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Chapter 7
INVERTEBRATA
Unicellular organisms are considered to be in the phylum Protozoa. Multicellular
animals called metazoa are divided into two groups those that have backbones
are called vertebrates; those that do not have backbones are called
invertebrates. More than 95% of all members of the animal kingdom are
invertebrates. They have many diverse forms and ways of life.
Learning objectives:
*
Invertebrates.
*
Diversity among invertebrates.
*
Important features of major groups of invertebrates.
*
Identification of invertebrates around you and their placement in proper
groups.
The Kingdom Animalia is broadly divided into two major groups:
i)
ii)
7.1
Invertebrata: Animals without back bone,
Chordata/Vertebrata: Animals having
supporting, rod like structure) in their body.
a
back
bone
(a
INVERTEBRATES
Animals, which do not have back bone in the bodies, are termed as
invertebrates. They are greater in number, but usually smaller in size than
vertebrates. Invertebrates are divided into many groups or phyla. Some of the
important phyla are described below:
1. Phylum Protozoa:
Phylum protozoa consists of unicellular (single celled), microscopic animals like
organisms, which are now not treated as true animals. They occur either as freeliving (e.g. Amoeba, Paramecium, Euglena, etc.) or as parasites, which depend
upon other living organisms for their nourishment and shelter. Common
protozoan parasites are Plasmodium and Entamoeba histolytica which cause,
important human diseases malaria and dysentery, respectively. Free-living
protozoa are chiefly found in water, both in fresh as well as in marine water.
Paramecium lives in fresh water pond. Its surface is covered by thousands of
very fine, hair-like structures called cilia which help in locomotion. Paramecium
has two nuclei, one is larger called macronucleus while the other one is smaller
called micronucleus. Outside the cell membrane, a flexible, non-living covering
or pellicle provides it a definite shape.
According to the recent classification protozoa is not included in kingdom
animalia but has been placed in kingdom Protoctista (Protista).
2.
Phylum Porifera:
Porifera are the simplest group of multi -cellular (many celled) animals. Their
bodies consist of loose aggregates of cells and lack any kind of tissues. The
entire body has numerous minute pores or ostia. Porifera are commonly termed
as sponges. All sponges are aquatic (live in water) and sessile (fixed at one
place throughout their life). The surrounding water enters into the body through
ostia, which leads through channels into a hollow space inside the body, where
the inner cells of sponges filter out the microscopic food particles. The water
then leaves the body of sponge through another opening, the osculum.
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Sponges have an internal skeleton of hard structures called spicules, which are
formed of calcium carbonate or silica. The spicules provide support and shape to
the multi-cellular body of a sponge. Common examples of sponge are Sycon,
Venus flower basket and bath sponge.
Sycon has a flask-like shape and contains needle-like spicules of calcium
carbonate. It is a colonial form which is found in shallow water in being attached
by a sticky secretion to some submerged solid object like rocks, shells of
molluscs and corals. It is one to three inches in length-and grey or light brown in
colour. The free end of each opening, the osculum.
3.
Phylum Cnidaria (Coelenterata):
Coelenterates are aquatic animals. Most of them are marine but some are found
in fresh water. Cells of coelenterates are organized to form tissues. They are
called diploblastic because their cells are arranged in two layers; an outer
ectoderm and an inner endoderm. In between these two layers are a jelly like,
non-living layer called mesoglea. Many of the coelenterates arc sessile (remain
fixed) (e.g. corals) while others are motile e;g Jelly-fish. They occur as free
living animals, cither living singly e.g. Hydra or in the form of a colony e.g.
Obelia.
Hydra is a fresh water coelenterate which can be better seen with a microscope.
Its body is cylindrical with several thread-like tentacles at its one end encircling
an opening called mouth. Both, the tentacles and the ectoderm bear a number of
poisonous, stinging cells for defence and for paralyzing the prey. The tentacles
are used for capturing the prey. The mouth opens into a hollow space or
coelenteron that lies inside the body. The prey is digested in the coelenteron.
The end of body of hydra opposite to the mouth is normally kept attached to
some object in water.
Jelly fishes live in the sea. They have an umbrella like body with thicker body
walls. They swim freely in water. The mouth is located in the center of the downside of umbrella. It is encircled by four long arms each bearing numerous
stinging cells. The long arms are used for capturing small animals.
4.
Phylum Platyhelminthes:
Platyhclminthes arc commonly called flat -worms because of their flat body.
They occur mostly as parasites of animals as well as man. Howover, some are
free-living such as Planaria which- lives in fresh, water streams. Platyhelminthes
are triploblastic animals because of the development of a middle layer of cells,
the mesoderm, in between ectoderm and endoderm.
Liver fluke is a flat leaf shaped organism about 3 cm in length. It is a parasite in the
liver of sheep, goat and cattle. It attaches itself with its host with the help of two
suckers, one around mouth at the anterior tip while the other slightly behind the first
on the ventral side of the body. It has a bifurcated, highly branched digestive system.
The anus is absent in them. Each liver fluke has both male and female reproductive
organs. It causes considerable damage to the infected cattle.
Tape worms live in the intestine of their vertebrate hosts. Their bodies are long,
ribbon-like and divided into series of segments. The anterior end is some what
rounded and head like. It is provided with four suckers and rings of minute hooks
which help them to attach with their host. Tape worm lacks digestive system,
mouth and anus. It absorbs the digested food of its host through its body
surface. Every segment has a set of male and a female reproductive organs.
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5.
Phylum Nematoda:
Nematodes or round worms have long, smooth, cylindrical body, which is
pointed at both the ends. The body is unsegmented. Nematodes have a complete
and one way digestive tube with mouth at the anterior tip while anus near the
posterior tip.
They are free-living as well as parasites of animals, man and even plants. The
most common round worm infecting man is Ascaris lumbricoides commonly
called human round worm. It lives in the intestine of man. Sexes are separate in
round worm and male is slightly smaller in length than female.
6.
Phylum Annelida:
Animals in this group have elongated, segmented body. The segmentation in annelids
is both external as well as internal. Internally, many organs are repeated in every
segment of the body. Annelids occur in water as well as on land. Most of them are
free living e.g. earth-worm, etc. while few are ectoparasites e.g. leech, etc. They
have well developed organ-systems in their bodies. However, respiration occurs
through their general body surface. An important feature of annelids is their
advanced closed type circulatory system i.e blood circulates in blood vessels
being pumped by. a number of simple hearts.
The leech lives in ponds. It attaches with the help of its suckers to man and
other animals that come in contact with water. It sucks the blood of its host.
7.
Phylum Arthropoda:
Phylum Arthropoda is the largest phylum of Kingdom Animalia. They have
jointed limbs (legs). Their body is covered externally with a hard covering
called exo-skeleton which is made up of a complex chemical called chitin.
Arthropods occur on land as well as in water. The body of an arthropod is usually
divisible into three distinct regions; an anterior head, a middle thorax and a
posterior abdomen. Scorpions, spiders, crabs, prawns, centipedes and insects
are the common arthropods.
Insects are the most abundant animals on earth. They all have three pairs of
legs, two pairs of wings and one pair of sensory hair-like antenna. Many of them
have wings to fly. Insects have special types of mouth parts (appendages for
food handling) according to their mode of feeding, for example mouth parts of
mosquito, housefly, butter fly and cockroach have different types of mouth parts.
Many insects such as mosquitoes hatch out of their eggs in a premature form
and pass through different stages of development to reach the adult stage. This
phenomenon is called metamorphosis.
Metamorphosis is of two types:
i)
Complete metamorphosis
ii)
Incomplete metamorphosis
i)
Complete metamorphosis:
In complete metamorphosis, the young one, the larva, which hatches out of egg
is morphologically completely different from adult. It feeds actively and then
encloses itself in a cover to become pupa. After going through farther changes,
the pupa comes out of its capsule, it is now termed adult. Thus, (The life cycles
complete after passing through these insect passes through egg, larva, pupa and
adult stages. Insects such as butter flies, mosquitoes, house flies, etc. undergo
complete metamorphosis.
ii)
Incomplete metamorphosis:
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In this type, the young one, called nympn which emerges out of egg is a
miniature young one which closely resembles the adult except that it lacks wings
and its reproductive organs are immature. After the development of wings and
maturity of reproductive organs, the nymph becomes adult. Incomplete
metamorphosis is observed in insects like grass-hopper, cockroach, termite, etc.
Common insects are house-fly, butter-fly, mosquito, cockroach, honey-bee,
termite, etc.
8.
Phylum Mollusca:
Animals of this group have a soft, fleshy body enclosed in a thin fleshy cover
called mantle. Many molluscs are covered over externally by a hard, non-living
shell made up of calcium carbonate. They mostly occur in water but some are
found on land. A fleshy organ or foot is given out of the shell on the ventral side
of the body. It helps in locomotion. The structure of shell plays an important role
in identification and further grouping of molluscs. Common examples of molluscs
are snail, pearl oyster, squid, octopus, and fresh water mussel. Squids are the
largest invertebrates.
Snails have spirally coiled shells and are commonly seen in lawns and gardens.
Fresh water mussels are aquatic and have a shell composed of two plates joined
together by a hinge joint.
9.
Phylum Echinodermata:
Echinodermates have rough, spiny skin. They are found only in the sea. They are
pentamerous and have a unique water vascular system which pumps the
external water throughout their bodies. This vascular system operates the soft,
tube shaped feet (tube feet) which enable the animal to glide over the rocks in
water. They do not have head, eyes or even brain. Common examples of phylum
echinodermata are star-fish-, sand-dollar, sea-cucumber, sea urchins etc.
Star-fish has a plate-like central disc with five or more arms extended out from
it. Sea Urchin has a ball-like body without arms. Its body is covered over
externally by long spines.
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SUMMARY
Invertebrates do not have back bones in their bodies.
Invertebrates are divided into a number of smaller groups or phyla.
Major phyla of invertebrates are Protozoa, Porifera, Coelentcrata,
Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca and
Echinodermata.
Protozoa are unicellular animal like organisms which are now placed in
kingdom Protoctista (Protista).
Porifera or sponges are multicellular but without
tissues. Coclentcrates are multicellular and diploblastic.
Platyhelminthes are flat and triploblastic.
Nematodes have elongated, un-segmented body pointed at both the
ends. Annelids are commonly known as segmented worms.
Arthropoda is the largest phylum. They have jointed legs. Around 95% of
the arthropods are insects. Insects have 3 pairs of jointed legs.
Molluscs have soft fleshy body enclosed in a soft membranous cover
called mantle which may be covered over by a hard shell.
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EXERCISE
1.
Fill in the blanks with appropriate terms:
i)
ii)
iii)
iv)
v)
vi)
vii)
2.
Write whether the following statements are true or false:
i)
ii)
iii)
iv)
3.
ii)
iii)
iv)
It is not multicellular.
(a) Porifera (b) Arthropoda
(c) Nematoda (d) Protozoa.
These are exclusively marine.
(a) Protozoa (b) Annelida
(c) Echinodermata (d) Mollusca.
Spiders do not have
(a) Jointed legs
(b) Wings (c) Exoskeleton
(d)Head
Which of the following is free-living?
(a) Plasmodium
(b) Tapeworm
(c) Hydra
(d) Liver fluke.
Write detailed answers to the following questions:
i)
ii)
iv)
iv)
5.
Dysentery is caused by Plasmodium.
Nematodes are segmented worms.
Spider, prawn, scorpion belong to phylum Annelida.
Water vascular system is a feature of sponges.
Encircle the appropriate answer:
i)
4.
The external, protective covering which develops in
arthropods is called __________.
Malaria is caused by _________.
Animals fixed at one place are called ________.
_________ is a parasite in the liver of sheep.
House flics, mosquitoes, butterflies belong to class ________.
Molluscs move by means of a fleshy organ called _________.
Echinodermates have ________ skin.
State and explain important characters of round worms
and segmented worms.
Discuss important characters of Phylum Arthropoda.
Give general characters of echinodermates.
Discuss important features of diploblastic animals. .
Write short answers to the following questions:
i)
Sponges are sessile. How do they get food while staying at
one place?
ii)
You have studied different groups of animals. Make a list of few
tripoblastic invertebrates beginning from the simplest forms.
iii)
iv)
What do you know about insects?
What is exoskeleton? Name two phyla of animals
having exoskeleton.
Define parasite. Name five parasites and the phyla to which
they belong.
Write short notes on Hydra and tape worm.
v)
vi)
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