Respiration in invertebrate
Respiratory System Principles
1. Movement of an oxygen-containing medium so it contacts a moist membrane
overlying blood vessels.
2. Diffusion of oxygen from the medium into the blood.
3. Transport of oxygen to the tissues and cells of the body.
4. Diffusion of oxygen from the blood into cells.
5. Carbon dioxide follows a reverse path.
The Respiratory System and Gas Exchange
Cellular respiration involves the breakdown of organic molecules to produce ATP. A
sufficient supply of oxygen is required for the aerobic respiratory machinery of Kreb's
Cycle and the Electron Transport System to efficiently convert stored organic
energy into energy trapped in ATP. Carbon dioxide is also generated by cellular
metabolism and must be removed from the cell. There must be an exchange of gases:
carbon dioxide leaving the cell, oxygen entering. Animals have organ systems involved
in facilitating this exchange as well as the transport of gases to and from exchange areas.
Bodies and Respiration
Single-celled organisms exchange gases directly across their cell membrane. However,
the slow diffusion rate of oxygen relative to carbon dioxide limits the size of single-celled
organisms. Simple animals that lack specialized exchange surfaces have flattened,
tubular, or thin shaped body plans, which are the most efficient for gas exchange.
However, these simple animals are rather small in size.
Large animals cannot maintain gas exchange by diffusion across their outer surface. They
developed a variety of respiratory surfaces that all increase the surface area for exchange,
thus allowing for larger bodies. A respiratory surface is covered with thin, moist
epithelial cells that allow oxygen and carbon dioxide to exchange. Those gases can only
cross cell membranes when they are dissolved in water or an aqueous solution, thus
respiratory surfaces must be moist.
Methods of Respiration
Single-celled organisms exchange gases directly across their cell membrane. Sponges
and jellyfish lack specialized organs for gas exchange and take in gases directly from the
surrounding water. Flatworms and annelids use their outer surfaces as gas exchange
surfaces. Arthropods, annelids, and fish use gills; terrestrial vertebrates utilize
internal lungs.
Figure 1: showing booklung and tracheae
The Body
Surface (cutaneous respiration)
Flatworms and annelids use their outer surfaces as gas exchange surfaces. Earthworms
have a series of thin-walled blood vessels known as capillaries. Gas exchange occurs at
capillaries located throughout the body as well as those in the respiratory surface.
Gills greatly increase the surface area for gas exchange. They occur in a variety of animal
groups including arthropods (including some terrestrial crustaceans), annelids, fish, and
amphibians. Gills typically are convoluted outgrowths containing blood vessels covered
by a thin epithelial layer. Typically gills are organized into a series of plates and may be
internal (as in crabs and fish) or external to the body (as in some amphibians).
Gills are very efficient at removing oxygen from water: there is only 1/20 the amount of
oxygen present in water as in the same volume of air. Water flows over gills in one
direction while blood flows in the opposite direction through gill capillaries. This
countercurrent flow maximizes oxygen transfer.
Figure II
Tracheal Systems
Many terrestrial animals have their respiratory surfaces inside the body and connected to
the outside by a series of tubes. Tracheae are these tubes that carry air directly to cells for
gas exchange. Spiracles are openings at the body surface that lead to tracheae that branch
into smaller tubes known as tracheoles. The tubes branch repeatedly so that extremely
fine tubules, tracheoles, reach the individual cells or small groups of cells inside the
body. As you can imagine, the surface they provide for respiratory exchange is very
large Body movements or contractions speed up the rate of diffusion of gases from
tracheae into body cells. However, tracheae will not function well in animals whose body
is longer than 5 cm.
Figure III
Respiratory system in an insect.
FIGURE IV -- Generalized Diagram of Insect Tracheal System
FIGURE V -- Schematic Diagram of One Part of the Tracheal System Showing
Relationships of Spiracle, Trachea and Tracheoles to the Body Wall and Internal
Lungs are ingrowths of the body wall and connect to the outside by as series of tubes and
small openings. Lung breathing probably evolved about 400 million years ago. Lungs are
not entirely the sole property of vertebrates, some terrestrial snails have a gas exchange
structures similar to those in frogs.
Arthropods have open circulatory systems with a prominent heart receiving blood from
the hemocoel and pumping it into vessels for distribution to the body. Depending on the
type of respiratory organ the arthropod possesses, the circulatory system may or may not
be important in the transport of oxygen to the body tissues. Those animals in which the
blood is not important for the circulation of respiratory gases (eg. insects) may have no
respiratory pigment.
protozoan, porifera and coelenterate ---Exchange of respiratory gases (CO 2 and O 2 )
by diffusion through body surface.
platyhelminthes and nemahelminthes---Anaerobic respiration. Energy (ATP) produce
by glycolysisGlycogen is broken down into volatile fatty aci, CO 2 and energy. CO 2 is
released ot through body surface.
ANNELIDA --- respiratory organ is lacking. Gas exchange through skin (cutaneous
respiration), gills (branchial respiration), parapodia.
Parapodium is highly involved in (polychaetae) this process.
Each parapodium has a capillary network and is richly supplied with blood. Body wall
dorso-ventrally supplied with blood capillaries.
Haemocoelomic fluid(blood) obtain oxygen through this network. Respiratory pigment
haemoglobin enhance the intake of oxygen by haemocoelomic fluid.
No special structure in Hirudinae and oligochaetae but by cutaneous respiration.
ARTHROPODA -Aquatic arthropods generally have gills for respiration except for a few extremely small
species in which there are no special respiratory structures.
Terrestrial arthropods use several different respiratory organs, the most unique of which
is the tracheal system.
Two type of respiration in arthropods are Aquatic respiration and Aerial respiration.
Aquatic respiration--- use the dissolved oxygen. Aquatic respiration is carried out by
the following ways:
Gills are delicate feather-like outgrowth of the thoracic appendages eg palaemon( prawn)
and penaeus(scorpion), crab and tracheal gills are found in mayfly, damselfly and
stonefly. Larvae have gills (blood gills and book gills) and crustacean through body
Gills are highly vascularised.
Aerial respiration: utilizes the oxygen from the air. This mode found in terrestrial
arthopods. Following are the aerial respiratory organs.
a. Tracheal system ----mostly found insects, cetipedes , millipedes and many arachnids.
b. Book lung ----- scorpion. (Highly vascularised chamber)
c. simple lung----- terrestrial coconut crab
d. Air tube-----Terrestrial crustacean.
MOLLUSCA—Mollusca lead different modes of life. There are terrestrial (aerial
respiration), aquatic (aquatic respiration) and amphibious mollusk (aerial and aquatic
Molluscs respire through the following organs:
 Skin and mantle
 Ctenidia
 Pulmonary sac or lungs
 Trachea.
 Body surface
Skin and mantle
Skin and mantle are richly supplied with blood vessels.if skin is used for respiration is
called cutaneous respiration and if mantle is used for respiration is called mantle of pallial
respiration. Eg. Aplysia, Dentalium etc.
Ctenidia are the comb like outgrowth of the body surface or mantle. They are also called
branchia or gill. Ctenidial respiration are found in chiton gastropods, plecypods,
cephalopods. Ctenidium has central axis lamellae or filament. Entire gills surrounded by
the cilia
Pulmonary sac or lungs
Pulmonary or lung respiration is present in both terrestrial and amphibious. Respiration is
called pulmonary or aerial respiration. Pulmonary sac is bag like structure formed by the
mantle. Hang from the roof of the mantle cavity.
It is a peaculiar features for some pulmonate molluscs. Some pulmonate mollusks gives
of air breathing tubes called tracheae.
Body surface
It is alternative means to other form of respiration
Some mollusks also used its integument for respiration called cutaneous respiration.
ECHINODERMATA—respiration is carried out by thousand of dermal branchia or
papulae. Dermal branchia are transparaent outgrowth on the skin, present on the aboral
surface projecting through minute opening. Their cavities are in continuous with the
coelom and they are covered by cilia. Dissolved O2 is extracted in the dermal papulae
then diffuse into coelom.
Gases exchange take place through thin wall of tube foot. Exchange of gases take place
between the water and coelomic fluid.
Generalization of respiration in invertebrate
1. Direct diffusion of gases to lung, gill and dermal papulae.
2. No respiratory organ in lower but higher has complex structure for respiration
3. Direct absorption of o2 but in higher o2 absorbed into coelomic fluid then
distributed to the various tissue.
4. Aquatic take dissolve o2 but higher either dissolved o2 or o2 directly from air.
5. Single respiratory organ higher more than one respiratory organ
What is the adaptive significance of the difference in the respiratory organ structure
between organisms living in terrestrial versus aquatic environments?
A surface that absorbs oxygen must be kept wet. This is not a problem for aquatic
organisms as they are surrounded by water. But terrestrial organisms would lose large
amounts of water to the dry air via evaporation from their respiratory surfaces. Therefore
most terrestrial animals have their respiratory surfaces recessed deep inside the body to
minimize the loss of water through evaporation.
Hard exoskeleton system (arthropods) and scale in reptile are the clear examples to
minimize the loss of water freely
Prasad, S.N. (1980). Life of Invertebrates. New Delhi: Vikas Publishing House.
Pvt. Ltd.
Armugum N.. (2004). A text book of invertebrate. Sara publication, new Delhi
Taylor D.J., Grea N.P.O., Stout G.W., (1998). Biological Science, Cambridge
University Press, U.K.

Respiration in invertebrate