured bands situated on the ventral side of the kidneys. The cortex produces a hor­mone, called cortin and the medulla secretes the adrenalin.

(f) Pancreas:

This gland contains two types of glandular structures. The exocrine part secretes pancreatic enzymes while the endocrine part secretes hormones.

(g) Gonads:

Besides the production of germ cells, the gonads produce sex hormones.

(h) Bidder’s organ:

The biological nature of Bidder’s organ is uncertain. Many workers claim the endocrine nature of Bidder’s organ.

 Type # 10. Excretory System:

During metabolism various nitrogenous waste products are formed in the body. These waste products should be eliminated. Because these waste products are not only useless but detrimental to the body if retained inside. The process concerning the extraction of nitroge­nous waste products and their removal from the body is called excretion.

Nitrogenous waste products such as urea, uric acid and excess of water with some salts are excreted as urine.

The organs responsible for this function are:

(a) A pair of kidneys,

(b) A pair of ureters,

(c) An urinary bladder,

(d) The cloaca and

(e) The vent.

Kidneys:

The kidneys are two in number. Each kidney has a compact elongated struc­ture and is attached to the dorsal side of the body wall. These are retroperitoneal in posi­tion, i.e., not enclosed within the peritoneum. The kidneys are of mesonephric type. The edges of the kidneys are marked by notches (Fig. 7.22).

Each kidney is composed innume­rable microscopic uriniferous tubules (or nephrons). Each uriniferous tubule has a proxi­mal two walled cup-like structure, known as Bowman’s capsule. A small afferent vessel from the renal artery enters into the capsule and breaks up into a network of capillaries to form the glomerulus. From the glomerulus an efferent vessel emerges out and pours the blood into the renal vein.

The Bowman’s cap­sule together with the glomerulus constitutes the Malpighian body (or renal corpuscle). The other end of the Bowman’s capsule communi­cated with the tubule is supplied by capillaries from renal portal vein. These tubules open into the collecting tubules which in turn open into the ureter.

Numerous minute openings, called nephrostomes are present on the ventral side of the kidneys. The ureters unite posteriorly to form a common duct which opens into the cloaca.

Opening into the ventral side of the cloaca there is a bilobed urinary bladder. The bilobed urinary bladder is formed by the fusion of two urinary bladders. Other excretory products are carbon- dioxide and water formed as by-product of internal respiration. They are eliminated by lungs and skin.

Physiology of urine formation:

The blood, containing excretory products is brought to the kidney by renal artery and the renal portal vein. On its way through glomerulus, water containing nitrogenous waste products and other substances is filtered out into the capsu­lar space.

The filtrate then passes through the convoluted tubule where the useful sub­stances are reabsorbed and go back into the circulating blood. The fluid from the tubules is collected by the collecting tubules and is drained into ureters as urine.

Urine then drips into the cloaca and is stored temporarily in the urinary bladder. When fully filled with urine, the urinary bladder contracts to eject the urine through vent. With all probabilities, the nephrostomes drain away excess of water and wastes from the body cavity.

Type # 11. Reproductive System:

The sexes are separate. Sexual dimor­phism is present in toad. During breeding sea­son, a cushion-like thumb pad appears at the base of each innermost finger of the hand of male. Male is provided with a vocal sac open­ing into the buccal cavity.

The impression of the vocal sac can be seen from outside. During breeding season abdomen of females become distended to accommodate the increasing number of eggs. In both the sexes, several yellowish outgrowths, called fat bodies, are present anterior to the kidneys. The fat bodies are the storehouse of fatty substances which are utilised during hibernation and also during germ cell formation.

Female reproductive system:

The female reproductive system consists of two ovaries and two oviducts. Each ovary is an irregular much folded sac and is attached with the ven­tral side of the kidney by a thin fold of mem­brane, called mesovarium. During breeding season, the ovaries become greatly enlarged.

The female gonoduct (or oviduct) is a very long coiled tube and is placed on the lateral side of the body. Each oviduct is differentiated into three parts (Fig. 7.22A). Anteriorly, the oviduct opens directly into coelom by a funnel like opening, called oviducal funnel. So oviduct has got no direct connection with the ovary. The median portion is extensively coiled and glandular.

The posterior part becomes dilated to act a thin-walled uterus (or ovisac). The two uteri unite to form a common median tube which in turn opens to the dorsal side of the cloacal chamber. The eggs, after maturation, are discharged into the oviduct through the opening of the oviducal funnel.

Male reproductive system:

The male repro­ductive system comprises of two elongated white bodies, called testes (Fig. 7.23A). Each testis is attached with the ventral side of the kid­ney by a thin fold of membrane, called mesorchium.

Each testis is made up of a collection of fine seminiferous tubules which manufacture spermatozoa or male germ cells. The seminife­rous tubules are connected with the collecting tubules of kidney by fine ducts, called vasa efferentia.

The vasa efferentia open into the ureter, which in the male, is known as the urinogenital duct. Through the urinogenital duct, both the male gametes and urine are poured into the cloaca (Fig. 7.23B). Just in front of the testis and attached to the anterior end of each kidney, there is a small rounded body, called Bidder’s organ.

It is regarded as an undeveloped ovary and may be functional if the testes are removed. This peculiar structure is also present in the female in an ill-developed condition. This organ is regarded by some as an ovotestis.