CBSE Class 9 Science Diversity in Living Organisms Assignment Set B

DIVERSITY AND CLASSIFICATION
Diversity means = variety. Different places on the earth have their own typical kinds of living beings. This gives rise to the need of classification.
So “Diversity is the need of classification”.
• Classification is the theory and practice of classifying organisms. It is the systematic arrangement of organisms on the basis of their relatedness.
• Classification is the arrangement of organisms into sets or groups according to the similarities and dissimilarities present between them.

(a) Advantages / Significance of Classification :
• It establishes hierarchy of groups of organisms on the basis of their common features.
It makes the systematic study easier.
It is essential to understand the interrelationship amongst different groups of organisms.
It serves as a base for the development of there biological sciences as well as different fields of applied biology like public health, environment etc.

(b) Taxonomic Hierarchy : It is the framework by which taxonomic groups are arranged in definite order from higher to lower categories. the hierarchical order of classifying of classifying organisms is : Kingdom à phylum à class à order à family à genus à species

(i) Taxon : it refers to any rank or category in the hierarchial order of classification. E.g. kingdom, phylum etc. the highest tacxon is kingdom while the lowest taxon is species.
(ii) Species : These are the groups of organisms having similar morphological characters which can freely interbreed & produce their own kind. It is the basic unit of classification.

(c) Nomenclature : It is the system of naming an individual. Nomenclature is done on the basis of a set of rules stated in the ICN i.e. international Code of Nomenclature.

(i) Binomial nomenclature : It is a system of naming the organisms in such a way that each of their names contain two components first is genus and the second one is species. E.g. scientific name of human is Homo sapiens. Scientific name of crow is Corvus splendus. Homo and Corvus are the genus while sapiens and splendus are the names of species. This system was introduced by carolus Linnaeus in his book Systems Naturae. Who is also called Father of Taxonomy.
(ii) Certain convections are followed while writing the scientific names
• The name of the genus begins with a capital letter.
• The name of the species begins with a small letter.
• When printed, the scientific name is given in italics.
• When written by hand, the genus name and the species name have to be underlined separately.

(d) System of Classification :

(i) Artificial system: It was based on some superficial similarities.
i.e. only one or few characters were taken .e.g. on the basis of habital and ability to fly.

(ii) Natural system: It was based on natural affinity i.e. more than one natural characters were used.

(iii) Phylogenetic system: It was based on evolutionary sequence as well as genetic relationship amongst the organisms.

Classification of Organisms :

(iv) Two kingdom system: It was given by Carolus Linnaeus in 1758. organisms were divided into plant kingdom and animal kingdom. Fungi, Bacteria and Euglena could not find an appropriate position.

(v) Three kingdom system: It was given by Earnst Haeckel. In this kingdom protista was also included along with plant kingdom and animal kingdom.

(vi) Four kingdom system: It was given by Copeland. Kingdom Monera was also included in this system of classification.

(vii) Five kingdom system: It was given by Robert Whittaker. According to him organisms were divided into five kingdoms.

(A) Kingdom Monera : Unicellular, prokaryotic, microscopic, most ancient, can live in deep oceans, hot spring, deserts, high salt concentration etc. they include bacteria, filamentous and photosynthetic blue green algae etc.

(B) Kingdom Protista: Unicellular, colonial, eukaryotic. They

include photosynthetic algae, decomposers (slime moulds) and protozoa (predators) etc.

(C) Kingdom Fungl: Unicellular or multicellular eukaryotic organisms, they are heterotrophic, parasitic or saprotrophic.

(D) Kingdom Plantae: They are multicellular, eukaryotic, autotrophic (photosynthetic), some are heterotrophic and parasitic. They include photosynthetic algae, green plants etc.

(E) Kingdome Animal: Multicellular, eukaryotic, heterotrophic.

BASIS OF CLASSIFICATION
• Complexity of structure
• Mode of nutrition
• Level of organization
• Plant kingdom was divided in two sub kingdoms by Eichler Sub kingdom Cryptogamae : (Cryptos = hidden gamous = marriage) :
• These are also called as lower plants, flowerless or seedless plants.
• Their reproductive organs are hidden till they reproduce.

(i) Division Thallophyta :
• Thallus : Undifferentiated plant body i.e. absence of root, stem & leaves.
• There is no vascular system. 
• Reproductive organ are single-celled and there is no embroyo formation after fertilization.
• Dominant gametophyte.
• Three classes of thallophyta are :

(A) Class Algae :
Characters :
These are aquatic or terrestrial, fresh water or marine. Autotrophic, photosynthetic containing various pigments like chlorophyll, carotenoids, xanthophylls etc.
• Unicellular, colonial, filamentous.
• Cell wall of cellulose, e.g. blue green algae (Nostoc) , Green algae (Ulothrix, Spirogyra) Brown algae, red algae etc.

(B) Class Fungi :
Characters :
• These are heterotrophic.
• They lack chlorophyll but have cell wall of chitin (fungus cellulose)..
• They can be parasitic or saprotrophic
• Their body is filamentous called as mycelium.
• Reserve food material consists of glycogen. E.g. moulds (Rhizopus), Yeasts (Saccharomyces) Mushroom, (Agaricus)..

(C) Lichen :
Characters:
It is symbiotic relationship between algae and fungi.
Algal part is Phycobiont and fungal part is mycobiont.
They grow on rocks, tree trunks, grounds etc. e.g. parmellis, Alectoria etc.

(ii) Division Bryophyta :
Bryophytes are called terrestrial amphibians (amphibians of plant kingdom) because they require moist soil surface for awimming of their sperm & supply of water to all parts.
They are the first amongst land plants which occur in damp & shady habital
Plant body is of primitive from i.e. differentiated only in stem & leaves.
Vascular tissue & mechanical tissue are absent in them.
Male gamete is flagellated.
Sex organs are jacketed & multicellular.
Fertilization produces embryo.
They show heteromorphic type of alternation in generation. E.g. liverworts (Riccia, Marchantia), hornworts (Anthoceros) and mosses (Funaria).

(iii) Division Pteridophyta :
Characters:
They are seedless vascular plants, primitive tracheophytes or vascular cryptogarr
Plant body is differentiated into true stem, leaves & roots.
Vascular tissue are present.
Sperms are flagellated.
Embryo stage is present.
Gametophytes are small, exosporic or endosporic.
Sex organs are jacketed & multicellular, e.g. fems (Dryopteris, Pteris), club moss (Lycopodium), Horsetail (Equisetum).

(b). Sub kingdom: Phanerogamae: (phaneros = visible: gamous = marriage.)
These are higher plants having flowers and seeds both.
Body differentiated into true stem, leaves and root.
Vascular system i\s well developed.
Sex organ are multicellular.
Embryo develops from fertilized egg.
It is divided into two divisions :

(i) Division Gymnospermae :
Characters:
Seeds are not enclosed in fruits. These are naked seeded.
They have well developed vascular system but xylem lacks vessels and phloem lack companion cells.
They occupy an intermediate position between the pteridophytes and the angiosperms.
Plants are commonly tall trees or shrubs.
The flowers are represented by unisexual cones, often both being present on the same plant. E.g. Cycas, pinus (commonly known as pine)

(ii) Division : Angiospermae :
Characters:
These are seed bearing.
These are represented by trees, herbs, shrubs.
Body well differentiated into root, system.
Seeds remain enclosed in ovary.
It is divided into two classes on the basis of number of  cotyledons.

(A) Class : Dicotyledonae :
Their seeds have 2 cotyledons in the embryo.
Leaves are dorsiventral and show reticulate venation.
Tap root is present. E.g. neem, Peepal, Mango, pea, Mustard.

(B) Class :Monocotyledonae
Their seeds have one cotyledon in embryo.
Leaves are isobilateral, with parallel venation.
Fibrous root system is present, e.g. wheat, Maize, Onion.

ANIMAL KINGDOM
(a) Basis of Classification :
• Organization and differentiation of cells to form tissues and organs.
• Body symmetry.
• Formation to body cavities and blood vascular system.
• Features of embryonic development.

ON THE ABOVE BASIS ANIMAL KINGDOM IS DIVIDED INTO 11 PHYLA

(a) Phylum Protozoa :
• Characters:
• They are unicellular, eukaryotes
• These are the simplest & the most primitive animals. their body organization is of “Protoplasmic Level”.
• They are of different shapes i.g. irregular elongated or rounded.
• They have different types of locomotory organs like cilia, flagella, pseudopodia (false feet)etc.
Nutrition is of different types like holozoic, holophytic, mixotrophic.
Digestion is intracellular & it : takes place in food vacuole.
Excretion & respiration occurs through general body  Surface by the process of simple diffusion.
Reproduction may be sexual or asexual.
Alternation of generation is found. E.g., Amoeba,
Entamoeba, plasmodium, Euglena, Paramecium.

(b) Phylum Porifera :
Characters:
These are pore bearing organisms i.e. with porous body.
Also called as sponges.
Aquatic, mostly marine.
These are sessile and sedentary (attached to substratum).
Occur in different shapes i.e. vase-like, rounded, sac like etc.
Body perforated by numerous pores called ostia which open into a canal system having canals and chambers lined with coenocytes (flagellated) and have a large size water outlet called oscula.
Their cavity is called spongocoel.
Endoskeleton is made of needle like spicules made of calciumcarbonate and silica or spongocoel.
Hermaphrodite, Asexual reproduction by budding. E.g., Sycon, pongilla, Euplectella.

(c) Phylum – Coelenterata or Chidaria :
Characters:
These are sac like structures. They have a body Cavity called gastrovascular cavity or coelenteron. It has single opening for ingestion and egestion both. Aquatic, mostly marine.
Multicellular, diploblastic, radially symmetrical.
They have special organs called tentacles, chidoblast.
Or nematocyst cells. They are specialized for stinging.
They paralyse the prey by releasing poison.
These are polymorphic i.e. they occur in two forms.
(i) Polyp is asexual, cylindrical, sedentary and diploid.
(ii) Medusa is sexual, umbrella shaped, free swimming and diploid.
Asexual reproduction by budding and sexual reproduction by gametes.
Some of them have exoskeleton of CaCO3 . they are called Corals, they live in colonies and when they die they form coral reefs, or islands. E.g. hydra, jelly fish.'

(d) Phylum – Ctenophora :
Characters:
Body is transparent with radial symmetry.
They possess comb plates that are ciliated and 8 in no. these help in locomotion. They also possess tentacles.
Marine, solitary and free swimming. E.g. cestum.

(e) Phylum – Platyhelminthes : (Platys = Flat; helminth = worm)
Characters:
Generally called as flatworms.
Bilaterally symmetrical, triploblastic, dorsoventrally flattened. Acoelomates.
Their digestive cavity has a single opening with mouth only and anus is absent.
They possess hooks and suckers.
They have flame cells or protonephridia for excretion.
Mode of nutrition is parasitic.
Reproduction is of both types i.e. asexual and sexual
These are hermaphrodite. E.g. Planaria, Fasciola (liver fluke)

(f) Phylum – Nematoda (Aschelminthes) :
Characters:
Also called as roundworms.
Bilaterallysymmetrical, unsegmented triploblastic.
These are pseudocoelomic.
Their alimentary canal is tubular having both mouth and anus.
Most are free living, some live in moist soil, some are fresh water while some are marine.
Some are parasites on plants & animals.
They lack circulatory system.
Reproduction is sexual and sexes are separate. E.g.
Ascaris (round worm), filarial worm.

(g) Phylum – Annelida (Annulus = ring ; segments)
Characters:
Their body is triploblastic, bilaterally symmetrical, soft, elongated, vermiform, cylindrical and dorsoventrally flattened.
Body is metamerically segmented. Head is formed by joining of some anterior body segments.
Exoskeleton is absent, body is covered by thin cuticle.
Eucoelomata i.e. They have true body cavity which first appeared in this phylum.
Well developed alimentary canal is present.
They have closed circulatory system.
Locomotion is with the help of chitinous projections
Called chastae (setae).
Excretion by nephridia.
Nervous system has dorsal brain.
Most are aquatic, marine or fresh water, some are terrestrial.
They reproduce sexually e.g. Earthworm, Leech.

(h) Phylum – Arthropoda (Arthros à jointed, poda = legs)
Characters:
These are the organisms with jointed appendages.
This is the largest phylum in animal kingdom.
body triploblastic, bilaterally symmetrical and metamerically segmented.
They have an exoskeleton made up of protein and Chitin (Moulting à Periodic shedding off of the exoskeleton to induce the growth).
They have a complete alimentary canal with mouth & anus.
Respiration occurs through general body surface, gills, trachea and book lungs.
They have open circulatory system with dorsal heart & arteries.
Body cavity is called haemocoel.
Excretion by coelomducts, malphigian tubules, green glands, coxal glands.
Sexes are separate .
Each segment has paired lateral and jointed appendages. E.g. Palaemon (prawn) ,
Cancer (crab) ,Periplanata (cockroach) , Anopheles (mosquito) & Aranea (spider) etc.

(i) Phylum Mollusca :
Characters:
It is second largest group of animals, body soft,
Unsegmented, bilaterally symmetrical and without Appendages.
Body divided into a head, foot and visceral mass. A
Thin skin covering the body is called as mantle, which
Secretes a calcareous shell.
Body cavity is haemocoel.
Respiration by gills (Ctenidial) in aquatic forms but in terrestrial forms space between mantle and body wall called as mantle  cavity act as lungs.
For ingestion they have tongue like structure “radual”
They also possess a digestive glandcalled hepatopancreas
Open circulatory system.
Excretion by metanephridia present near heart.
Reproduction is sexual and sexes are separate.
E.g. Chiton (8 Calcareous pieces) , Pila, Melix
(torsion univalve) , Dentalium (tusk like shell) , Unio, Mytilus (Bivalve), Octopus.

(j) Phylum Echinodermata :
Characters:
These are marine animals, their body is triploblastic,
Eucoelomata, unsegmented.
Their body has spines arising from exoskeleton of Calcium.
Adults are radially symmetrical while larvae are Bilaterally symmetrical.
Head is absent, oral and aboral surfaces have five Radial ambulacra.
Excretory organs are absent.
A complex system of water containing tubes and Bladders passing though pores of skin called water
Vascular system is present. From this tube like structure Arise, these tubes look like feet and are called as tube Feet that helps in locomotion Reproduction can be asexual, sexual or by regeneration. E.g. Asterias
(star fish) , echinus (sea urchin) , Holothuria (sea cucumber) , Antedon (feather star).

PHYLUM HEMICHORDATA

Characters:
• They are placed in between nochordates and Chordates as they possess some characters of both.
• They include worm like, unsegmented, bilaterally Symmetrical animals which are exclusively marine.
• Their body is divided into three regions proboscis, Collar & trunk.
• They do not possess notochord, which is a flexible, Rod like structure running trough the length of the body,Above alimentary canal.
• They possess gill slit or gill cleft which is meant for Respiration.
• They possess nerve cord in collar region but it is notA true dorsal cord. E.g. Balanoglossus (tongueWorm).

Characters:
• This is the most advanced group of animals.
• Notochord is present at some stages of life, supported by a tubular hollow dorsal nerve cord.
• In higher chordates i. E. Vertebrates, the notochord is replaced by vertebral column.
• A set of gill slits is also present at certain stage of life, also called as pharyngeal gill clefts.
• Tail is also present behind the anal aperture that is post anal tail.
• They also possess a proper circulatory system.

CHORDATA IS FURTHER DIVIDED AS FOLLOWS

(a) Protochordata :
Characters:
• They are termed as lower chordates.
• They do not possess brain, cranium, vertebral column, jaw and paired appendages.
• Notochord is present atleast in some stages along with other diagnostic chordate characters (dorsal hollow never cord, gill slits, post and tail).
• They are triploblastic, bilaterally symmetrical, enterocoelomic, organ system level organization.
• They are marine animals.
• Protochordata are further divided into two groups :

(i) Subphylum : Urochordata
Characters:
They are exclusively marine animals where pharynx possess several gill slits.
• They are commonly known as tunicates.
• The notochord is present in the tail of the larva and disappears in the adult.
• The dorsal hollow never cord is present in the larva only. it is replaced by a dorsal ganglion in the adult.
• The pharynx has gill slits.
• The larva (tadpole) undergoes retrogressive metamorphosis,
i.e., changes from a better developed larva to a less developed adult.
e.g. Herdmania (sea squirt), Doliolum, Pyrosomea.

(ii) subphylum : Urochordata
Characters:
• Animals are fish like without a head.
• Animals possess all the characters of chordates i.e.,
• A notochord which extends upto the entire length of the body and persists throughout the life.
• A nerve cord (without a distinct brain).
• Numerous well developed gill sli
• A post anal tail throughout life. e.g. branchiostoma (Amphioxus; Lancelet)
• Adults are radially symmetrical while larvae are bilaterally symmetrical.
• Head is absent, oral and aboral surfaces have five radial ambulacra.
• Excretory organs are absent.
• A complex system of water containing tubes and bladders passing through pores of skin called water vascular system is present. from this tube like structure arise, these tubes look like feet and are called as tube feet that helps in locomotion
• Reproduction can be asexual, sexual or by regeneration. e.g. Asterias (star fish), Echinus (sea urchin), Holothuria (sea cucumber), Antedon (feather star),

SUBPHYLUM VERTEBRATA OR CRANIATA
• Majority of chordates are included in this phylum.
• Characters:
• They are advanced animals, having a cranium (brain box) around the brain. Nervous system is well developed.
• Notochord is replaced by a vertebral column (backbone) in the adults. Endoskeleton is highly developed.
• There are two pairs of limbs or appendages.
• Head is well differentiated.
• The heart is situated ventrally. The circulatory system is closed consisting of bold vascular system and lymphatic system. Red Coloured pigment hemoglobin is present in red blood corpuscles.
• Respiratory organs may be gills (in aquatic animals), skin, buccopharyngeal cavity (in amphibians) or lungs (in land animals).
• Excretion occurs through kidneys.
• Sexes are separate.

This subphylum is subdivided into seven classes. They are:
(Jawless vertebrates) (Vertebrates with well  developed jaws)
1. Cyclostomata
2. Chondrichthyes 3. Osteichtlyes 4. Amphibians 5. Reptiles 6. Aves 7. Mammals

(a) Class Cyclostomata :
(Gr. Cyclos = circular, stome = mouth ; the circular
Mouthed fishes) these are the most primitive Vertebrates.
Characters:
• Animals are jawless and possess a circular mouth.
• They are ectoparasites on fish and use mouth to stick to fish ; the mouth is therefore suctorial.
• Notochord is present in the form of a cylindricalrod.
• Head and brain are poorly developed.
• Cartilaginous endoskeleton is present.
• Respiration occurs through gills contained in pouches.
• Heart is two-chambered consisting of one auricle and one ventricle.
• Gonad is single and fertilization is external. e.g. petromyzon (lamprey), Myxine (hag

(b) Class Chondrichthyes :
(Gr. Chondros = cartilage; ichthys = fish, the cartillagenous fish).
Characters:
• Skeleton is cartilaginous, hence the name Chondrichthyes is given.
• The body is either laterally compressed and spindle shaped, or dorso ventrally flattened and disc shaped.
• Mouth is ventral in position on the head.
• Jaws are well-developed.
• Respiration occurs through gills.
• The skin is covered with placoid scales (exoskeleton).
• Heart is two chambered, consisting of an auricle and a ventricle.
• Lung or air bladder is absent.
• They reproduce by laying eggs (oviparous) or produce eggs which hatch inside the mother’s body (ovoviviparous).
• Fertilization is internal.
• Mostly marine and large in size (upto 10-20 meters long). e.g. sharks, rays and skates. Scoliodon (Indian shark, dog fish), Torpedo (electric ray), Trygon (sting ray), Rhinobatus (guitar fish).

(c) Class Osteichthyes :
Characters:
• Skeleton is cartilaginous, in the embryonic stage, but is replaced by bones in adult form (bony endoskeleton), hence the name osteichthyes is given.
• Body is generally spindle shaped.
• The exoskeleton, if present, consists of cycloid or ctenoid scales.
• The mouth is terminal (anterior) on the head.
• Gills are covered by bony flap called as the operculum.
• A swim bladder (or air bladder) is usually present. it helps in floating.
• Heart is two chambered.
• Fertilization is mostly external.
• They live both in fresh and sea water.
• In size, they vary from 10 mm to 4 meters. e.g. Labeo,
Hippocampus (sea horse), Anabas (climbing perch), Muraena (eel), Protopterus (lung-fish).

(d) Class Amphibia :
(Gr. amphi = both, bios = life, the vertebrates leading two lives/ dual life)
• The amphibians are the first land vertebrates, Amphibious party terrestrial and partly aquatic.
Characters:
• They are amphibious in nature ; found in fresh water and moist places.
• Skin is smooth or rough, rich in glands which keep it moist ; skin with pigmented cells, i.e., chromatophores.
• Body is without scales.
• Endoskeleton is mostly bony, notochord does not persist in adults.
• Head and trunk are distinct ; neck and tall may or may not be present.
• Limbs tetrapods(four-limbed), pentadactyi type (five-fingered).
• Respiration occurs by lungs, skin or buccal lining, are present at least during Laval stage for reapiration.
• Heart three chambered with two auricles and a ventricle, red blood corpuscles are large, biconvex, oval and nucleated.
• Brain is not much developed, cranial nerves are 10 pairs.
• Sexes are separate, i.e., dioecious, male without copulatory organ.
• Eggs with gelatinous covering, usually laid in water.
• Fertilization is external.
• Development is indirect with a tadpole larva which undergoes metamorphosis to become adult.
E.g. Salamanders, newts, frogs and toads. Salamandra (salamander), ecturous (mud puppy), Triturus (newt), Rana (frog), Bufo (toad).

(e) Class Reptilia :
(L. reptare = to creep ; creeping vertebrates).
• These are first truly terrestrial animals living in warmer regions.
Characters:
• Body is divisible into head, neck and trunk. tail is well developed in some, while it is reduced in others.
• Two pairs of pentadactyl limbs are present ; but in snakes limbs are reduced or absent.
• Body is covered with epidermal horny scales.
• Skin is dry, impermeable and devoid of glands.
• Respiration takes place by lungs only. gills are absent.
• Heart is incompletely four- chambered, having two auricles and incompletely divided ventricle. in crocodile, heart is completely four chambered.
• Sexes are separate.
• Fertilization is internal(characteristics of land animals).
• The embryo always lies in a fluid filled sac called amnion.
• They is no larval stage in development. e.g. Testudo(tortoise),
Chelone(turtle) Draco (flying lizard), Chameleon, Hemidactylus (wall lizard), Naja (cobra) etc.

(f) Class Aves :
(L. Aves = birds).
• The birds are described as ‘feathered reptiles’ that have developed the power of flight.
Characters:
• The body is covered with soft feathers (feathery exoskeleton).
• The body is divisible into head, neck, trunk and tail.
• There are two pairs of limbs. the fore limbs are modified to form wings (in flying birds) or are reduced (as in non-flying birds). Hind limbs are strongly developed for perching, walking.
• Endoskeleton is light. the bones have got air cavities. this makes the bird light.
• Jaws are modified to form a strong beak.
• Teeth are absent.
• Respiration is by lungs only. Lungs have additional bag like membranous extensions called asair sacs.
• Heart is completely four chambered.
• Sexes are separate.
• Birds are oviparous, i.e. egg laying.
• Fertilization is internal. Fertilized eggs are laid with a yolk (stored food) and with a hard calcareous shell.
• Like reptiles and mammals, they have the embryonic membranes namely the amnion, chorion, yolksac and allantois.
• High degree of parental care is exhibited.
• There is no larval stage in development. e.g. Columba (pigeon), Pavo(peacock), Corvus (crow), Passer (sparrow), Struthio (ostrich), Kiwi and penguin are flightless birds.

(g) Class Mammalla :
(L. mamma = breast; the mammals)
• Mammalia is the most evolved group of organisms and are found in diverse habitats ranging from deserts, polar ice caps, oceans, mountains, forests and grasslands.
• They are named mammals as all of them possess mammary glands (milk producing glands). Mammals are the only animals which feed their young ones with milk.
Characters:
• Skin is covered with an exoskeleton of hair. Hair are provided with sweat glands which help in the regulation of body temperature. in aquatic mammals, hair being negligible, the subcutaneous layer of fats provides insulation.
• Mammals have two pairs of pentadactyl limbs.
• The body cavity is unequally divided into two parts by a muscular partition called as diaphragm.
• Eyes are provided with movable lids.
• Ears have fleshy external ears or pinnae.
• Teeth are embedded in sockets (thecodont). Two sets of teeth develop in the life time of a mammals Milk teeth and permanent teeth (diphyodont).
• Teeth are of different types (heterodont).
• Respiration occurs by lungs.
• Heart is our chambered. R.B.Cs are non nucleated and usually circular.
• Sexes are separate. Gonads are paired. Testes lie commonly in the scrotal sacs outside the abdomen.
• Fertilization is internal. Eggs are small, microscopic without shells and are retained in uterus of female for development.
• Embryonic membranes (amnion, chorion, yolk sac and allantois) present.
• They give birth to living young ones and are called as viviparous.
The young ones are fed on milk from mammary glands.

(h) Important Groups of Mammals :
Mammals are divided into three main groups,
(i) Egg-laying mammals (monotremes) : These mammals show characters of both reptiles and mammals. They jay hard shelled eggs (oviparous) e.g. spiny ant eater, Duck – billed platypus
(ii) Marsupial mammals (pouched mammals) : Pouched or marsupial mammals (Latin marsupium = pouch) They are viviparous. The young ones, when born, are only three cm. long. Hence they are cared in pouch called marsupium present on the mother’s abdomen. in the pouch, they feed on the mother milk e.g. Kangaroo(Macropus), Kola bear,
(iii) Placental mammals (true mammals) : These mammals with true placenta. The embryo is retained in the uterus. These are the very successful group of land animals, occurring in diverse climatic condition. e.g. Mole, bat, lion, tiger, camel, giraffe, whale, dolphin, monkey, humans etc.