• Every organism needs energy to perform its basic metabolic function. This energy can be obtain from food and sun. As sun is a universal source of energy but only autotrophic organism can obtain energy from sun. The energy which is obtain by autotrophic organism can now be taken from various heterotrophic organism. For gaining energy from any source heterotrophic organism must perform two basic processes first feeding and other is digestion.
  • Feeding refer to as obtaining food from environment when one feels hungry or require energy from outside. Feeding in simple terms refer to as eating. Now there is a difference between eating and obtaining energy. For converting food to useful fuel for our body it must be digested first.
  • Digestion involves multiple processes from obtaining food to converting it to absorbable form. This processes includes movement of food, grinding of food, mixing of food with acid and alkaline and action of various digestive enzymes. In other word digestion is complex process that allow complex food particle to break down in simple organic fuel that can be utilised by our body
FIGURE DEPICTING :- Single opening of gastrovascular cavity in case of hydra


  • Body of the porifera is organised in such a manner as to form complex system of pores and canal. This system is generally refer to as canal system. Various components of canal system are as follow.

Ostia or dermal pores

  • The external body surface bear two or more opening for the entry of the outside water into the body of sponge. This pores are known as ostia or dermal pores.  The diameter of ostia can be reduce for regulation of amount of water to be enter with the help of contractile cells or myocytes around them.

Incurrent canal

  • These canals are invaginated folds of bady wall. These communicate outside through ostia but end blindly at their inner ends. Pinacocytes lines these canals.


  • Incurrent canal communicates with radial canals through intercellular spaces called prosopyles.

Radial canal

  • Evagination of body wall forms chambers lined by flagellated choanocytes. These chambers are called flagellated or radial canal. Incurrent canal and radial canal are arranged parallel and alternate with each other both vertically and radially.
  • Radial canal ends blindly at their outer ends but lead their inner ends into the spongocoel.


  • Opening of radial canal into the spongocoel are called apopyle.


  • It is the large central cavity of body.


  • Spongocoel leads to outside through terminal opening of the osculum. They are also provided by sphincter for regulation of rate of water.

Current of water

  • Flow of water inside canal system is maintained by continuous beating of flagella of collar cells lining the radial canals. Every beats of flagellum consist of normal active stroke and recovery stroke.
FIGURE DEPICITNG Water canal system of Sycon
FIGURE DEPICITNG :- Water canal system of Sycon
  • Porifera are generally filter feeder relying on marine organism [ planktons ] and organic material. These enter the body with the water current through ostia that allows the entry of only small particles. Inside the flagellated chamber the beating, of flagella of collar cells causes the water to circulate through the collar, that allow the food particles to adhere on them. Collar cell consist of microvilli that act as a filter for  food particles which gradually moves towards their bases.
  • Consequently the food particles are engulf by the pseudopodial action of choanocytes at the bases of their collar and then taken upto the food vacoules. Choanocytes are single layer flagellated collar cells.
  • The atmosphere in the food vacuole is first acidic and then alkaline. Here the food undergoes the partial digestion and partly the food is passed onto wandering amoebocytes  in mesenchyme. Within amoebocytes digestion of food is completed and indigested food is eliminated with the outgoing water current  of water. Amoebocytes also distributes the digested food to all other cells of the body, while some of them [thesocytes] stores some food for future use.
  • A number of enzymes have been isolated from sponges. They include protein starch and fat digesting enzymes.



  • Hydra is almost exclusively carnivore.
  • Coelenterates are almost first animals to use projectiles, called nematocysts for capturing animals. A hungry hydra wait for its food to come to it. It normally rests at its basal disc attached to some objects while its body and tentacles are extend out in water, controlling considerable amount of hunting territory. When a small passing organism touches its tentacles, dozens of nematocyst are discharges onto it at once.
  • The penetrants puncture the victim and inject paralyzing hypnotoxin. The tentacles, holding the prey now contract and bend inwards drawing the paralysed prey towards the mucus lined mouth which opens widely to swallow it. Contraction of hypostome [mouth of hydra] and body wall [peristaltic movement] pressure the food down into the gastrovascular cavity.
  • Hydra engulfs only those animals which have glutathione in their body because glutathione is necessary to initiate feeding reactions.
FIGURE DEPICTING Capturing and ingestion of cyclop by Hydra
FIGURE DEPICTING :- Capturing and ingestion of cyclop by Hydra


  • Digestion is both extracellular as well as intracellular and can occur in two stages.


  • First the prey is murdered with the help of action of digestive juices secreted by the gland called gastrodermis.  Expansion and contraction of the body wall leads to churning movement. And lashing movement of flagella of nutritive muscle cell thoroughly mixes the digestive juice with broken food.
  • Protein is partly digested into polypeptide by a proteolytic enzyme similar to trypsin. This type of digestion occur outside the gastrodermal cells outside the cavity is called extracellular digestion and is purely proteolytic.

Intracellular digestion

  • Nutritive muscle cells engulf small fragments of food by the means of pseudopodia and digest it within the food vacuole. It undergo both acidic and alkaline phases and digestion of protein is completed by other proteolytic enzymes.



  • Migration occur during feeding. It has been found that the hungry  flukes migrates into smaller bile ducts and capillaries for feeding. They suck up food, lymph, bile and tissue pieces by the oral sucker from the walls of bile passages. Oral suckers and muscular pharynx serves as an efficient suctorial apparatus.


  • It lacks anus hence it have incomplete Alimentary canal. Mouth is located at the anterior end surrounded by oral suckers. It leads into the ovoid pharynx having the small narrow lumen and thick walls provided with radial muscles and pharyngeal glands. It is followed by the short narrow oesophagus. Oesophagus opens into the large intestine. Intestine bifurcates into the left and right main branches. Each branches gives numerous side branches called caeca or diverticula. Caeca of outer side is further branched.
  • Digestion which is extracellular takes place in intestine. Distribution of digested food is accomplished by diverticula of intestine aided by mesenchyme. Waste material diffuse into the surrounding mesenchyme or egested through the mouth. Monosacchride sugars such as glucose and fructose are diffuse directly into the body of fluke through its general surface. Tegument play a role in absorption of amino acids. This occur due to number of fine folds on tegument.
  • Reserved food is glycogen and fats stored in mesenchyme and muscles.
FIGURE DEPICTING Digestive system of liver fluke
FIGURE DEPICTING :- Digestive system of liver fluke


Feeding and digestion  of Ascaris lumbricoides

  • Its food consist of blood and fully and partially digested occurring in the fluid form in the host gut. It is sucked by the rhythmic pumping action of pharynx. Digestion is completely extracellular in intestine and aided by enzymes protease, amylase and lipases. Digested food is absorbed by intestinal the intestinal cells. And distributed by the pseudocoelomic fluid. Excess of food is stored by the syncytial epidermis in the form of glycogen and fats.
  • Some intestinal cells also engulfs small solid particles by the phagocytosis and digest them intracellularly. Defaecation of undigested food whenever it occurs is facillated by the depressor ani muscles which raise the dorsal wall of rectum and posterior lip of anus or cloaca.
FIGURE DEPICTING [A] Alimentary canal, [B] T.S of pharynx,[C] T.S of intestine, [D] Brush border of intestinal wall
FIGURE DEPICTING [A] Alimentary canal [B] T.S of pharynx [C] T.S of intestine [D] Brush border of intestinal wall


  • Different organism require different amount of energy for their survival hence pattern of feeding and quality of food changes from one phylum to another. When food quality changes mechanism digestion also differ among them. Here we have discuss the difference in pattern of feeding and digestion among the lower invertebrates.
  • Porifera are simple organism they perform feeding as well as their digestion through water canal system.
  • Hydra may swallow prey larger than itself, such as young fish and tadpole. For feeding purpose they uses tentacles whereas digestion is performed intracellularly as well as extracellular.
  • Liver fluke are endoparasites  they feed with the help of their suckers while perform digestion in their intestine.
  • Ascaries are also parasites their food is blood or partially digested food in fluid of their host. They posses well develop digestive system and various enzymes for digestion of their food.

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