10th Science Unit 13 Structural Organisation of Animals Book Back Answers
10th Science Unit 13 Structural Organisation of Animals book back answers for all 36 questions in the 2026-27 Samacheer Kalvi (Tamil Nadu) textbook.
This unit takes a close look at the body plan of two animals: the Indian cattle leech and the rabbit. You study the leech's shape, segments, suckers, openings, body wall, movement and digestion, plus the rabbit's organ systems and teeth. Book-back questions ask you to name structures, explain functions like the role of hirudin, and compare invertebrate and vertebrate organisation.
Answers at a glance
I. Choose the correct answer
II. Fill in the blanks
III. Identify whether the statements are True or False. Correct the false statement
- Q1False – An anticoagulant present in saliva of leech is called hirudin.
- Q2False – The vas deferens serves to transport the sperms.
- Q3False – Diastema is a gap between the incisors and premolar teeth in rabbit.
- Q4False – The cerebral hemispheres of rabbit are connected by a band of nerve tissue called corpus callosum.
I. Choose the correct answer
Leech has no parapodia or setae; it moves by looping (crawling) and swimming.
Looping movement is brought about by the contraction and relaxation of body muscles, the suckers only giving attachment.
d) Contraction and relaxation of muscles
The body of leech is metamerically segmented into 33 segments.
Such body segments of annelids are called metameres or somites; proglottids and strobila belong to tapeworm.
a) Metameres (somites)
The central nervous system of leech has a nerve ring and a paired ventral nerve cord.
The nerve ring is made of the suprapharyngeal ganglion (brain), circumpharyngeal connective and subpharyngeal ganglion — so pharyngeal ganglia are parts of the nervous system.
b) Nervous system
The nerve ring surrounds the pharynx.
The brain (suprapharyngeal ganglion) lies above the pharynx, while the subpharyngeal ganglion lies below it.
c) Pharynx
Metamerism is the segmentation of the body.
The leech body is metamerically divided into 33 segments, the last seven fusing to form the posterior sucker.
b) 33 segments
Mammals occupy the highest group in the animal kingdom and have a hairy body covering.
They maintain a constant body temperature, so they are warm blooded (not cold blooded or poikilothermic).
b) Warm blooded
II. Fill in the blanks
Leech has two suckers, one at each end of the body.
The posterior sucker is formed by the fusion of the last seven segments, i.e. segments 27 to 33.
last seven (27th – 33rd)
The existence of two sets of teeth in the life of an animal is called ______ dentition.
Rabbit has milk teeth in young ones and permanent teeth in adults.
Having two sets of teeth in a lifetime is called diphyodont dentition.
diphyodont
The sucker at the anterior end is ventral in position and occupies the first five segments.
It is called the anterior sucker or oral sucker and helps in feeding, attachment and locomotion.
anterior sucker (oral sucker)
Leech is an ectoparasite that feeds on the blood of fishes, frogs, cattle and human.
Such a blood sucking habit is termed sanguivorous.
sanguivorous (blood sucking)
Kidneys are dark red, bean shaped organs in the abdominal cavity, each made of several nephrons.
They separate nitrogenous waste from the blood and excrete it as urea.
Kidneys (nephrons)
The peripheral nervous system of rabbit has 12 pairs of cranial nerves and 37 pairs of spinal nerves.
37 pairs of
III. Identify whether the statements are True or False. Correct the false statement
The salivary glands of leech secrete a protein that prevents the coagulation of blood while it sucks blood.
The textbook names this anticoagulant as hirudin, not heparin.
False – An anticoagulant present in saliva of leech is called hirudin.
Vas deferens is a duct of the male reproductive system.
It carries spermatozoa (sperms) from the epididymis/testis, not the ovum.
The ovum is carried by the oviduct (fallopian tube) in the female.
False – The vas deferens serves to transport the sperms.
Canines are absent in rabbit, leaving a toothless space in the jaw.
This gap lies between the incisors and the premolars, and is called diastema.
It helps in mastication and chewing of food in herbivores.
False – Diastema is a gap between the incisors and premolar teeth in rabbit.
The cerebral hemispheres of rabbit are connected by band of nerve tissue called corpora quadrigemina.
The forebrain of rabbit has a pair of cerebral hemispheres.
The right and left cerebral hemispheres are joined by a transverse band of nerve tissue called corpus callosum.
Corpora quadrigemina is not the connecting band mentioned in the text.
False – The cerebral hemispheres of rabbit are connected by a band of nerve tissue called corpus callosum.
IV. Match columns I, II and III correctly
| Organs | Membranous Covering | Location |
|---|---|---|
| Brain | pleura | abdominal cavity |
| Kidney | capsule | mediastinum |
| Heart | meninges | enclosed in thoracic cavity |
| Lungs | pericardium | cranial cavity |
Brain – it is situated in the cranial cavity and is covered by three membranes (duramater, arachnoid, piamater) together called meninges.
Kidney – the dark red, bean-shaped kidneys lie in the abdominal cavity and each is covered by a capsule.
Heart – the pear-shaped heart is enclosed by a double layered membrane called pericardium and lies in the mediastinum, between the two lungs.
Lungs – each lung is enclosed by a double membranous pleura and is enclosed in the thoracic cavity.
Brain – meninges – cranial cavity; Kidney – capsule – abdominal cavity; Heart – pericardium – mediastinum; Lungs – pleura – enclosed in thoracic cavity
V. Answer in a sentence
Hirudinaria granulosa belongs to Phylum Annelida, Class Hirudinea.
It is an ectoparasite that sucks the blood of cattle, fishes, frogs and humans.
The common name of Hirudinaria granulosa is the Indian cattle leech.
Leech has no special respiratory organs; respiration takes place through the skin.
A dense network of tiny blood vessels (capillaries) containing haemocoelic fluid extends between the cells of the epidermis.
Oxygen dissolved in water diffuses through the skin into the haemocoelic fluid and carbon dioxide diffuses out.
The skin is kept moist and slimy by mucus secretion, which also prevents drying.
Leech respires through its moist skin by simple diffusion of gases into and out of the haemocoelic fluid.
The number of each kind of tooth on one side of the upper and lower jaws is counted.
In rabbit: Incisors , Canines , Premolars , Molars .
Canines are absent, leaving a gap called diastema.
Dental formula of rabbit = , written as .
Leech is hermaphrodite, having both male and female organs in the same animal.
The testes are spherical sacs called testes sacs, one pair in each segment from the 12th to the 22nd segment.
Eleven pairs of testes are present in leech (segments 12 to 22).
Rabbit is a herbivore and canine teeth are completely absent in its jaws.
Because of the absence of canines, a gap is left between the incisors and the premolars.
This gap is called the diastema, and it helps in mastication and chewing of food.
Diastema is the gap formed between the incisors and premolars due to the absence of canine teeth in rabbit.
The trachea (wind pipe) divides into two branches called bronchi.
One bronchus enters each lung and divides further into bronchioles which end in alveoli.
The pair of lungs is attached to the two bronchi, one bronchus entering each lung.
The buccal cavity of leech leads into a muscular pharynx.
The blood of the host is sucked by the pumping action of this muscular pharynx.
The muscular pharynx acts as the suction pump in leech.
In rabbit the nervous system has CNS, PNS and ANS.
CNS consists of the brain and the spinal cord.
CNS stands for Central Nervous System (brain and spinal cord).
Rabbit's teeth are not all alike; they are of different types.
Four kinds of teeth occur — incisors, canines, premolars and molars (canines absent in rabbit).
Such dentition with different kinds of teeth is called heterodont.
Because rabbit possesses different kinds of teeth (incisors, premolars and molars), its dentition is called heterodont.
The leech attaches itself firmly to the host by its posterior sucker.
It makes a triradiate or Y-shaped incision on the skin of the host with the three jaws protruded through the mouth.
The muscular pharynx sucks the blood and salivary secretion is poured into the wound.
The saliva contains hirudin, which prevents clotting of blood, and an anaesthetic substance so the host does not feel the bite.
The sucked blood is stored in the crop chambers and their diverticula.
Leech attaches by its suckers, cuts a Y-shaped wound with its three jaws and sucks blood by its muscular pharynx, hirudin in the saliva preventing clotting.
VI. Short answer questions
The trachea (wind pipe) carries air from the larynx to the lungs, and its walls are supported by rings of cartilage.
These cartilage rings keep the trachea always open and prevent it from collapsing during breathing.
So they help in the free passage of air to and from the lungs during inspiration and expiration.
Air thus reaches the bronchi, bronchioles and alveoli without any block.
Cartilage rings support the tracheal wall, keep it from collapsing and allow free passage of air to the lungs.
Show steps (6)
Blood sucking: blood is sucked by the muscular pharynx.
Suckers: anterior and posterior ends bear suckers by which the leech attaches itself firmly to the body of the host.
Jaws: three jaws inside the mouth make a painless Y-shaped (triradiate) wound in the skin of the host.
Hirudin: salivary glands secrete hirudin, which prevents clotting of blood, so a continuous supply of blood is maintained.
Parapodia and setae are completely absent.
Storage: blood stored in the crop and its diverticula nourishes the leech for several months, so elaborate digestive juices and enzymes are not needed.
Suckers for attachment, jaws for a painless Y-shaped wound, muscular pharynx for sucking, hirudin to stop clotting, loss of parapodia and setae, and blood storage in the crop are the parasitic adaptations of leech.
VII. Long answer questions
How is the circulatory system designed in leech to compensate the heart structure ?
Show steps (8)
No true heart or blood vessels: In leech there is no true heart and no true blood vessels. Circulation is carried out by a haemocoelic system.
Haemocoelic channels: The blood vessels are replaced by channels called haemocoelic channels or canals, which are filled with a blood-like fluid.
Coelomic fluid acts as blood: The coelomic fluid present in these channels contains haemoglobin, so it can carry respiratory gases like blood.
Four longitudinal channels: There are four longitudinal channels running along the length of the body.
Dorsal and ventral channels: One channel lies above (dorsal to) the alimentary canal and one lies below (ventral to) the alimentary canal.
Lateral channels act as heart: The other two channels lie on either (lateral) side of the alimentary canal. These two lateral channels serve the function of the heart and possess inner valves, which keep the flow of fluid in one direction.
Connection of channels: All the four channels are connected together posteriorly in the 26th segment, so the fluid circulates continuously through the body.
Link with respiration: A dense network of tiny capillaries containing haemocoelic fluid extends between the cells of the epidermis, so the circulating fluid picks up oxygen diffusing through the skin and gives out carbon dioxide.
Leech has no true heart or blood vessels; circulation is by a haemocoelic system of four longitudinal channels filled with haemoglobin-containing coelomic fluid, of which the two lateral channels bear inner valves and function as the heart, all four joining in the 26th segment.
Show steps (6)
Two methods: Locomotion in leech takes place in two ways — (i) looping or crawling movement and (ii) swimming movement.
Role of muscles: The body wall has a muscular layer formed of circular and longitudinal muscles. Contraction and relaxation of these muscles bring about movement.
Role of suckers: Leech has an anterior (oral) sucker and a posterior sucker. Both suckers serve for attachment to the substratum during movement.
Looping or crawling movement: This movement is brought about by the contraction and relaxation of muscles. The two suckers fix and release alternately on the substratum, so the body is drawn forward in a series of loops.
Swimming movement: In water, leeches swim very actively by performing undulating (wave-like) movements of the flattened body.
No locomotory appendages: Parapodia and setae are completely absent in leech, so locomotion depends entirely on the body muscles and the two suckers.
Leech moves by looping or crawling on a substratum, using contraction and relaxation of body muscles with alternate attachment of its two suckers, and by undulating swimming movements in water.
Show steps (9)
Testes: The male reproductive system of rabbit consists of a pair of ovoid testes. They are enclosed by scrotal sacs in the abdominal cavity.
Seminiferous tubules: Each testis consists of numerous fine tubules called seminiferous tubules, in which sperms are produced.
Epididymis: The network of seminiferous tubules leads into a coiled tubule called the epididymis, which stores the sperms.
Vas deferens: The epididymis leads into the sperm duct called vas deferens.
Urethra: The vas deferens of both sides join the urethra just below the urinary bladder. The urethra runs backward and passes into the penis.
Penis: The penis is present on the ventral side of the anus and serves as the copulatory organ.
Accessory glands: There are three accessory glands — the prostate gland, Cowper's gland and perineal gland. Their secretions are involved in reproduction.
Sexual dimorphism: Rabbit exhibits sexual dimorphism, so the male and female sexes are separate and morphologically different.
Labelled diagram: Draw the outline as in Figure 13.11 and label — testis, scrotal sac, epididymis, vas deferens, urinary bladder, ureter, kidney, urethra, prostate gland, Cowper's gland and perineal gland.
The male reproductive system of rabbit consists of a pair of testes in scrotal sacs containing seminiferous tubules, which open through the epididymis and vas deferens into the urethra leading to the penis, aided by prostate, Cowper's and perineal glands.
VIII. Higher Order Thinking Skills (HOTS)
Arjun is studying in tenth standard. He was down with fever and went to meet the doctor. As he went to the clinic he saw a patient undergoing treatment for severe leech bite. Being curious, Arjun asked the doctor why leech bite was not felt as soon as it attaches to the skin ? What would have been the reply given by the doctor?
Show steps (9)
The animal involved: The Indian cattle leech (Hirudinaria granulosa) is an ectoparasite that is sanguivorous, i.e. it sucks the blood of fishes, frogs, cattle and human beings.
How it attaches: It has two suckers — the anterior (oral) sucker occupying the first five segments and the posterior sucker formed by fusion of the last seven segments. It first attaches itself firmly to the victim by the posterior sucker.
How the wound is made: The mouth lies in the middle of the anterior sucker and the buccal cavity bears three jaws with a single row of minute teeth. These jaws protrude and make a triradiate or Y-shaped incision in the skin of the host.
The reason the bite is not felt (main answer): Along with saliva, the leech injects an anaesthetic substance into the wound. This anaesthetic numbs the site, so the host does not feel the bite when the leech attaches and begins to feed.
Painless wound: Because of this anaesthetic, the three jaws cause only a painless Y-shaped wound — the person comes to know of the leech only later, when he sees it or when bleeding continues.
Why bleeding continues: The salivary glands also secrete a protein called hirudin, which prevents coagulation (clotting) of the blood. So a continuous supply of blood is maintained and the wound keeps oozing even after the leech drops off.
Where the blood goes: The blood is sucked by the muscular pharynx and stored in the ten chambers of the crop and its diverticula, which can hold a large quantity of blood that is digested slowly.
Parasitic adaptation: Thus the anaesthetic secretion and hirudin are important parasitic adaptations of the leech — they let it feed undisturbed for a long time without the host reacting.
Doctor's advice: Hence the bite is painless at first; the wound must be cleaned and pressed to stop the prolonged bleeding caused by hirudin.
The doctor would reply that while biting, the leech injects an anaesthetic substance with its saliva which numbs the skin, so the Y-shaped wound made by its three jaws is painless; its saliva also contains hirudin, an anticoagulant that keeps the blood flowing for continuous feeding.
Shylesh has some pet animals at his home. He has few rabbits too, one day while feeding them he observed something different with the teeth. He asked his grandfather, why is it so? What would have been the explanation of his grandfather?
Show steps (8)
What Shylesh observed: The rabbit's teeth are not all alike — there are large chisel-like teeth in front, grinding teeth at the back, and a wide toothless gap in between.
Two sets of teeth (diphyodont): The grandfather would explain that a rabbit has two sets of teeth in its life — milk teeth in the young ones and permanent teeth in the adult. This condition is called diphyodont dentition.
Different kinds of teeth (heterodont): The teeth are of different types, so the dentition is called heterodont. In mammals there are four kinds of teeth — incisors (I), canines (C), premolars (PM) and molars (M).
Function of teeth: Teeth are hard, bone-like structures used to cut, tear and grind the food materials.
Canines are absent: In rabbit the canine teeth are completely absent, because it is a herbivore feeding on grass and vegetables like turnips, carrots and lettuce; it does not need canines to tear flesh.
Diastema: The gap between the incisors and the premolars, left by the missing canines, is called the diastema. It helps in mastication and chewing of food in herbivorous animals — the food can be turned over and ground properly.
Dental formula: The teeth of a mammal are represented by a dental formula, counting each kind of tooth in the upper and lower jaw of one side. In rabbit it is , written as .
Conclusion: So the difference Shylesh noticed is due to the rabbit's heterodont, diphyodont dentition with no canines and a diastema — an adaptation for its herbivorous food habit.
The grandfather would explain that a rabbit has diphyodont (two sets) and heterodont (different kinds) dentition with canines absent and a diastema between incisors and premolars — a herbivorous adaptation, its dental formula being .
IX. Value based questions
Leeches do not have an elaborate secretion of digestive juices and enzymes -Why ?
Leech is sanguivorous — it feeds only on the blood of its host, which is already a simple, ready-to-absorb liquid food needing little breakdown.
The blood is sucked by the muscular pharynx and stored in the large crop and its lateral diverticula, which can hold blood five times more than the leech's body weight.
This stored blood gives nourishment for several months; it may take more than a year for complete digestion and absorption of a single full meal.
Since the food is simple and digested very slowly in the stomach by a proteolytic enzyme, no elaborate secretion of digestive juices and enzymes is needed.
Because the leech feeds on blood (a simple liquid food) which is stored in the crop and digested slowly over months, it needs only a proteolytic enzyme and hence no elaborate digestive secretions.
How is the digestive system of rabbit suited for herbivorous mode of feeding?
Show steps (7)
Rabbit is a herbivore feeding on grass and vegetables like turnips, carrots and lettuce, and its alimentary canal and teeth are modified for this.
Heterodont and diphyodont dentition: it has different kinds of teeth (incisors, premolars, molars) in two sets — milk teeth and permanent teeth — suited for cutting and grinding plant food.
The chisel-like incisors cut the grass and vegetables, while the broad premolars and molars grind and crush the plant material.
Canines are absent, and the gap left between the incisors and premolars is the diastema, which helps in mastication and chewing of food in herbivores. Dental formula is .
A muscular tongue on the floor of the buccal cavity helps in mixing and rolling the food.
A large thin-walled caecum is present at the junction of the small and large intestine; it contains bacteria that help in the digestion of cellulose present in plant food.
Well developed digestive glands — salivary glands, gastric glands, liver, pancreas and intestinal glands — pour out secretions that digest the food in the alimentary canal.
Cutting incisors, grinding premolars and molars with a diastema, absence of canines, a muscular tongue and a large cellulose-digesting caecum make the rabbit's digestive system well suited for a herbivorous diet.
Quick check
In leech locomotion is performed by
Question 1 of 5Show answer
Answer: (4) Contraction and relaxation of muscles
Leech has no parapodia or setae; it moves by looping (crawling) and swimming.
Looping movement is brought about by the contraction and relaxation of body muscles, the suckers only giving attachment.
Mammals are ______________ animals.
Question 2 of 5Show answer
Answer: (2) Warm blooded
Mammals occupy the highest group in the animal kingdom and have a hairy body covering.
They maintain a constant body temperature, so they are warm blooded (not cold blooded or poikilothermic).
Pharyngeal ganglion in leech is a part of
Question 3 of 5Show answer
Answer: (2) Nervous system
The central nervous system of leech has a nerve ring and a paired ventral nerve cord.
The nerve ring is made of the suprapharyngeal ganglion (brain), circumpharyngeal connective and subpharyngeal ganglion — so pharyngeal ganglia are parts of the nervous system.
The body of leech has
Question 4 of 5Show answer
Answer: (2) 33 segments
Metamerism is the segmentation of the body.
The leech body is metamerically divided into 33 segments, the last seven fusing to form the posterior sucker.
The segments of leech are known as
Question 5 of 5Show answer
Answer: (1) Metameres (somites)
The body of leech is metamerically segmented into 33 segments.
Such body segments of annelids are called metameres or somites; proglottids and strobila belong to tapeworm.