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1

Aminul Sir

Assistant Professor, CESC
01912 - 85 33 95

Biology 2nd Paper : Zoology

CHAPTER TWO [Revised: 2023]
Subject code:

Animal’s Identity
Hydra – Hydra vulgaris

1

7

9

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Topic - 1 

i) Discover & Naming

Introduction :

ii) Systematic position / Classification
iii) Habit & Habitat
iv) Hydra species in Bangladesh
...


‘Hydra’ named dragon from Greek myth

ii) Internal structure : Mesogloea & Gastrovascular cavity i
...
Coelenteron
...

ii) Cells of Gastrodermis or Endodermis : 5 types of cells
...


Topic - 5



Topic - 6 

Food intake / Feeding and Digestion
...


Topic - 7 

A) Asexual reproduction and

Reproduction :

B) Sexual reproduction
...


Topic - 9 

Practical : Observation of the permanent slide / model of Hydra –
i) Physical structure i
...
whole mount
ii) Longitudinal section (L
...
)
iii) Transverse section (T
...


             

2
Topic – 1

Introduction :



i) Discover & Naming
ii) Systematic position / Classification
iii) Habit & Habitat

Aminul Sir

iv) Hydra species in Bangladesh
...
Hydra is a simple fresh water inhabitant under
phylum Cnidaria
...

2) In 1744, Swiss naturalist Abraham Trembley (1710-1784) first discovered the Hydra
...
[ In some books - In 1702, Antonie van Leeuwenhoek first discovered the Hydra
...

4) Among its other characteristics, ‘Regeneration’ is one
...
Nephilim (মহাবীর) Hercules killed it later on
...
vulgaris

= Colourless / Yellowish-brown in colour

H
...
viridis) = Green in colour
H
...
fusca)
-

= Brown in colour
...


iii) Habit & Habitat :
1) Hydra is a solitary (i
...
free living), polypoid (i
...
in the form of a polyp), fresh water animal
...

2) Though the other individuals under phylum Cnidaria are marine, only 20 species of Cnidaria live in fresh water
...

3) Hydra is a carnivorous animal
...

4) They perform respiration and excretion through diffusion process by using their body wall
...

[ P
...
O
...
According to ‘Encyclopedia of Flora and Fauna of Bangladesh: Vol
...
viridis) = Green in colour
3) H
...
fusca)

= Brown in colour
...
e
...


i) External morphology : Hypostome, Trunk & Pedal disc
Hydra’s body is radial symmetric, soft, tubular, 10–30 mm in length & 1 mm in width
...
e
...
e
...
‘Mouth aperture’ is positioned at open end & closed end is attached with any
substratum
...
A mature Hydra’s body can be divided into 3 parts –
1) Hypostome

Mouth aperture

2) Trunk
3) Pedal disc

Tentacles

Hypostome
Testes
Bud
Ovary

Pedal disc

Fig
...
Hypostome is conical body part, can contract & expand
...
Food is taken and
undigested food residues are expelled through mouth aperture
...
The following parts are
found at the Trunk –
i) Tentacle: Encircling the mouth aperture, longer than body height, 6–10 numbers of narrow & hollow, thread-like
structures are called ‘Tentacles’
...
Each battery carries different types of Nematocyst
...


[ P
...
O
...
In such environment, one or
more buds are formed from almost the middle part of the body
...
Budding is
a type of regular asexual reproduction process in Hydra
...
During Autumn, Late autumn and Winter, when there is
scarcity of food in nature, Hydra produces the temporary sex organ i
...
gonad in the body
...
Testis) and ‘Ovaries’ are developed at upper half and lower half of the Trunk, respectively
...

3) Pedal disc: Pedal disc is round and flattened part of the body and is positioned at the posterior to the Trunk
...
Besides, epidermal cells of Pedal disc form
‘Bubble’, so that Hydra can float on water surface
...
This type of locomotion is known as Gliding or Amoeboid movement
...
e
...
In mature animals, these two germ layers are transformed into ‘Epidermis’ and
‘Gastrodermis’, respectively
...
Sometime, several cells
and fibres are lying at Mesogloea
...
Hydra’s body is made up of ‘Body wall’ and
central ‘Gastrovascular cavity / Coelenteron’
...
Actually, Mesogloea is created from the secretion of epidermis and gastrodermis and its diameter is 0
...

Mesogloea of tentacle is thinnest but thickest in pedal disc
...

Functions :
1) Mesogloea acts as a ‘Basement membrane’ for the cells of epidermis and gastrodermis
...

2) It contains the fibres of Sensory & Nerve cells and the ‘myoneme’ named contractile myofibrils of Musculo-epithelial cell
...

4) Its elasticity helps to expand the body after contraction
...
e
...
e
...
It communicates with the outer side of the body through only a single oral pore ‘Mouth
aperture’ and that’s why, Coelenteron also called ‘Blind gut’ or ‘Blind sac’
...


[ P
...
O
...

2) It takes part in extra-cellular digestion or inter-cellular digestion, egestion etc and due to this, it is called ‘Gastrovascular
cavity’
...
e
...

4) It contains water and thus works as ‘Hydroskeleton’
...
: L
...
of Hydra

Fig
...
S
...


i) Cells of Epidermis : 7 types of cells
A thin & flexible ‘Cuticle’ encloses the epidermis and forms the outermost layer of Hydra
...
The outer side of the cell is
expanded, while inner end narrow
...
Two contractile-expandable fibres called ‘Myoneme’ lies parallelly to the body axis
...
At one side,
these cell work as body covering; another side, function as muscle and due to these, the cells are called ‘Musculo-Epithelial cells’
...
T
...
]

6
Functions :
1) Protect the body by forming epithelium like epithelial cells
...

3) Mucous particles secrete cuticle and make the body slippery
...


Aminul Sir

Assistant Professor, CESC
01912 - 85 33 95

1
...
Cnidocyte

Cuticle
2
...
Sensory cell

Epidermis

Mesogloea
4
...
: T
...
showing the cells of Epidermis of Hydra (Partial)

2) Interstitial cell: In between the Musculo-epithelial cells and adjoining to mesogloea, a cluster of round or triangular cells,
are known as ‘Interstitial cells’
...

Functions :
1) When necessary, this cell transforms into any type of epidermal cell
...

3) After several days, they convert into other cells and replace old cells of the body
...
They are more abundant at tentacles, hypostome & pedal disc
...
Distal end of the cell
gives rise to fine ‘sensory hair’, while ‘nodulated fine fibre’ originates from the proximal end and attaches with nerve fibres
...


4) Nerve cell: Nerve cell consists of a cell body and two or more nodulated fine branched ‘Nerve fibres (process)’
...
Nerve fibres of the nerve cells join together, build ‘Nerve-network’
...

[ P
...
O
...
They are transformed epidermal cells and
elongated in shape
...

These fibres arrange radially at mesogloea from the centre of pedal disc
...


Assistant Professor, CESC
01912 - 85 33 95

2) Secretes mucus (sticky juice) which helps Hydra to attach with any substratum
...

4) Gland cells of pedal disc make pseudopod and move slowly to cover the very short distance
...


6) Germ cell: Germ cells are lying at gonad
...
Each sperm has - a swollen ‘Head’ with
nucleus, ‘Middle piece’ with centriole and a movable ‘Tail’
...

Functions :
1) Participates in sexual reproduction
...
Developing
Cnidocyte is known as ‘Cnidoblast’
...


Food vacuole
Sensory cell

Functions :
1) Nematocyst of Cnidocyte is used to take food, movement and self-protection
...
The following 5 types of cells form the Endodermis –
1) Nutritive cell or Musculo-Epithelial cell: These cells are more abundant and spread all over the gastrodermis
...
Contractile muscle fibres are originated from proximal end of the cell and
stay at mesogloea by forming right angle
...


ii) Pseudopodial nutritive cell : This cell has pseudopod at its inner open edge (towards coelenteron)
...
T
...
]

8
Functions :
1) Muscle fibres of the cell contract and expand the body
...

3) Flagellated nutritive cells, through their lashing movement convert the food into small particles
...

4) Pseudopodial nutritive cells engulf small food particles and perform intra-cellular digestion
...
Actually,
these cells came from epidermis
...

Functions :
1) When necessary, this cell transforms into any type of endodermal cell
...
Each cell is narrow & elongated
...

Functions :
1) Probably, Sensory cell verifies the quality of food that entered with water in the coelenteron and sends the information to
the nerve cell
...
Nerve cell consists of a cell body and two or
more fine branched ‘Nerve fibres (process)’
...

Functions :
1) Co-ordinate and create appropriate response against stimulus, received by the Sensory cell
...
They are more abundant at mouth aperture (hypostome) and inner body trunk and are less at inner pedal disc
...

Gland cells are 2 types –

H
...
: Make a comparative study between
the cells of Epidermis and Endodermis
...

ii) Enzyme secreting gland cell: These gland cells are lying at all inner trunk and secrete enzymes for digestion
...

2) Gland cells of inner trunk secrete enzyme, which helps to digest the food
...


i) Structure of a Typical Cnidocyte :
The stinging structure bearing epidermal cell of Hydra which exists all over the epidermis, especially in between or inside the
musculo-epithelial cells of tentacle, but absent at the pedal disc, is called ‘Cnidocyte’
...

A typical Cnidocyte resembles round or oval or pear or cup or top shaped
...
Other structures of a typical Cnidocyte is described below –

Open string
Barbule

Cnidocil

Barb

Closed operculum
Butt / Shaft

Butt / Shaft

Open operculum

Barb
Nematocyst
Coiled string
Muscle fibre

Nematocyst
Lasso
Cytoplasm
Nucleus

Fig
...
: Everted Cnidocyte

1) Membrane: Each Cnidocyte cell is covered by a bi-layered membrane
...

Nucleus is positioned at the basal region of the cell
...
nema = thread and kystis = bag / sac
...
It is composed of two parts – i) Capsule, which is filled up by protein & phenol
made toxic liquid ‘Hypnotoxin’ and the – ii) String, which remains winded/coiled within the sac
...
At the butt, 3 large spines are called ‘Barb’ and spirally arranged small spines are called ‘Barbules’
...

3) Operculum: The upper portion of nematocyst and capsule are covered with a lid, called ‘Operculum’
...

4) Cnidocil: Above on the Cnidocyte, a hard, small, sensory thorn-like structure presents at one corner, called ‘Cnidocil’
...
Cnidocil acts as ‘trigger’, therefore the winded string everts outside the cnidocyte
...
Later, all the fibres combine,
connected with mesogloea through the base of Cnidocyte
...


10
ii) Types of Nematocyst :
Based on the characteristics of ‘String’, in 1965 scientist Werner identified 23 types of Nematocyst from Cnidarian
...
Its string is long, hollow and the tip is open
...
Capsule is filled up by protein &
phenol made toxic liquid ‘Hypnotoxin’
...
Thus, prey
becomes paralyzed
...
Its string is short, wide, elastic, spineless and the tip is
closed
...
It helps to encircle the target / prey
...
Butt or shaft
is not well developed
...

4) Stereoline glutinant / Atrichous isorhiza: This Nematocyst is the smallest among the 4 types
...
It also secretes sticky juice which helps Hydra to locomote and captures the prey
...

Whenever, any prey comes closer to Hydra’s tentacles, the combine influence of mechanical stimulus (i
...
movement of prey) and
chemical substances of the body, increase the permeability of capsule membrane only for water
...
To make it dilute, water rushes
into the capsule and the intra-capsular pressure rises
...
e
...

After opening of operculum, three barbs come out, make a hole, by puncturing the integument of prey
...
Finally, hypnotoxin releases inside the body of
prey through the opening of string
...

Nematocyst-thread being used once, it never be returned into the cnidocyte or does not use for second time
...
This cnidocyte goes to gastrovascular cavity or coelenteron and
digested along with the food
...


11
Topic - 5



Food intake / Feeding and Digestion
...
The aquatic animals which can be easily paralyzed by the nematocysts, are
Hydra’s main food, such as - larvae of insects, Cyclops viridis & Daphnia cuculata named crustacean arthropod, small worms,
segmented animals (annelids) & fish egg
...


Process of Food intake: During the time of food intake, tentacles and the nematocysts present in the tentacles work
combinely
...
Volvent nematocyst coils
the appendage (e
...
limbs) of prey and stops its movement
...
Barbs of Stenotile / Penetrant nematocyst puncture the prey’s integument and make a hole
...
Thus, prey becomes paralyzed
...
At last, the prey (food) reaches to the gastrovascular cavity i
...
coelenteron
...
In Hydra, digestion takes
place through 2 steps –
1) Extra-cellular digestion: The digestion process takes place outside the cell, is called ‘Extra-cellular digestion’ or ‘Inter-cellular
digestion’
...
At first, prey dies due to the influence of enzyme
...
This time, flagella of flagellated nutritive cells lash and mix the food with enzyme
...
Pepsin
enzyme converts protein into polypeptide
...

2) Intra-cellular digestion: Due to contraction & expansion of the body wall, food pieces turn into more small particles
...
As a result, a food
vacuole is formed in the cytoplasm
...
Cytoplasm
secretes ‘Trypsin’ which converts polypeptide into amino acids
...
‘Amylase’ converts
amylose (simple carbohydrate, made up of 200–1,000 glucose molecules) into glucose
...

The simplest food ingredients produced by digestion, reach to all body cells through diffusion process
...


12
Topic - 6



Locomotion : Looping, Somersaulting, Gliding / Amoeboid movement,
Floating, Drowning, Swimming, Climbing, Crawling,
Tentacular movement / Walking, Body contraction & expansion
...
Due to different stimuli, such as – food capturing, self protection, defense against adverse environmental
condition, reproduction etc Hydra has to locomote
...
Since it does not have any specific locomotive organ, it
moves with the help of pedal disc, contractile & expandable muscle fibres (myoneme fibres) of Musculo-Epithelial cells contractile
& expandable muscle fibres (myoneme fibres) of Musculo-Epithelial cells, tentacles and three types of nematocysts
...
Muscle fibres receive the information from
nerve-network and start to contract & expand
...

Aminul Sir

Assistant Professor, CESC
01912 - 85 33 95

In Hydra, following types of locomotion are seen –

1) Looping: Generally, Hydra uses ‘Looping’ type locomotion to cover long distance, slowly
...
Then, turns it down to the substratum i
...
a loop is formed towards the direction
of movement
...
Pedal disc is lifted & placed near the
hypostome; and follow the sequence repeatedly, Hydra leaves the place
...
This type of movement is almost like as Caterpillar’s movement
...
: Looping movement

Fig
...
Prior to start this movement,
Hydra bends its body down to the substratum i
...
a loop is formed towards the direction of movement
...
Instead of placing the pedal disc forward, it is rather lifted the trunk upward, to stand
straight based on tentacles
...
Finally, it stands on
the pedal disc to free its tentacles
...
Then, gland cells create pseudopod on that surface and move slowly to cover the very short distance
...


4) Floating: In aquatic environment, gland cells of pedal disc of epidermis form ‘Bubble’, so that Hydra can float on water
surface
...


5) Drowning: Whenever bubble ruptures, Hydra becomes heavy and drowns in water
...


Aminul Sir

Assistant Professor, CESC
01912 - 85 33 95

6) Swimming: In aquatic environment, Hydra can swim freely by moving its tentacles along the water current / wave
...


8) Crawling: Sometimes, Hydra grabs any substance or aquatic plants
...


9) Tentacular movement / Walking: Before starting this movement, Hydra bends its body down to the substratum i
...
a loop is
formed towards the direction of movement
...
Then, Hydra walks by
using the tentacles
...


             

14
Topic - 7



Reproduction :
A)

Asexual reproduction

and

B) Sexual reproduction
...
In Hydra, reproduction is of two types – A) Asexual reproduction & B) Sexual reproduction
...
Here, only one parent is required
...


i) Budding: It is a regular and common asexual reproduction process in Hydra
...
It is done through the following steps –

1) At the beginning of this process, interstitial cells of middle or lower region of epidermis divide rapidly and form a small
protrusion
...

3) The coelenteron cavity of bud is communicated with the mother Hydra’s body to draw the nourishment, until it grows into a
young Hydra, with oral pore, hypostome and tentacles
...
Pedal disc is developed at the detached part of young Hydra
...

5) After detached young Hydra moves for a while, then attaches itself with a sub-merged substratum, collects food and leads
a free life
...
Around 3 (three) weeks required to complete the entire
process
...
For any accidental cause, if
Hydra’s body is divided into two or more fragments, then from each fragment a new Hydra will be grown
...
In 1744, Abraham Trembley for the first time discovered the regeneration power of the Hydra
...
Therefore,
Hydra has no normal death
...

Aminul Sir

Assistant Professor, CESC
01912 - 85 33 95

B) Sexual Reproduction: The reproduction process which involves the fusion of two opposite gametes (i
...
Sperm & Ovum) to
create a zygote in a process called ‘Fertilization’
...
e
...


i) Formation of temporary gonad & gametes i
...
Gametogenesis: Most of the Hydra are dioecious i
...
unisexual
...
e
...
Despite of being monoecious, self fertilization does not occur in that
particular Hydra because the gonads do not mature at the same time
...
e
...


There is no permanent gonad on Hydra’s body
...
e
...
Male sex organ i
...
gonad, is called ‘Testis’ and formation of sperm inside the testis, is
called ‘Spermatogenesis’
...
e
...

[ P
...
O
...
The cells inside the Testis undergo repeated mitosis cell division, form ‘Spermatogonia (2n)’
...
Each spermatocyte divides through meiosis to form 4
haploid ‘Spermatid (n)’ which develop & modify to form 4 active ‘Sperm (n)’
...
At maturity, the sperms (n) rupture the nipple of Testis and come out in water for swimming
...


Fig
...
S
...
: T
...
of Ovary of Hydra

b) Oogenesis: Generally, a few interstitial cells of epidermis of lower half but just above to pedal disc divide several times and
form one or two round ‘Ovary’
...

The oogonium lying at the centre of ovary, enlarge quickly by engulfing the other oogonia and transform into a large ‘Oocyte (2n)’
...
Polar bodies die out shortly, while the ootid modifies to form an active ‘Ovum (n)’
...
Ovum remains surrounded by slippery gelatinous
layer
...
e
...
e
...
Mature sperms (n)
making a group, rupture the nipple of testis, come out in water in search of ovum
...
On the other hand, if the ovum is not fertilized by sperm within few days, it destroys gradually
...
After fertilization diploid ‘Zygote (2n)’ is formed
...
Germinal development starts keeping the zygote attached to the
body of mother Hydra
...
e
...


2) Blastula: Cells of Morula are arranged themselves in one layer by making a hollow at the centre
...
Cells of Blastula are called ‘Blastomere’ and the central hollow cavity is called ‘Blastocoel’
...
Gastrula consists of outer
‘Ectoderm’, inner ‘Endoderm’ & primitive ‘Coelenteron’
...
Mesogloea is formed in between them, by the jelly-like secretions of both the layers
...


Fig
...
This stage is called ‘Cyst’
...


5) Hydrula: At the onset of spring in favourable temperature, embryo becomes elongated within the cyst
...
Hydrula comes out water by rupturing the chitinized spiny outer coat of cyst and leads a free life
...


i) Regeneration of Hydra :
Regeneration is the ability of an organism to replace its lost or damaged body parts
...
In
1744, Swiss naturalist Abraham Trembley (1710-1784) first observed the regeneration power in Hydra
...
Nephilim
(মহাবীর) Hercules killed it later on
...
In this process, interstitial cells of the broken part divide swiftly and create hypostome,
tentacles & mouth aperture at oral end and pedal disc at aboral end
...

Aminul Sir

ii) Division of labour in Hydra :

Assistant Professor, CESC
01912 - 85 33 95

For proper functioning of a multicellular organism, different cells of the body are specialized structurally and physiologically to
perform all the activities
...
Amongst animal kingdom, this phenomenon is first
seen in Hydra as well as Cnidarians
...
Therefore, cells of the body arranged in epidermis & gastrodermis and perform their activities separately
...

2) Interstitial cell: Plays active role in bud formation, creation of testis & ovary including, reforming any part of the body as well
as regeneration
...

3) Sensory cell: Receives environmental stimuli (such as - light, heat etc) and send it to Nerve cell
...
Co-ordinate among the cells and create
appropriate response for each stimulus, received by the Sensory cell
...
T
...
]

19
Gastrodermal or endodermal Gland cells of mouth aperture (hypostome) secrete mucus, which makes the food slippery and
helps to swallow / ingest; Gland cells of inner trunk secrete enzyme, which helps to digest the food
...

7) Cnidocyte: Used in movement, capturing of prey & self protection
...

2) Coelenteron / Gastrovascular cavity: Works as both body cavity and alimentary canal; takes part in extra-cellular digestion
or inter-cellular digestion; contains water and thus works as ‘Hydroskeleton’
...

4) Trunk: Holds bud and reproductive organ i
...
testis & ovary
...

Aminul Sir

Assistant Professor, CESC
01912 - 85 33 95

iii) Symbiosis in Hydra :
Greek ‘sym’ means ‘together’ & ‘bios’ means ‘living’
...
e
...
The two species of
individuals participate in symbiosis, are called ‘Symbiont’
...
viridis)] and
Zoochlorella named unicellular alga show this type of symbiotic association
...
They are benefitted from each other through the following ways –

Ways Zoochlorella gets benefit –
1) Shelter: Zoochlorella gets shelter in the musculo-epithelial cells of gastrodermis of Hydra
...

3) Food production: Nitrogenous waste products derived from the metabolic activities of Hydra, are used for protein synthesis
...
T
...
]

20
Ways Hydra gets benefit –
1) Food availability: The excess food produced by Zoochlorella through photosynthesis, Hydra takes that food and fulfill his
demand of carbohydrates
...

3) CO2 absorption: CO2 produced during respiration in Hydra, is taken by Zoochlorella for photosynthesis and thus, Hydra gets
relief to remove the CO2 from the body
...

Thus, Hydra becomes easily free from the removal of these nitrogenous wastes

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