• All animal whether big or small, aquatic, aerial or terrestrial, vertebrates or invertebrates require oxygen for performing basic cellular activities and obtaining energy. Whereas as needs carbon dioxide to be removed from our body. For obtaining energy and removing waste it is necessary to perform respiraton.
  • There are generally three common ways through which invertebrates can perform the process  of respiration. These are cutaneous respiration which is mainly performed by lower invertebrates and annelids other is respiration through trachea and gills.
  • Gills are evaginations of the body surface. Some open directly to the environment; others, as in fishes, are enclosed in a cavity. Gills are respiratory organ generally present in water invertebrates. Whereas trachea as a respiratory organ is mostly present in arthropods that too especially in insects.
FIGURE DEPICTING :- Spiracle respiratory organ in case of cockroach


Pheretima posthuma: The Indian earthworm

Respiration takes place by the diffusion of gasses through general body surface. Gaseous exchange that is intake of oxygen and removal of CO2 takes place between blood capillaries of outer epidermis and surface film of moisture contributed by secreted mucus, excreted wastes and coelomic fluid. Haemoglobin dissolve in plasma of blood act as respiratory pigment, transporting O2 to the body tissue.


Periplaneta americana: The common cockroach


  • Respiratory system is very well develop hence it compensate poorly develop circulatory system. It consist of following parts


  • Haemocoel  contains network of elastic, closed and branching air tubes known as tracheae. There are three pairs of large parallel tracheal trunks, one dorsal ,one ventral, and one  lateral. Which are connected together by transverse commissures. Tracheae are formed by invagination of outer integuments hence they made up of outer epithelial wall lined by inner chitinous cuticles. The cuticular lining that prevents intima or taenidia which prevents the tracheal tubes from collapsing. When cockroach is dissected under water, the tracheae, filled with air, presents a glistering appearance.


  • The ultimate finer branches of tracheae are called tracheoles which comes in contact with individual body cells. They have diameter of only 1 micron. Their cavity are intracellular that is each trachiole is made up of  a single cell. Their wall are very thin and devoid of cuticular spiral thickening, instead they are lined by a protein called trachein. They are permeable to water. Their tips are usually filled with a fluid in which oxygen dissolve and diffuses to the tissue.
  • However, some zoologist think that tracheoles end blindly and remain without fluid. Thus the elaborate tracheal system carries oxygen directly to all the body cells. This very well compensate for inability of blood to transport oxygen due to the absence of respiratory pigments.


  • The main tracheal trunks open to the exterior on a body surface through 10 pairs of segmentally arranged aperture termed as spiracles or stigmata. Two pairs of spiracles are thoracic one in between pro and mesothorax  and other between meso and metathorax. Eight pairs of spiracles are abdominal one pair in each of the first eight abdominal segments. They are present laterally in a soft cuticle between  terga and sterna. A spiracles are guarded by bristle of hairs to keep dirt out. Each spiracles internally leads into a short tracheal chamber of artrium from which arises a main tracheal trunk.
FIGURE DEPICTING Respiratory system in case of cockroach
FIGURE DEPICTING :- Respiratory system in case of cockroach


  • Alternate contraction and expansion of abdominal muscles causes rhythmic contraction and relaxation of abdomen. Such movement causes change in diameter of tracheae and force air in and out of tracheal tubes through spiracles. First and third pair of spiracles always remains open while remaining eight opens only during inspiration. Respiratory movement depends upon activity of insect and temperature. Greater the respiratory activity the more vigorous pumping in and out of air.
  • Respiratory movements are coordinate and regulated by nerve centres in thoracic ganglia which are stimulated by low O2 and higher CO2 concentration in tissue fluid.
  • Gaseous exchange takes place by simple diffusion between air in tracheae and dissolve gas in blood or tracheolar fluid which has been shown to rise and fall. When insect is resting the tips of tracheoles remains filled with fluid so that O2 diffuses slowly into the body and fluid. Vice versa occur when insect perform vigorous activity.
  • Oxygen entering tissues brings about oxidation of energy rich food molecules with release of energy and production of carbon dioxide and water. Some carbon dioxide removes from the body through spiracle while major part is removed through cuticular covering.
FIGURE DEPICITNG [A] Tracheae with fluid at rest [B] tracheae without fluid after work
FIGURE DEPICITNG :- [A] Tracheae with fluid at rest [B] tracheae without fluid after work


Unio (Lamellidens) : freshwater Mussel


The process of respiration is performed by two structure ; the mantle and gills or ctenidia. The gills are derived from mantle itself.


  • They are suspended from visceral mass and a mantle lobe. Present on either side of the body and lies in a mantle cavity.
  • Gills are of typical  eulamellibranch type. Each gills consist of two half gills or demibranch or laminae lying side by side. The inner laminae are broader than outer one except that they don’t differ in structure. Each lamina is made up of rectangular plates called lamellae.  These lie side by side attach to their anterior, ventral and posterior edges. The narrow cavity between the two pockets are partition by several thin ventral septa, the interlamellar junction, into the number of vertical compartments called water tubes. All the water tubes of lamina opens dorsally into a common suprabranchial chamber extending above the lamina.
  • Lamina is made up vertical V shaped rods called gill filaments.  The successive V shaped gill filaments are connected by numerous longitudinal bars of connective tissue called inter-filamentar junctions. The inter filamentar junction between the adjacent gill filaments bound microscopic openings, the ostia, through which mantle cavity communicates with water tubes of lamina. Each V shaped gill filaments bears notch at its angle; the notch of the successive gill filament line up to form longitudinal food groove which run along the ventral edge of lamina.
  • Unio posses condition of homorhabdic in which all the gill filament of lamina are same. Each gill filament of Unio is composed mainly of connective tissue and is supported by the skeleton of chitinous rods; the entire structure is bounded by extremely cilliated epithelium. These cilia are of three main type
  • Frontal cilia lining the external ridge like a face of the filament
  • Lateral cilia lines the lateral sides of filament which bounds an ostium
  • Latero frontal cilia lines either sides between the frontal and lateral cilia
FIGURE DEPICTING T.S. OF Body through middle region of gills
FIGURE DEPICTING :- T.S. OF Body through middle region of gills
  • The gills receives deoxygenated blood from the kidney through the afferent branchial vessel which runs through the ctenidial axis and gives out branches into the interlamellar junctions. These branches open into an efferent branchial vessel which also runs through the ctenidial axis. During its flow from the afferent to the efferent blood vessel the blood is oxygenated. The oxygenated blood is finally conveyed to the heart by the efferent blood vessel.
  • Each gill divide the mantle cavity of its sides into
  • Spacious intra branchial chamber that lies below the attached dorsal margin of the lamellae and communicates with the exterior through the inhalent siphon
  • Two narrow longitudinal supra branchial chambers partitioned from each other by ctenidial axis.


  • The mantle lobe in addition to secrete shell, also carry on respiration. Being thin and highly vascular, they are well suited for the purpose.


  • Lateral cilia of gill filament and those lining of mantle, set up the current of water which enters  the infra branchial chamber through the inhalent siphon. The incoming water contains dissolve oxygen and microorganism which constitute food of an organism. From the infra branchial chamber the water enters the water tubes of the gills laminae through the ostia; the food particles donot enters the water tubes. In water tubes the water flows upward and enters the supra branchial chambers. In these chambers it flows backward and reaches the cloacal chambers from where it leaves the body through the exhalent siphon.
  • As the water flow through the infra branchial chamber and water tubes, the blood flowing through the mantle and gills take up the dissolve oxygen and return carbon dioxide to it. The outgoing current of water in addition to carbon dioxide, carries awa the nitrogenous excretory products and faecal matter as well.


Asterias : A sea star

  • Respiration is accomplished by numerous dermal  branchiae or papullae present on a aboral surface. Papullae are thin walled, contractile outgrowth of skin. Their cavities are continous with coelom. Oxygen dissolve in the surrounding sea water diffuses into the papullae and carbon dioxide diffuse out. A respiratory water current is set up to pass over the papillae by cilia lining their outer surface. Cilia lining their cavities, cause the coelomic fluid to flow into them.
  • Some gaseous exchange also takes place through a thin walled tubefeet.


  • Annelids perform respiration through their skin. Gaseous exchange takes place easily through their body surface there is no need of especial respiratory organs or system.
  • In case arthropods respiratory system is well develop, as seen in cockroach respiratory system even compensate for their circulatory system. A well develop tracheal system is present. Contraction and relaxation of abdominal muscles allow inspiration and expiration of air. Respiration is coordinated by activity as well as by temperature.
  • In case of mollusc complex respiratory system is present. As they are habitat is aquatic there respiratory organ is gills. Gills receive deoxygenated blood from kidney the afferent branchial vessel and oxygenated blood is finally conveyed to the heart by the efferent blood vessel.
  • Numerous papullae are present on a aboral surface in case of sea star. Papullae are thin walled and outgrowth of skin only. These papullae allow gaseous exchange between water and body.

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