100 BIOLOGY

In the preceding chapters you came across a large variety of organisms,

both unicellular and multicellular, of the animal kingdom. In unicellular

organisms, all functions like digestion, respiration and reproduction

are performed by a single cell. In the complex body of multicellular

animals the same basic functions are carried out by different groups of

cells in a well organised manner. The body of a simple organism like

Hydra is made of different types of cells and the number of cells in each

type can be in thousands. The human body is composed of billions of

cells to perform various functions. How do these cells in the body work

together? In multicellular animals, a group of similar cells alongwith

intercellular substances perform a specific function. Such an organisation

is called tissue.

You may be surprised to know that all complex animals consist of

only four basic types of tissues. These tissues are organised in specific

proportion and pattern to form an organ like stomach, lung, heart and

kidney. When two or more organs perform a common function by their

physical and/or chemical interaction, they together form organ system,

e.g., digestive system, respiratory system, etc. Cells, tissues, organs and

organ systems split up the work in a way that exhibits division of labour

and contribute to the survival of the body as a whole.

7.1 ANIMAL TISSUES

The structure of the cells vary according to their function. Therefore, the

tissues are different and are broadly classified into four types : (i) Epithelial,

(ii) Connective, (iii) Muscular and (iv) Neural.

TRUCTURAL

RGANISATION IN

NIMALS

HAPTER

7.1 Animal Tissues

7.2 Organ and Organ

System

7.3 Earthworm

7.4 Cockroach

7.5 Frogs

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7.1.1 Epithelial Tissue

We commonly refer to an epithelial tissue as epithelium (pl.: epithelia).

This tissue has a free surface, which faces either a body fluid or the outside

environment and thus provides a covering or a lining for some part of the

body. The cells are compactly packed with little intercellular matrix. There

are two types of epithelial tissues namely simple epithelium and

compound epithelium. Simple epithelium is composed of a single layer

of cells and functions as a lining for body cavities, ducts, and tubes. The

compound epithelium consists of two or more cell layers and has protective

function as it does in our skin.

On the basis of structural modification of the cells, simple epithelium

is further divided into three types. These are (i) Squamous, (ii) Cuboidal,

(iii) Columnar (Figure 7.1).

The squamous epithelium is made of a single thin layer of flattened

cells with irregular boundaries. They are found in the walls of blood vessels

and air sacs of lungs and are involved in functions like forming a diffusion

boundary. The cuboidal epithelium is composed of a single layer of

cube-like cells. This is commonly found in ducts of glands and tubular

parts of nephrons in kidneys and its main functions are secretion and

absorption. The epithelium of proximal convoluted tubule (PCT) of

nephron in the kidney has microvilli. The columnar epithelium is

composed of a single layer of tall and slender cells. Their nuclei are located

at the base. Free surface may have microvilli. They are found in the lining

of stomach and intestine and help in secretion and absorption. If the

columnar or cuboidal cells bear cilia on their free surface they are called

ciliated epithelium (Figure 7.1d). Their function is to move particles or

mucus in a specific direction over the epithelium. They are mainly present

in the inner surface of hollow organs like bronchioles and fallopian tubes.

Figure 7.1

Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar

(d) Columnar cells bearing cilia

(a)

Flattened cell

Cube-like cell

Tall cell

(b)

(d)

(c)

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Some of the columnar or cuboidal cells

get specialised for secretion and are called

glandular epithelium (Figure 7.2). They

are mainly of two types: unicellular,

consisting of isolated glandular cells (goblet

cells of the alimentary canal), and

multicellular, consisting of cluster of cells

(salivary gland). On the basis of the mode of

pouring of their secretions, glands are

divided into two categories namely

exocrine and endocrine glands. Exocrine

glands secrete mucus, saliva, earwax, oil,

milk, digestive enzymes and other cell

products. These products are released

through ducts or tubes. In contrast,

endocrine glands do not have ducts. Their

products called hormones are secreted

directly into the fluid bathing the gland.

Compound epithelium is made of more

than one layer (multi-layered) of cells and thus

has a limited role in secretion and absorption

(Figure 7.3). Their main function is to provide

protection against chemical and mechanical

stresses. They cover the dry surface of the skin,

the moist surface of buccal cavity, pharynx,

inner lining of ducts of salivary glands and of

pancreatic ducts.

All cells in epithelium are held together with little intercellular material.

In nearly all animal tissues, specialised junctions provide both structural

and functional links between its individual cells. Three types of cell junctions

are found in the epithelium and other tissues. These are called as tight,

adhering and gap junctions. Tight junctions help to stop substances

from leaking across a tissue. Adhering junctions perform cementing to

keep neighbouring cells together. Gap junctions facilitate the cells to

communicate with each other by connecting the cytoplasm of adjoining

cells, for rapid transfer of ions, small molecules and sometimes big molecules.

7.1.2 Connective Tissue

Connective tissues are most abundant and widely distributed in the body

of complex animals. They are named connective tissues because of their

special function of linking and supporting other tissues/organs of the

body. They range from soft connective tissues to specialised types, which

Figure 7.2 Glandular epithelium : (a) Unicellular

(b) Multicellular

unicellular

gland

Multicelluar

gland

(a)

)

Figure 7.3 Compound epithelium

Multi-

layered

cells

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Fat storage

area

Nucleus

Macro-

phage

Mast

cell

Fibroblast

Collagen

fibers

Plasma

Membrane

include cartilage, bone, adipose, and blood. In all

connective tissues except blood, the cells secrete fibres of

structural proteins called collagen or elastin. The fibres

provide strength, elasticity and flexibility to the tissue.

These cells also secrete modified polysaccharides, which

accumulate between cells and fibres and act as

matrix (ground substance). Connective tissues are

classified into three types: (i) Loose connective tissue,

(ii) Dense connective tissue and (iii) Specialised

connective tissue.

Loose connective tissue has cells and fibres loosely

arranged in a semi-fluid ground substance, for example,

areolar tissue present beneath the skin (Figure 7.4). Often

it serves as a support framework for epithelium. It

contains fibroblasts (cells that produce and secrete fibres),

macrophages and mast cells. Adipose tissue is another

type of loose connective tissue located mainly beneath the

skin. The cells of this tissue are specialised to store fats.

The excess of nutrients which are not used immediately

are converted into fats and are stored in this tissue.

Fibres and fibroblasts are compactly packed in the

dense connective tissues

. Orientation of fibres show a

regular or irregular pattern and are called dense regular

and dense irregular tissues. In the dense regular

connective tissues, the collagen fibres are present in rows

between many parallel bundles of fibres. Tendons, which

attach skeletal muscles to bones and ligaments which

attach one bone to another are examples of this tissue.

Dense irregular connective tissue has fibroblasts and

many fibres (mostly collagen) that are oriented differently

(Figure 7.5). This tissue is present in the skin. Cartilage,

Figure 7.4 Loose connective tissue : (a) Areolar tissue (b) Adipose tissue

(a)

(b)

fibres

Collagen fibre

(a)

(b)

Figure 7.5 Dense connective tissue:

(a) Dense regular

(b) Dense irregular

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bones and blood are various types of specialised

connective tissues.

The intercellular material of cartilage is solid and pliable

and resists compression. Cells of this tissue (chondrocytes)

are enclosed in small cavities within the matrix secreted by

them (Figure 7.6a). Most of the cartilages in vertebrate

embryos are replaced by bones in adults. Cartilage is

present in the tip of nose, outer ear joints, between adjacent

bones of the vertebral column, limbs and hands in adults.

Bones have a hard and non-pliable ground substance

rich in calcium salts and collagen fibres which give bone

its strength (Figure 7.6b). It is the main tissue that provides

structural frame to the body. Bones support and protect

softer tissues and organs. The bone cells (osteocytes) are

present in the spaces called lacunae. Limb bones, such as

the long bones of the legs, serve weight-bearing functions.

They also interact with skeletal muscles attached to them

to bring about movements. The bone marrow in some bones

is the site of production of blood cells.

Blood is a fluid connective tissue containing plasma,

red blood cells (RBC), white blood cells (WBC) and platelets

(Figure 7.6c). It is the main circulating fluid that helps in

the transport of various substances. You will learn more

about blood in Chapters 17 and 18.

7.1.3 Muscle Tissue

Each muscle is made of many long, cylindrical fibres

arranged in parallel arrays. These fibres are composed of

numerous fine fibrils, called myofibrils. Muscle fibres

contract (shorten) in response to stimulation, then relax

(lengthen) and return to their uncontracted state in a

coordinated fashion. Their action moves the body to adjust

to the changes in the environment and to maintain the

positions of the various parts of the body. In general,

muscles play an active role in all the movements of the body.

Muscles are of three types, skeletal, smooth, and cardiac.

Skeletal muscle tissue is closely attached to skeletal

bones. In a typical muscle such as the biceps, striated

(striped) skeletal muscle fibres are bundled together in a

parallel fashion (Figure 7.7a). A sheath of tough connective

tissue encloses several bundles of muscle fibres (You will

learn more about this in Chapter 20).

Platelets

WBC

RBC

(a)

(b)

(c)

Figure 7.6 Specialised connective

tissues : (a) Cartilage

(b) Bone (c) Blood

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The smooth muscle fibres taper at both ends (fusiform) and do not

show striations (Figure 7.7b). Cell junctions hold them together and they

are bundled together in a connective tissue sheath. The wall of internal

organs such as the blood vessels, stomach and intestine contains this type

of muscle tissue. Smooth muscles are ‘involuntary’ as their functioning

cannot be directly controlled. We usually are not able to make it contract

merely by thinking about it as we can do with skeletal muscles.

Cardiac muscle tissue is a contractile tissue present only in the heart.

Cell junctions fuse the plasma membranes of cardiac muscle cells and

make them stick together (Figure 7.7c). Communication junctions

(intercalated discs) at some fusion points allow the cells to contract as a

unit, i.e., when one cell receives a signal to contract, its neighbours are

also stimulated to contract.

7.1.4 Neural Tissue

Neural tissue exerts the greatest control over

the body’s responsiveness to changing

conditions. Neurons, the unit of neural

system are excitable cells (Figure 7.8). The

neuroglial cell which constitute the rest of

the neural system protect and support

neurons. Neuroglia make up more than one-

half the volume of neural tissue in our body.

When a neuron is suitably stimulated,

an electrical disturbance is generated

which swiftly travels along its plasma

Nucleus

Striations

Junction

between

adjacent

cells

Nucleus

Striations

Smooth

muscle

fibers

Figure 7.7 Muscle tissue : (a) Skeletal (striated) muscle tissue (b) Smooth muscle tissue

(a)

(b)

(c)

Figure 7.8 Neural tissue (Neuron with

neuroglea)

Dendrite

Cell

body

with

nucleus

Axon

Neuroglea

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membrane. Arrival of the disturbance at the neuron’s endings, or output

zone, triggers events that may cause stimulation or inhibition of adjacent

neurons and other cells (You will study the details in Chapter 21).

7.2 ORGAN AND ORGAN SYSTEM

The basic tissues mentioned above organise to form organs which in turn

associate to form organ systems in the multicellular organisms. Such an

organisation is essential for more efficient and better coordinated activities

of millions of cells constituting an organism. Each organ in our body is

made of one or more type of tissues. For example, our heart consists of all

the four types of tissues, i.e., epithelial, connective, muscular and neural.

We also notice, after some careful study that the complexity in organ and

organ systems displays certain discernable trend. This discernable trend

is called evolutionary trend (You will study the details in class XII). You

are being introduced to morphology and anatomy of three organisms at

different evolutionary levels to show their organisation and functioning.

Morphology refers to study of form or externally visible features. In the

case of plants or microbes, the term morphology precisely means only

this. In case of animals this refers to the external appearance of the organs

or parts of the body. The word anatomy conventionally is used for the

study of morphology of internal organs in the animals. You will learn the

morphology and anatomy of earthworm, cockroach and frog representing

invertebrates and vertebrates.

7.3 EARTHWORM

Earthworm is a reddish brown terrestrial invertebrate that inhabits the

upper layer of the moist soil. During day time, they live in burrows made

by boring and swallowing the soil. In the gardens, they can be traced by

their faecal deposits known as worm castings. The common Indian

earthworms are Pheretima and Lumbricus.

7.3.1 Morphology

Earthworms have long cylindrical body. The body is divided into more

than hundred short segments which are similar (metameres about

100-120 in number). The dorsal surface of the body is marked by a dark

median mid dorsal line (dorsal blood vessel) along the longitudinal axis of

the body. The ventral surface is distinguished by the presence of genital

openings (pores). Anterior end consists of the mouth and the prostomium,

a lobe which serves as a covering for the mouth and as a wedge to force

open cracks in the soil into which the earthworm may crawl. The prostomium

is sensory in function. The first body segment is called the peristomium

(buccal segment) which contains the mouth. In a mature worm, segments

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14-16 are covered by a prominent dark band of glandular tissue called

clitellum. Thus the body is divisible into three prominent regions –

preclitellar, clitellar and postclitellar segments (Figure 7.9).

Four pairs of spermathecal apertures are situated on the ventro-lateral

sides of the intersegmental grooves, i.e., 5

-9

segments. A single female

genital pore is present in the mid-ventral line of 14

segment. A pair of

male genital pores are present on the ventro-lateral sides of the 18

segment. Numerous minute pores called nephridiopores open on the

surface of the body. In each body segment, except the first, last and

clitellum, there are rows of S-shaped setae

, embedded in the epidermal

pits in the middle of each segment. Setae can be extended or retracted.

Their principal role is in locomotion.

7.3.2 Anatomy

The body wall of the earthworm is covered externally by a thin non-cellular

cuticle below which is the epidermis, two muscle layers (circular and

longitudinal) and an innermost coelomic epithelium. The epidermis is made

Figure 7.9 Body of earthworm : (a) dorsal view (b) ventral view (c) lateral view

showing mouth opening

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up of a single layer of columnar epithelial cells

which contain secretory gland cells.

The alimentary canal is a straight tube and

runs between first to last segment of the body.

(Figure 7.10). A terminal mouth opens into the

buccal cavity (1-3 segments) which leads into

muscular pharynx. A small narrow tube,

oesophagus (5-7 segments), continues into a

muscular gizzard (8-9 segments). It helps in

grinding the soil particles and decaying leaves,

etc. The stomach extends from 9-14 segments.

The food of the earthworm is decaying leaves and

organic matter mixed with soil. Calciferous

glands, present in the stomach, neutralise the

humic acid present in humus. Intestine starts

from the 15

segment onwards and continues

till the last segment. A pair of short and conical

intestinal caecae project from the intestine on the

segment. The characteristic feature of the

intestine after 26

segment except the last

-25

segments is the presence of internal

median fold of dorsal wall called typhlosole. This

increases the effective area of absorption in the

intestine. The alimentary canal opens to the

exterior by a small rounded aperture called anus.

The ingested organic rich soil passes through the

digestive tract where digestive enzymes

breakdown complex food into smaller absorbable

units. These simpler molecules are absorbed

through intestinal membranes and are utilised.

Pheretima exhibits a closed type of

blood vascular system, consisting of blood

vessels, capillaries and heart. (Figure 7.11). Due

to closed circulatory system, blood is confined

to the heart and blood vessels. Contractions

keep blood circulating in one direction. Smaller

blood vessels supply the gut, nerve cord, and

the body wall. Blood glands are present on the

, 5

and 6

segments. They produce blood

cells and haemoglobin which is dissolved in

blood plasma. Blood cells are phagocytic in

nature. Earthworms lack specialised breathing

devices. Respiratory exchange occurs through

moist body surface into their blood stream.

Mouth

Pharynx

Oesophagus

Gizzard

Stomach

Pre-typhlosolar

part of intestine

Intestinal

caecum

Lymph gland

Typhlosolar

part of intestine

Intestinal lumen

Typhlosole

Figure 7.10 Alimentary canal of earthworm

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The excretory organs occur as segmentally

arranged coiled tubules called nephridia

(sing.: nephridium). They are of three types:

(i) septal nephridia, present on both the sides of

intersegmental septa of segment 15 to the last

that open into intestine, (ii) integumentary

nephridia, attached to lining of the body wall of

segment 3 to the last that open on the body

surface and (iii) pharyngeal nephridia, present

as three paired tufts in the 4

, 5

and 6

segments (Figure 7.12). These different types of

nephridia are basically similar in structure.

Nephridia regulate the volume and composition

of the body fluids. A nephridium starts out as a

funnel that collects excess fluid from coelomic

chamber. The funnel connects with a tubular part

of the nephridium which delivers the wastes

through a pore to the surface in the body wall

into the digestive tube.

Nervous system is basically represented by

ganglia arranged segmentwise on the ventral

paired nerve cord. The nerve cord in the anterior

region (3

and 4

segments) bifurcates, laterally

encircling the pharynx and joins the cerebral

ganglia dorsally to form a nerve ring. The cerebral

ganglia alongwith other nerves in the ring

integrate sensory input as well as command

muscular responses of the body.

Figure 7.11 Closed circulatory system

Figure 7.12 Nephridial system in earthworm

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110 BIOLOGY

Sensory system does not have eyes but does

possess light and touch sensitive organs (receptor

cells) to distinguish the light intensities and to feel

the vibrations in the ground. Worms have

specialised chemoreceptors (taste receptors) which

react to chemical stimuli. These sense organs are

located on the anterior part of the worm.

Earthworm is hermaphrodite (bisexual), i.e.,

testes and ovaries are present in the same

individual (Figure 7.13). There are two pairs of

testes present in the 10

and 11

segments.

Their vasa deferentia run up to the 18

segment

where they join the prostatic duct. Two pairs of

accessory glands are present one pair each in

the 17

and 19

segments. The common prostate

and spermatic duct (vasa deferentia) opens to

the exterior by a pair of male genital pores on

the ventro-lateral side of the 18

segment. Four

pairs of spermathecae are located in 6

-9

segments (one pair in each segment). They receive

and store spermatozoa during copulation. One

pair of ovaries is attached at the inter-segmental

septum of the 12

and 13

segments. Ovarian

funnels are present beneath the ovaries which

continue into oviduct, join together and open on

the ventral side as a single median female genital

pore on the 14

segment.

A mutual exchange of sperm occurs between

two worms during mating. One worm has to find

another worm and they mate juxtaposing

opposite gonadal openings exchanging packets

of sperms called spermatophores. Mature sperm

and egg cells and nutritive fluid are deposited in

cocoons produced by the gland cells of clitellum.

Fertilisation and development occur within the

cocoons which are deposited in soil. The ova

(eggs) are fertilised by the sperm cells within the

cocoon which then slips off the worm and is

deposited in or on the soil. The cocoon holds the

worm embryos. After about 3 weeks, each cocoon

produces two to twenty baby worms with an

average of four. Development of earthworms is

direct, i.e., there is no larva formed.

Figure 7.13 Reproductive system of earthworm

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Earthworms are known as ‘friends of farmers’ because they make

burrows in the soil and make it porous which helps in respiration and

penetration of the developing plant roots. The process of increasing fertility

of soil by the earthworms is called vermicomposting. They are also used

as bait in game fishing.

7.4 COCKROACH

Cockroaches are brown or black bodied animals that are included in

class Insecta of Phylum Arthropoda. Bright yellow, red and green coloured

cockroaches have also been reported in tropical regions. Their size ranges

from ¼ inches to 3 inches (0.6-7.6 cm) and have long antenna, legs and

flat extension of the upper body wall that conceals head. They are

nocturnal omnivores that live in damp places throughout the world. They

have become residents of human homes and thus are serious pests and

vectors of several diseases.

7.4.1 Morphology

The adults of the common species of cockroach, Periplaneta americana

are about 34-53 mm long with wings that extend beyond the tip of the

abdomen in males. The body of the cockroach is segmented and divisible

into three distinct regions – head, thorax and abdomen (Figure 7.14).

The entire body is covered by a hard chitinous exoskeleton (brown in

colour). In each segment, exoskeleton has hardened plates called sclerites

(tergites dorsally and sternites ventrally) that are joined to each other by

a thin and flexible articular membrane (arthrodial membrane).

Figure 7.14 External features of cockroach

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Head is triangular in shape and lies anteriorly at right angles to the

longitudinal body axis. It is formed by the fusion of six segments and

shows great mobility in all directions due to flexible neck (Figure 7.15).

The head capsule bears a pair of compound eyes. A pair of thread like

antennae arise from membranous sockets lying in front of eyes. Antennae

have sensory receptors that help in monitoring the environment. Anterior

end of the head bears appendages forming biting and chewing type of

mouth parts. The mouthparts consisting of a labrum (upper lip), a pair

of mandibles, a pair of maxillae and a labium (lower lip). A median flexible

lobe, acting as tongue (hypopharynx), lies within the cavity enclosed by

the mouthparts (Figure 7.15b). Thorax 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 walking legs. The first pair of wings arises from mesothorax

and the second pair from metathorax. Forewings (mesothoracic) called

tegmina are opaque dark and leathery and cover the hind wings when at

rest. The hind wings are transparent, membranous and are used in flight.

The abdomen in both males and females consists of 10 segments. In

females, the 7

sternum is boat shaped and together with the 8

and 9

sterna forms a brood or genital pouch whose anterior part contains female

gonopore, spermathecal pores and collateral glands. In males, genital pouch

or chamber lies at the hind end of abdomen bounded dorsally by 9

and

terga and ventrally by the 9

sternum. It contains dorsal anus, ventral

male genital pore and gonapophysis. Males bear a pair of short, thread-

like anal styles which are absent in females. In both sexes, the 10

segment

bears a pair of jointed filamentous structures called anal cerci.

Ocellus

Compound eye

Mandible

Maxilla

Labrum

Labium

(a)

Figure 7.15 Head region of cockroach : (a) parts of head region (b) mouth parts

Grinding

region

Incising

region

Mandible

Maxilla

Labrum

Hypopharynx

Labium

Mandible

Maxilla

(b)

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7.4.2 Anatomy

The alimentary canal present in the body cavity

is divided into three regions: foregut, midgut

and hindgut (Figure 7.16). The mouth opens

into a short tubular pharynx, leading to a

narrow tubular passage called oesophagus.

This in turn opens into a sac like structure

called crop used for storing of food. The crop

is followed by gizzard or proventriculus. It has

an outer layer of thick circular muscles and

thick inner cuticle forming six highly chitinous

plate called teeth. Gizzard helps in grinding the

food particles. The entire foregut is lined by

cuticle. A ring of 6-8 blind tubules called

hepatic or gastric caeca is present at the

junction of foregut and midgut, which secrete

digestive juice. At the junction of midgut and

hindgut is present another ring of 100-150

yellow coloured thin filamentous Malpighian

tubules. They help in removal of excretory

products from haemolymph. The hindgut is

broader than midgut and is differentiated into

ileum, colon and rectum. The rectum opens

out through anus.

Blood vascular system of cockroach is an

open type (Figure 7.17). Blood vessels are

poorly developed and open into space

(haemocoel). Visceral organs located in the

haemocoel are bathed in blood (haemolymph).

The haemolymph is composed of colourless

plasma and haemocytes. Heart of cockroach

consists of elongated muscular tube lying

along mid dorsal line of thorax and abdomen.

It is differentiated into funnel shaped chambers

with ostia on either side. Blood from sinuses

enter heart through ostia and is pumped

anteriorly to sinuses again.

The respiratory system consists of a

network of trachea, that open through 10 pairs

of small holes called spiracles present on the

lateral side of the body. Thin branching tubes

(tracheal tubes subdivided into tracheoles)

carry oxygen from the air to all the parts. The

Salivary gland

Pharynx

Salivary

reservoir

Oesophagus

Crop

Gizzard

Hepatic caeca

Mesenteron

or midgut

Malpighian

tubules

Ileum

Colon

Rectum

Figure 7.16 Alimentary canal of cockroach

Anterior aorta

Alary muscles

Chambers

of heart

Figure 7.17 Open circulatory system of cockroach

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114 BIOLOGY

opening of the spiracles is regulated by the sphincters. Exchange of gases

take place at the tracheoles by diffusion.

Excretion is performed by Malpighian tubules. Each tubule is lined

by glandular and ciliated cells. They absorb nitrogenous waste products

and convert them into uric acid which is excreted out through the hindgut.

Therefore, this insect is called uricotelic. In addition, the fat body,

nephrocytes and urecose glands also help in excretion.

The nervous system of cockroach 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 of cockroach is spread throughout the body. The

head holds a bit of a nervous system while the rest is situated along the

ventral (belly-side) part of its body. So, now you understand that if the

head of a cockroach is cut off, it will still live for as long as one week. In

the head region, the brain is represented by supra-oesophageal ganglion

which supplies nerves to antennae and compound eyes. In cockroach,

the sense organs are antennae, eyes, maxillary palps, labial palps, anal

cerci, etc. The compound eyes are situated at the dorsal surface of the

head. Each eye consists of about 2000 hexagonal ommatidia

(sing.: ommatidium). With the help of several ommatidia, a cockroach can

receive several images of an object. This kind of vision is known as mosaic

vision with more sensitivity but less resolution, being common during

night (hence called nocturnal vision).

Cockroaches are dioecious and both sexes have well developed

reproductive organs (Figure 7.18). Male reproductive system consists of

a pair of testes one lying on each lateral side in the 4

-6

abdominal

segments. From each testis arises a thin vas deferens, which opens into

ejaculatory duct through seminal vesicle. The ejaculatory duct opens into

male gonopore situated ventral to anus. A characteristic mushroom-

shaped gland is present in the 6

-7

abdominal segments which functions

as an accessory reproductive gland. The external genitalia are represented

by male gonapophysis or phallomere (chitinous asymmetrical structures,

surrounding the male gonopore). The sperms are stored in the seminal

vesicles and are glued together in the form of bundles called

spermatophores which are discharged during copulation. The female

reproductive sysytem consists of two large ovaries, lying laterally in the

– 6

abdominal segments. Each ovary is formed of a group of eight

ovarian tubules or ovarioles, containing a chain of developing ova.

Oviducts of each ovary unite into a single median oviduct (also called

vagina) which opens into the genital chamber. A pair of spermatheca is

present in the 6

segment which opens into the genital chamber.

Sperms are transferred through spermatophores. Their fertilised eggs

are encased in capsules called oothecae. Ootheca is a dark reddish to

blackish brown capsule, about 3/8" (8 mm) long. They are dropped or

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glued to a suitable surface, usually in a crack or crevice of high relative

humidity near a food source. On an average, females produce 9-10

oothecae, each containing 14-16 eggs. The development of P. americana

is paurometabolous, meaning there is development through nymphal

stage. The nymphs look very much like adults. The nymph grows by

moulting about 13 times to reach the adult form. The next to last nymphal

stage has wing pads but only adult cockroaches have wings.

Many species of cockroaches are wild and are of no known economic

importance yet. A few species thrive in and around human habitat. They are

pests because they spoil food and contaminate it with their smelly excreta.

They can transmit a variety of bacterial diseases by contaminating food material.

Testis

Phallic gland

Small tubules

Long tubules

Seminal vesicle

Vas deferens

Ejaculatory duct

Right phallomere

Ventral phallomere

Anal cercus

Caudal style

Pseudopenis

Titillator

Left phallomere

Ovary

Oviduct

Common oviduct

or vagina

Collaterial glands

Genital chamber

Vestibulum

Genital

pouch

Spermatheca

gonapophyses

]

Figure 7.18 Reproductive system of cockroach : (a) male (b) female

(a)

(b)

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116 BIOLOGY

7.5 FROGS

Frogs can live both on land and in freshwater and belong to class

Amphibia of phylum Chordata. The most common species of frog found

in India is Rana tigrina.

They do not have constant body temperature i.e., their body

temperature varies with the temperature of the environment. Such animals

are called cold blooded or poikilotherms. You might have also noticed

changes in the colour of the frogs while they are in grasses and on dry

land. They have the ability to change the colour to hide them from their

enemies (camouflage). This protective coloration is called mimicry. You

may also know that frogs are not seen during peak summer and winter.

During this period they take shelter in deep burrows to protect them

from extreme heat and cold. This is known as summer sleep (aestivation)

and winter sleep (hibernation) respectively.

7.5.1 Morphology

Have you ever touched the skin of frog? The skin is smooth and slippery

due to the presence of mucus. The skin is always maintained in a moist

condition. The colour of dorsal side of body is generally olive green with

dark irregular spots. On the ventral side the skin is uniformly pale yellow.

The frog never drinks water but absorb it through the skin.

Body of a frog is divisible into head and trunk (Figure 7.19). A neck

and tail are absent. Above the mouth, a pair of nostrils is present. Eyes

are bulged and covered by a nictitating membrane that protects them

while in water. On either side of eyes a membranous

tympanum (ear) receives sound signals. The

forelimbs and hind limbs help in swimming,

walking, leaping and burrowing. The hind limbs end

in five digits and they are larger and muscular than

fore limbs that end in four digits. Feet have webbed

digits that help in swimming. Frogs exhibit sexual

dimorphism. Male frogs can be distinguished by the

presence of sound producing vocal sacs and also a

copulatory pad on the first digit of the fore limbs

which are absent in female frogs.

7.5.2 Anatomy

The body cavity of frogs accommodate different organ systems such as

digestive, circulatory, respiratory, nervous, excretory and reproductive

systems with well developed structures and functions (Figure 7.20).

The digestive system consists of alimentary canal and digestive glands.

The alimentary canal is short because frogs are carnivores and hence the

length of intestine is reduced. The mouth opens into the buccal cavity that

Figure 7.19 External features of frog

Eye

Fore limb

Hind limb

Head

Trunk

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leads to the oesophagus through pharynx. Oesophagus is a short tube

that opens into the stomach which in turn continues as the intestine, rectum

and finally opens outside by the cloaca. Liver secretes bile that is stored in

the gall bladder. Pancreas, a digestive gland produces pancreatic juice

containing digestive enzymes. Food is captured by the bilobed tongue.

Digestion of food takes place by the action of HCl and gastric juices secreted

from the walls of the stomach. Partially digested food called chyme is passed

from stomach to the first part of the small intestine, the duodenum. The

duodenum receives bile from gall bladder and pancreatic juices from the

pancreas through a 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 numerous finger-like folds in

the inner wall of intestine called villi and microvilli. The undigested solid

waste moves into the rectum and passes out through cloaca.

Frogs respire on land and in the water by two different methods. In

water, skin acts as aquatic respiratory organ (cutaneous respiration).

Dissolved oxygen in the water is exchanged through the skin by diffusion.

Figure 7.20 Diagrammatic representation of internal organs of frog showing

complete digestive system

Intestine

Ureter

Rectum

Cloaca

Urinary

bladder

Oesophagus

Liver

Stomach

Kidney

Gall

bladder

Fat bodies

Lung

Heart

Cloacal Aperture

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118 BIOLOGY

On land, the buccal cavity, skin and lungs act as the respiratory organs.

The respiration by lungs is called pulmonary respiration. The lungs are

a pair of elongated, pink coloured sac-like structures present in the upper

part of the trunk region (thorax). Air enters through the nostrils into the

buccal cavity and then to lungs. During aestivation and hibernation

gaseous exchange takes place through skin.

The vascular system of frog is well-developed closed type. Frogs have

a lymphatic system also. The blood vascular system involves heart, blood

vessels and blood. The lymphatic system consists of lymph, lymph

channels and lymph nodes. Heart is a muscular structure situated in the

upper part of the body cavity. It has three chambers, two atria and one

ventricle and is covered by a membrane called pericardium. A triangular

structure called sinus venosus joins the right atrium. It receives blood

through the major veins called vena cava. The ventricle opens into a sac-

like conus arteriosus on the ventral side of the heart. The blood from the

heart is carried to all parts of the body by the arteries (arterial system).

The veins collect blood from different parts of body to the heart and form

the venous system. Special venous connection between liver and intestine

as well as the kidney and lower parts of the body are present in frogs. The

former is called hepatic portal system and the latter is called renal portal

system. The blood is composed of plasma and cells. The blood cells are

RBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytes

and platelets. RBC’s are nucleated and contain red coloured pigment

namely haemoglobin. The lymph is different from blood. It lacks few

proteins and RBCs. The blood carries nutrients, gases and water to the

respective sites during the circulation. The circulation of blood is achieved

by the pumping action of the muscular heart.

The elimination of nitrogenous wastes is carried out by a well

developed excretory system. The excretory system consists of a pair of

kidneys, ureters, cloaca and urinary bladder. These are compact, dark

red and bean like structures situated a little posteriorly in the body cavity

on both sides of vertebral column. Each kidney is composed of several

structural and functional units called uriniferous tubules or nephrons.

Two ureters emerge from the kidneys in the male frogs. The ureters act as

urinogenital duct which opens into the cloaca. In females the ureters and

oviduct open seperately in the cloaca. The thin-walled urinary bladder is

present ventral to the rectum which also opens in the cloaca. The frog

excretes urea and thus is a ureotelic animal. Excretory wastes are carried

by blood into the kidney where it is separated and excreted.

The system for control and coordination is highly evolved in the frog. It

includes both neural system and endocrine glands. The chemical

coordination of various organs of the body is achieved by hormones which

are secreted by the endocrine glands. The prominent endocrine glands

found in frog are pituitary, thyroid, parathyroid, thymus, pineal body,

pancreatic islets, adrenals and gonads. The nervous system is organised

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into a central nervous system (brain and spinal

cord), a peripheral nervous system (cranial and

spinal nerves) and an autonomic nervous system

(sympathetic and parasympathetic). There are ten

pairs of cranial nerves arising from the brain. Brain

is enclosed in a bony structure called brain box

(cranium). The brain is divided into fore-brain,

mid-brain and hind-brain. Forebrain includes

olfactory lobes, paired cerebral hemispheres and

unpaired diencephalon. The midbrain is

characterised by a pair of optic lobes. Hind-brain

consists of cerebellum and medulla oblongata.

The medulla oblongata passes out through the

foramen magnum and continues into spinal cord,

which is enclosed in the vertebral column.

Frog has different types of sense organs, namely

organs of touch (sensory papillae), taste (taste

buds), smell (nasal epithelium), vision (eyes) and

hearing (tympanum with internal ears). Out of

these, eyes and internal ears are well-organised

structures and the rest are cellular aggregations

around nerve endings. Eyes in a frog are a pair of

spherical structures situated in the orbit in skull.

These are simple eyes (possessing only one unit).

External ear is absent in frogs and only tympanum

can be seen externally. The ear is an organ of

hearing as well as balancing (equilibrium).

Frogs have well organised male and female

reproductive systems. Male reproductive organs

consist of a pair of yellowish ovoid testes (Figure

7.21), which are found adhered to the upper part

of kidneys by a double fold of peritoneum called

mesorchium. Vasa efferentia are 10-12 in

number that arise from testes. They enter the

kidneys on their side and open into Bidder’s

canal. Finally it communicates with the

urinogenital duct that comes out of the kidneys

and opens into the cloaca. The cloaca is a small,

median chamber that is used to pass faecal

matter, urine and sperms to the exterior.

The female reproductive organs include a pair

of ovaries (Figure 7.22). The ovaries are situated

near kidneys and there is no functional

connection with kidneys. A pair of oviduct arising

Figure 7.22 Female reproductive system

Oviduct

Ovary

Ova

Ureter

Cloaca

Cloacal aperture

Urinary

bladder

Figure 7.21 Male reproductive system

Fat

bodies

Kidney

Urino

genital duct

Cloaca

Cloacal

aperture

Testis

Adrenal

gland

Urinary

bladder

Rectum

Vasa

efferentia

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120 BIOLOGY

from the ovaries opens into the cloaca separately. A mature female can

lay 2500 to 3000 ova at a time. Fertilisation is external and takes place in

water. Development involves a larval stage called tadpole. Tadpole

undergoes metamorphosis to form the adult.

Frogs are beneficial for mankind because they eat insects and protect

the crop. Frogs maintain ecological balance because these 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.

SUMMARY

Cells, tissues, organs and organ systems split up the work in a way that ensures

the survival of the body as a whole and exhibit division of labour. A tissue is

defined as group of cells along with intercellular substances performing one or

more functions in the body. Epithelia are sheet like tissues lining the body’s surface

and its cavities, ducts and tubes. Epithelia have one free surface facing a body

fluid or the outside environment. Their cells are structurally and functionally

connected at junctions.

Diverse types of connective tissues bind together, support, strengthen, protect,

and insulate other tissue in the body. Soft connective tissues consist of protein

fibres as well as a variety of cells arranged in a ground substance. Cartilage, bone,

blood, and adipose tissue are specialised connective tissues. Cartilage and bone

are both structural materials. Blood is a fluid tissue with transport functions.

Adipose tissue is a reservoir of stored energy. Muscle tissue, which can contract

(shorten) in response to stimulation, helps in movement of the body and specific

body parts. Skeletal muscle is the muscle tissue attached to bones. Smooth muscle

is a component of internal organs. Cardiac muscle makes up the contractile walls

of the heart. Connective tissue covers all three types of tissues. Nervous tissue

exerts greatest control over the response of body. Neurons are the basic units of

nervous tissue.

Earthworm, Cockroach and Frog show characteristic features in body

organisation. In Pheretima posthuma (earthworm), the body is covered by cuticle.

All segments of its body are alike except the 14

, 15

and 16

segment, which are

thick and dark and glandular, forming clitellum. A ring of S-shaped chitinous

setae is found in each segment. These setae help in locomotion. On the ventral

side spermathecal openings are present in between the grooves of 5 and 6, 6 and

7, 7 and 8 and 8 and 9 segments. Female genital pores are present on 14

segment

and male genital pores on 18

segment. The alimentary canal is a narrow tube

made of mouth, buccal cavity, pharynx, gizzard, stomach, intestine and anus.

The blood vascular system is of closed type with heart and valves. Nervous system

is represented by ventral nerve cord. Earthworm is hermaphorodite. Two pairs of

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testes occur in the 10

and 11

segment, respectively. A pair of ovaries are present

on 12 and 13

intersegmental septum. It is a protandrous animal with cross-

fertilisation. Fertilisation and development take place in cocoon secreted by the

glands of clitellum.

The body of Cockroach (Periplaneta americana) is covered by chitinous

exoskeleton. It is divided into head, thorax and abdomen. Segments bear jointed

appendages. There are three segments of thorax, each bearing a pair of walking

legs. Two pairs of wings are present, one pair each on 2

and 3

segment. There

are ten segments in abdomen. Alimentary canal is well developed with a mouth

surrounded by mouth parts, a pharynx, oesophagus, crop, gizzard, midgut,

hindgut and anus. Hepatic caecae are present at the junction of foregut and

midgut. Malpighian tubules are present at the junction of midgut and hindgut

and help in excretion. A pair of salivary gland is present near crop. The blood

vascular system is of open type. Respiration takes place by network of tracheae.

Trachea opens outside with spiracles. Nervous system is represented by

segmentally arranged ganglia and ventral nerve cord. A pair of testes is present in

-6

segments and ovaries in 2

-6

segments. Fertilisation is internal. Female

produces 9-10 ootheca bearing developing embryos. After rupturing of single

ootheca sixteen young ones, called nymphs come out.

The Indian bullfrog, Rana tigrina, is the common frog found in India. Body is

covered by skin. Mucous glands are present in the skin which is highly vascularised

and helps in respiration in water and on land. Body is divisible into head and trunk.

A muscular tongue is present, which is bilobed at the tip and is used in capturing

the prey. The alimentary canal consists of oesophagous, stomach, intestine and

rectum, which open into the cloaca. The main digestive glands are liver and pancreas.

It can respire in water through skin and through lungs on land. Circulatory system

is closed with single circulation. RBCs are nucleated. Nervous system is organised

into central, peripheral and autonomic. The organs of urinogenital system are kidneys

and urinogenital ducts, which open into the cloaca. The male reproductive organ is

a pair of testes. The female reproductive organ is a pair of ovaries. A female lays

2500-3000 ova at a time. The fertilisation and development are external. The eggs

hatch into tadpoles, which metamorphose into frogs.

EXERCISES

1. Answer in one word or one line.

(i) Give the common name of Periplanata americana.

(ii) How many spermathecae are found in earthworm?

(iii) What is the position of ovaries in cockroach?

(iv) How many segments are present in the abdomen of cockroach?

(v) Where do you find Malpighian tubules?

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122 BIOLOGY

2. Answer the following:

(i) What is the function of nephridia?

(ii) How many types of nephridia are found in earthworm based on

their location?

3. Draw a labelled diagram of the reproductive organs of an earthworm.

4. Draw a labelled diagram of alimentary canal of a cockroach.

5. Distinguish between the followings

(a) Prostomium and peristomium

(b) Septal nephridium and pharyngeal nephridium

6. What are the cellular components of blood?

7. What are the following and where do you find them in animal body.

(a) Chondriocytes

(b) Axons

8. Describe various types of epithelial tissues with the help of labelled diagrams.

9. Distinguish between

(a) Simple epithelium and compound epithelium

(b) Cardiac muscle and striated muscle

(d) Adipose and blood tissue

(e) Simple gland and compound gland

10. Mark the odd one in each series:

(a) Areolar tissue; blood; neuron; tendon

(b) RBC; WBC; platelets; cartilage

(d) Maxilla; mandible; labrum; antennae

(e) Protonema; mesothorax; metathorax; coxa

11. Match the terms in column I with those in column II:

Column I Column II

(a) Compound epithelium (i) Alimentary canal

(b) Compound eye (ii) Cockroach

(d) Open circulatory system (iv) Mosaic vision

(e) Typhlosole (v) Earthworm

(f) Osteocytes (vi) Phallomere

(g) Genitalia (vii) Bone

12. Mention breifly about the circulatory system of earthworm

13. Draw a neat diagram of digestive system of frog.

14. Mention the function of the following

(a) Ureters in frog

(b) Malpighian tubules

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