Plus One Zoology Notes Chapter 3 Structural Organisation in Animals is part of Plus One Zoology Notes. Here we have given Kerala Plus One Zoology Notes Chapter 3 Structural Organisation in Animals.
|Text Book||NCERT Based|
|Chapter Name||Structural Organisation in Animals|
|Category||Kerala Plus One|
Kerala Plus One Zoology Notes Chapter 3 Structural Organisation in Animals
All organisms, are made up of cells The cell is the basic structural and functional unit of life. In unicellular organism all, vital functions are performed by a single; cell. But in multicellular animals, a group of similar cells along with intercellular substances perform a specific function. Such an organization is called tissue.
- Tissues are a group of cells having the same structure, function, and origin.
- Structure of the cells vary according to their function and are broadly classified into four f types.
i. Epithelial tissue
ii. Connective tissue:
iii. Muscular tissue
iv. Neural tissue
- It covers the external surface of the body and the internal free surfaces of many organisms,
- The cells are compactly packed with little intercellular spaces.
- Epithelial tissues are of two types,
A. Simple epithelium
- It consists of a single layer of cells.
- It functions as a lining for body cavities, ducts, and tubes.
- Based on the structural modification of cells, it is divided into three types.
a. Squamous epithelium
b. Cuboidal epithelium
c. Columnar epithelium
a. Squamous epithelium
- It is made up of a single thin layer of flattened cells with irregular boundaries.
- Found in the walls of blood vessels and air sacs of lungs.
- They form diffusion boundaries
b. Cuboidal epithelium
- These are composed of a single layer of cube-like cells.
- These tissues are found in ducts of glands and tubular parts of nephrons in the kidney.
- Main functions are secretion and absorption.
- The epithelium of proximal convoluted tubule of the nephron in the kidney has microvilli.
c. Columnar epithelium
- These tissues are composed of a single layer of tall and slender cells.
- Their nuclei are located at the base.
- A free surface may have microvilli.
- They are found in the lining of the stomach and intestine, and cells help in secretion and absorption.
- If the columnar or cuboidal cells bear cilia on their free surface they are called ciliated epithelium.
- Present in the inner surface of hollow organs like bronchioles and fallopian tubes.
- Function: To move particles or mucus in a specific direction over the epithelium.
- Columnar or cuboidal cells are specialized for secretion to form glandular epithelium.
- These are found in the intestine, pancreas and submandibular salivary glands.
- They are of two types:
- Consisting of isolated glandular cells (Goblet cells of the alimentary canal).
- Consisting of a cluster of secretory cells (salivary gland).
- On the basis of their secretion, glands are divided into two categories:
I. Exocrine glands
II. Endocrine glands
|Exocrine glands||Endocrine glands|
|They secrete mucus, saliva, ear wax, oil, milk, digestive enzymes, and other cell products.||Their products called hormones are secreted directly into the fluid bathing the gland.|
|These are secreted through ducts or tubes.||Do not have ducts.|
B. Compound epithelium
- It is made up of more than one layer of cells (Multi-layered).
- Thus it has a limited role in secretion and absorption.
- Gives protection against chemical and mechanical stress.
- Compound epithelium covers the dry surface of the skin, the moist surface of a buccal cavity, pharynx, inner lining of ducts of salivary glands and of pancreatic ducts.
- All cells in the epithelium are held together with little intercellular material.
- Three types of cell junctions are found in the epithelium and other tissues. They are the following,
i. Tight junctions – They help to stop substances from leaking across a tissue.
ii. Adhering junctions – They perform cementing to keep neighboring cells together.
iii. Gap junctions – They facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, transfer of ions and small and sometimes big molecules.
- These tissues are widely distributed in the body of complex animals.
- They are named connective tissues because of their special function of linking and supporting other tissues or organs of the body.
- In all connective tissues except blood, the cells secrete fibers of structural proteins called collagen or elastin.
- These fibers provide strength, elasticity, and flexibility to the tissue.
- It includes cartilage, bone, adipose, and blood.
- Connective tissues are classified into three types:
a. Loose connective tissue
b. Dense connective tissue
c. Specialized connective tissue.
a. Loose connective tissue
It has cells and fibers loosely arranged in a semi-fluid ground substance. This tissue is of two types, :
i. Areolar tissue
ii. Adipose tissue
i. Areolar tissue
- It composed of cells, fibers, and matrix.
- Present beneath the skin.
- The cells contain fibroblasts, macrophages, and mast cells.
- The fibroblast is cells that produce fibers.
- Produce two types of proteins collagen and elastin.
ii) Adipose tissue
- Itisalooseconnectivetissuefound below the skin. The cells of this tissue are specialized to store fats.
- The excess nutrients are converted into fats and stored in adipose tissue.
b. Dense connective tissue
- The matrix is dense.
- The fibers and fibroblasts are arranged densely in the matrix.
- An orientation of fibers show a regular or irregular pattern and are called dense regular and dense irregular tissues.
- In Dense regular tissues, the collagen fibers are seen in rows between many parallel bundles of fibers, eg., Tendon, and ligaments
1. Tendon – Tissue that helps to join a muscle to a bone.
2. Ligaments – It helps to attach one bone to another.
- Dense irregular tissue is composed of fibroblasts, collagen fibers, and matrix.
- Fibroblasts and collagen fibers are oriented differently.
- This tissue is present in the skin.
c. Specialized connective tissue
- It is a connective tissue specialized to perform different functions.
- These are of three different types: Cartilage, bone, and blood.
- It is a solid, pliable, semi-rigid and flexible and resists compression. In the matrix, there are fluid-filled spaces called lacunae.
- The cells of this tissue called chondrocytes are enclosed in small cavities within the matrix secreted by them.
- Most of the cartilages in vertebrate embryos are replaced by bones in an adult.
- Cartilage is present in the tip of a nose, outer ear joints, between adjacent bones in the vertebral column, limbs, and hands in adults.
- It is a strong rigid and non-flexible tissue.
- It forms the framework of the body.
- Ground substance rich in calcium phosphate.
- In the matrix bone cells called osteocytes
Functions of bone
- It supports and protects softer tissues and organs. Limb bone provides a bearing function.
- They interact with muscle and helps elements.
- Bone marrow in some bones is the site of production of blood cells.
- Provides structural to the body.
- It is a fluid connective tissue.
- It consists of plasma, red blood cells (RBC), white blood cells (WBC) and platelets.
- Helps in the transport of various substances
- Each muscle is made of elongated cells, called muscle fibers.
- These fibers are composed of numerous 1 fine fibrils called myofibrils.
- Muscle fibers contract in response to stimulation, then relax and return to their uncontracted state in a coordinated fashion.
- Muscle helps in the movement of the body.
- On the basis of their location, structure, and function, there are three types of muscle fibers skeletal, smooth and cardiac muscles.
i. Skeletal muscle
- The tissues are attached to skeletal bones. Eg., Biceps.
- It bears striations in the form of alternate light and dark bands.
- The cells are long and cylindrical.
- Muscle fibers are bundled together in a parallel fashion.
- A sheath of tough connective tissue encloses several bundles of muscle fibers.
- The muscle has the ability to contract rapidly and is responsible for quick movements.
- Voluntary in nature.
ii. Smooth muscle
- The cells are long spindle-shaped, unbranched and uni-nucleated.
- Striations are absent.
- Cell junctions hold them together and they are bundled together In a connective tissue sheath!
- They are involuntary, a movement is not controlled by the mind.
- Found within the walls of all the tubular organs such as stomach, intestine, ureter, bronchi etc.
iii. Cardiac muscle
- The cells are cylindrical and branched.
- They are involuntary muscle, found in the heart.
- Cell junctions fuse the plasma membranes of cardiac muscle cells and make them stick together.
- It has the function of rhythmic contraction and relaxation throughout life.
- Nervous tissue is specialized to transmit messages in our body.
- Neurons are the structural and functional units of the nervous system.
- The tissue exerts the greatest control over the body’s responsiveness to changing conditions.
- It is concerned with receiving and transmit ting information and messages in the form of nerve impulses.
- Neuroglia makes up more than one half the volume of neural tissue in our body.
- Neuroglia ceils are non- nervous cells which protect and support neurons.
Organ and organ system
- In all multicellular organisms, the cells are organized to form tissues.
- Tissues are organized to form organs, eg., Heart
- The organs associate to form organ systems, eg., Circulatory system,
♦ Morphology and Anatomy of animals
- Morphology refers to the study of form or exexternallyisible features.
- Anatomy refers to the study of the morphology of internal organs in the animals.
Reddish brown terrestrial invertebrate that inhabits the upper layer of moist soil. During the daytime, they live in burrows made by boring and swallowing the soil. Their fecal deposits are known as worm castings. The common Indian earthworms are Pheretima and Lumbricus.
Morphology of Earthworm
- Body is long and cylindrical.
- Body is linearly divided into 100 to 120 segments or metamers.
- The dorsal surface of the body is marked by a dark median mid-dorsal line.
- The ventral surface is distinguished by genital openings (pores).
- The anterior end of the body consists of the mouth and prostomium.
- Prostomium – fleshy lobe, covering the mouth. It is sensory in function.
- The first body segment is called peristomium (buccal segment) which bears the mouth.
- In mature worms 14-16 prominent dark band of glandular tissues called clitellum is present.
- Their body is divided into three regions. They are:
iii. Postclitellar segments.
- On the ventrolateral side of the intersex- mental grooves, four pairs of spermathecal apertures are situated (5th-9th segments).
- Female genital pore is present in the mid-ventral line of 14th segment.
- A pair of male genital pores on the ventrolateral sides of the 18th segment.
- Numerous minute pores called nephridiopores open on the surface of the body.
- In each body segment, except first, last and clitellum, there are rows of S-shaped setae embedded in the epidermal pits in the middle of each segment.
- Setae are used for locomotion.
- The body wall of an earthworm is covered by a thin non-cellular cuticle.
- Below the cuticle is the epidermis, two muscle layers, and an innermost coelomic epithelium.
- A single layer of columnar epithelial cells, which contain secretory gland cells is present in the epidermis.
- The alimentary canal is a straight tube and runs between the first and last segments of the body.
- A terminal mouth opens into the buccal cavity (1-3 segments).
- They lead into the muscular pharynx.
- A small narrow tube, the esophagus (5-7 segments) leads into a muscular gizzard (8-9 segments).
Functions of gizzard
Grinding the soil particles, decaying leaves.
- Stomach extends from 9-14 segments.
- Calciferous glands present in the stomach, neutralize the humic acid present in the hummus.
(The food of earthworm is decaying leaves and organic matter mixed with soil.)
- Intestine starts from the 15th segment onwards and continues until the last segment.
- On the 26th segment a pair of short conical intestinal caecae project out.
- Between the segment 26-35, there is the internal median fold of the dorsal wall. This is called a typhlosole.
- Typhlosole increases the effective area of absorption in the intestine.
- The alimentary canal opens to the exterior by a small rounded aperture called anus.
- Pheretima exhibit a closed type of blood vascular system.
- It consists of Blood, blood vessels, capillaries and heart.
- Due to a closed circulatory system, the blood is confined to the heart and blood vessels.
- Smaller blood vessels supply blood to the gut, nerve cord and the body wall.
- Blood glands are present on the 4th, 5th, 6th segments.
- They produce blood cells and hemoglobin which is dissolved in blood plasma.
- Blood cells are phagocytic in nature.
- Earthworm lacks specialized breathing devices.
- A respiratory exchange occurs through moist, body surface into their bloodstream.
- Excretory organs are segmentally arranged coiled tubules called nephridia.
They are of three types
i. Septal nephridia
- Present on either side of the intersegmental septa
- It opens into the intestine.
- They are present from the 15th segment to the last.
ii. Integumentary nephridia
- Attached to the lining of the body wail and opening individually to the outside. They are present in all segments. 3rd to the last segment.
iii. Pharyngeal nephridia
- Present as three paired tuft in 4lh, 5th and 6th segments.
- Nephridia regulate the volume and composition of body fluids.
- In the nervous system, ganglia are arranged segment wise on the ventral paired nerve cord.
- The nerve cord in the anterior region (3rd and 4thsegments) form a nerve ring.
- The cerebral ganglia along with other nerves in the ring integrate sensory input as well as command muscular responses of the body.
- A sensory system does not have eyes, but does possess light and tough sensitive organs (receptor cells)
- Using receptor cells, worms distinguish light intensities and feel the vibrations in the ground.
- Chemoreceptors are present, which react to chemical stimuli.
- Sense organs are located on the interior part of the worm.
- Earthworms are hermaphrodites (bisexual) ie. testes and ovaries are present in the same individual.
- Two pairs of testes are present in the 10 th and 11th segment.
- Thevasadeferentiarun up to the 18th segment and joins the prostatic duct.
- Two pairs of accessory glands, one pair each in the 17th and 19th segment.
- The common spermatic and prostate duct through the male genital pore in the 18th segment.
- Four pairs of spermathecae are located in the 6th to the 9th segments.
- They receive and store spermatozoa during copulation.
- One pair of ovaries are attached at the intersegmental septum of 12th and 13th segments.
- Ovarian funnels are present beneath the ovaries.
- Female genital pore on the 14th segment.
- Fertilization and development occur within the cocoons which are deposited in the soil.
- The ova are fertilized by the sperm within the cocoon which then slips off the worm and is deposited in the soil.
- The cocoon holds the worm embryos.
- After about 3 weeks each cocoon produces 2-20 baby worms with an average of four. Development is direct ie. there are no larvae formed.
- Earthworms are known as ’ friends of farmers’.
- They make burrows in the soil and make it porous which help in respiration and penetration of the developing plant roots.
- The process of increasing the fertility of soil by the earthworms is called vermicomposting.
- They are also used as bait in game fishing.
They are brown or black bodied, nocturnal omnivores that live in damp places. They are serious pests and vectors of several diseases. Common species: Periplaneta americana
- The adults are about 34-53 mm long.
- Body is covered by a hard chitinous exoskeleton.
- In each segment, exoskeleton has hardened plates called sclerites.
- The body is segmented and divisible into three regions: i) Head ii) Thorax
- It is triangular in shape. Formed by the fusion of six segments.
- It attached to the thorax by a neck.
- Head bears threadlike antennae, a pair of compounds eyes and biting and chewing type of mouthparts.
- Mouthparts, consists of a labrum (upper lip), 2 mandibles, 2 maxillae, hypopharynx and a labium (lower lip).
- It consists of three parts: prothorax, mesothorax, and metathorax.
- The head is connected with thorax by a short extension of the prothorax known as the neck.
- Each thoracic segment bears a pair of of walking legs.
Two pairs of wings are found in cocockroach
- The first pair of wings or tegmina are opaque dark and leathery and cover the hind wings when at rest.
- The second pair is hind wings.T hey are transparent, membranous and are used in flight.
- It is the largest part of the body.
- It consists of 10 segments.
- In females, 7th sternum is boat shaped and together with 8lhand 9th sterna forms a brood / a genital pouch.
- The anterior part of the genital pouch contains female gonopore, spermathe- cal pores and collateral glands.
- In males the genital pouch lies at the hind end of the abdomen dorsally by 9th and 10th terga and ventrally by the 9,h sternum.
- Males bear a pair of short thead like anal styles which are absent in females.
- In both sexes , the 10th segment bears a pairof jointed filamentousstructures called anal cerci.
The alimentary canal is divided into three regions namely foregut, midgut and hindgut.
- It is lined by cuticle
- it consists of mouth which opens into a short tubular pharynx, leading into oe-sophagus.
- This in turn opens into a sac like structure called crop used for storing the food. The crop is followed by gizzard.
- Gizzard helps in grinding the food particles.
- It is not lined by cuticle. At the junction of the foregut and midgut has rings of 6 – 8 tubules called hepatic or gastric caecae, which secrete digestive juice.
- Malpighian tubules are present at the junctions of midgut and hindgut, helps in the removal of excretory products from haemolymph.
- It is broader than midgut and lined internally by cuticle.
- It includes ileum, colon and rectum.
- Rectum opens out through anus.
- Circulatory system is open type.
- Blood vessels are poorly developed and open into space (haemocoel).
- The visceral organs located in the haemocoel are bathed in blood.
- The haemolymph is composed of colour less plasma and haemocytes.
Haemolymph = Colourless plasma + haemocytes
- Heart consists of elongated muscular tube lying along mid-dorsal line of thorax and abdomen.
- It consists of thirteen funnel shaped cham-bers with ostia on either side.
- Blood from sinuses enter heart through ostia and is pumped anteriorly to sinuses again.
- It consists of network of trachea, spiracles and tracheoles.
- Spiracles are the 10 pairs of small holes present in the lateral side of the body.
- The opening of the spiracles are regulated by sphincters.
- The trachae are branched to form fine tubules called tracheoles.
- Exchange of gases takes place at the tracheoles by diffusion.
- Excretion is performed by malpighian tu-bules.
- Each tubule is lined by glandular and ciliated cells.
- They absorb nitrogenous waste products and converts them to uric acid and excretes out through the hindguts.
- Cockroaches are uricotelic.
- It consists of a series of fused, segmentally arranged ganglia joined by paired longitudinal connectives on the ventral side.
- Three ganglia lie in the thorax, and six in the abdomen.
- The nervous system speads throughout the body.
- In the head region, the brain is represented by supraoesophageal ganglion.
- Sense organs : Antennae, eyes, maxillary palps, labial palps, anal cerci etc. y Compound eyes are situated at the dorsal surface of the head.
- Each eye consists of about 2000 hexagonal ommatidia, using these, a cockroach can receive several images of an object. This is known as mosaic vision.
- It has more sensitivity but less resolution.
- Being common during night, vision in cock roach also called as nocturnal vision.
- Cockroaches are dioecious, which exhibit sexual dimorphism.
- Male reproductive system consists of a pair of testes lying one on each lateral side in the 4th – 6th abdominal segments.
- From each testis arises a thin tube called was deferens.
- It opens into ejaculatory duct through the seminal vesicle.
- The ejaculatory duct opens into male go- nopore situated ventral to anus.
- Accessory reproductive gland called mush room gland is present in 6-7 abdominal segments.
- The external genitalia is represented by male gonapophysis or phallomere (chitinous asymmetrical structures surrounding the male gonopore)
- The sperms are stored in seminal vesicle and are glued together in the form of bundles called spermatophores.
- The sperms are discharged during copulation.
- Female reproductive system consists of two large ovaries lying laterally in the 2nd – 6lh abdominal segments.
- Each ovary is made up of a group of eight ovarian tubules containing a chain of developing ova.
- Oviducts of each ovary unite into a single median oviduct (vagina) which opens into genital chamber.
- A pair of spermatheca is present in the 6th segment, which opens into the genital chamber.
- Sperms are transferred through spermato- phores.
- Fertilised eggs are encased in capsules called oothecae.
- Ootheca is a dark reddish brown capsule.
- Dropped to a crack or crevice of high relative humidity near the food source.
- On an average female produce 9-10 oothecae, each containing 14-16 eggs.
♦ Development is paurometabolous, it is through nymphal stage.
- The nymph look very much like adults.
- The nymph grows by moulting about 13 times to reach adult form.
- The next to last nymphal stage has wing pads. But only the adult has wings.
♦ Difference between male and female cockroach
|Low and narrow abdomen||Abdomen is short and broad.|
|Brood pouch is absent||Presence of broad pouch|
|Anal style is present in the 9th segment||Anal style is absent|
|Dorsally the 7th segment covers only the 8th segment in the abdomen.||Dorsally the 7th segment covers both the 8th and 9th segment in the abdomen|
|Ventrally 9 abdominal segment are visible.||Ventrally first 7 abdominal segments are visible|
- Cockroaches are pests.
- They contaminate and destroy food.
- Transmit bacterial diseases.
- They have no economic importance
Class : Amphibia
- Frog can live both on land and in fresh water.
- They are cold blooded animals (Poikilotherms)
ie., do not have a constant body temperature, varies according to atmospheric temperature.
- They exhibit camouflage (ability to change the colour of the skin to suit it’s surrounding, the protective colouration is mimicry.)
- During peak summer they show aestivation (summer sleep).
- During peak winter they show hibernation (winter sleep).
- Body is smooth and slippery due to the pres ence of mucus.
- Dorsal side of the body is generally olive green colour with dark irregular spots.
- Ventral side is pale yellow.
- The body is divisible into head and trunk.
- Neck and tail are absent.
- Above the mouth a pair of nostrils are present.
- Eyes are bulged and covered by a nictitating membrane that protects them while in water.
- Membranous tympanum(ear) receives sound signals.
- Fore limbs and hind limbs help in swimming, walking, leaping and burrowing. Five digits in hind limbs and four digits in the forelimbs.
- Feet have webbed digits that helps in swimming.
- They exhibit sexual dimorphism.
- Vocal sacs and copulatory pad are the dis-tinguishing characters of a male frog.
♦ Digestive system
- The digestive system consists of the alimentary canal and the digestive glands. The alimentary canal is short because frogs are carnivores hence the length of the intestine is reduced.
- Path of the food in the alimentary canal
- Enzymes produced by pancreas, liver and stomach are involved in digestive process.
- Digestion of food takes place by the action of HCI and gastric juices secreted from the walls of the stomach.
- Partially digested food called chyme is passed from the stomach to the first part of the intestine, the duodenum.
- The duodenum receives bile from the gall bladder and pancreatic juices from the pan-creas through the common bile duct.
- Bile emulsifies fat and pancreatic juices digest carbohydrates and proteins.
- Final digestion takes place in the intestine.
- Digested food is absorbed by the 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.
- Frog respire on land and in water by two different methods.
- In water – Cutaneous respiration (through skin).
- On land – Through buccal cavity, skin and lungs.
- The respiration by lungs is called pulmonary respiration.
- Lungs are a pair of elongated, pink coloured sac like structures present in the upper part of the trunk region (thorax).
- During aestivation and hibernation gaseous exchange takes place through the skin by diffusion.
- It consists of blood vascular system and lymphatic system.
A. Blood vascular system is closed type.
- It consists of heart, blood vessels and blood.
- Heart is the muscular structure situated in the upper part of the body cavity.
- It has three chambers: two atria and one ventricleand is covered by a membrane called pericardium.
- A triangular structure called sinus venosus joins the right atrium. It receives blood through venacava.
- Ventricle opens into a sac like conus arte-riosus, on the ventral side of the heart.
- Blood vessels consists of arteries and veins
- Arteries carries blood from heart to all part of the body.
- Veins collect blood from different parts of the body to the heart.
- Hepatic portal system – Special ve nous connections between the liver and the intestine.
- Renal portal system – The venous system between kidney and the s lower part of the body.
- The blood is composed of plasma and bood cells (RBC, WBC and platelets).
- RBCs are nucleated and contain a red col-oured pigment, haemoglobin.
- The blood carries nutrients, gases and water to the respective sites during circulation.
- Circulation of blood is achieved by the pumb- ing action of the muscular heart.
B. Lymphatic system consists of lymph, lymph channels and lymph nodes.
- Lymph lacks a few proteins and RBCs.
- It consists of a pair of kidneys, ureters, cloaca and urinary bladder.
- Kidneys are compact, dark-red and bean like structures situated a little posteriorly in the body cavity on both sides of the vertebral column.
- Functional and structural units of kidneys are nephrons or uriniferous tubules.
- The ureters act as urinogenital duct which opens into the cloaca.
- Males – two ureters emerge from the kidney.
- Females – ureters and oviducts open sep-arately in the cloaca.
- The thin walled urinary bladder is present which also opens into the cloaca.
- They are ureotelics (Excrete urea).
- Excretory wastes are carried by blood into kidney where it is excreted and separated.
Control and coordination
- It is highly evolved in frogs.
- It includes neural system and endocrine glands.
- Hormones control and coordinate the activities of various organ of the body – Chemical coordination.
Endocrine glands in frog
- Pineal body
- Pancreatic islets
- Nervous system organised into central nervous system, peripheral and autonomic nervous system.
i. Central nervous system (Brain and spinal cord).
ii. Peripheral nervous system (Cranial and spinal nerves)
iii. Autonomic nervous system (Sympathetic and parasympathetic)
- There are ten pairs of cranial nerves arising from the brain.
- Brain is enclosed in a structure called cranium.
- The brain is divided into three regions: fore – brain, mid – brain and hindbrain.
|Regions of brain||Parts|
|Forebrain||Olfactory lobes, cere
brain hemispheres and unpaired diencephalon
|Midbrain||A pair of optic lobes|
|Hindbrain||Cerebellum and me
- The medulla oblongata passes out through the foramen magnum and continues into spinal cord, which is enclosed in the vertebral column.
Sense organs in Frog
- Organs of touch – Sensory papillae
- Organs of taste – Taste buds
- Organs of smell – Nasal epithelium
- Organs of vision – Eyes
- Organs of hearing and balance – Tympanum with internal ears
- Out of these eyes and internal ears are well organised structures and rest are cellular aggregations around the nerve endings.
- External ears are absent, only tympanum can be seen externally.
- They have well organised male and female reproductive systems.
♦ Male reproductive system
- It consists of a pair of yellowish ovoid testes found adhered to the upper part of the 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 opens into Bidder’s canal.
- Finally it communicates with the urinogenitai duct that comes out of the kidneys and opens into the cloaca.
- Cloaca- small median chamberthat is used to pass faecal matter, urine and sperms to the exterior.
♦ Female reproductive system
- They include a pair of ovaries.
- Situated near the kidneys
- A pair of oviduct arising from the ovaries opens into the cloaca separately.
- A mature female can lay 2500 – 3000 ova at a time.
External fertilisation takes place in water.
The larvae is called tadpole. Tadpole un-dergoes metamorphosis to form the adult.
Importance of frog
Frogs are beneficial for mankind
- They eat insects and protect the crop.
- Maintain ecological balance because they serve as an important link of food chain and food web in the ecosystem.
- In some countries the muscular legs of frog are used as food by man.
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