DIGESTIVE SYSTEM AND FUNCTIONS At the end of this article, students should be able to:
Describe with appropriate illustration different types of alimentary tracts
Explain the feeding mechanism of some animal
Explain how some insectivorous plants feed


Digestive system includes the alimentary tracts or canal and all the glands and organs associated with the digestion and assimilation of food in animals.  Digestion is the breaking down of large molecules of food into simple and absorbable form for use by the animals.
The digestive system including the alimentary tracts varies from one animal to another. Organisms from simple to complex ones have different types of digestive system or alimentary tract. However, there is no definite alimentary canal in plants.


A typical mammalian alimentary tract or canal includes the following parts: mouth, pharynx, Oesophagus or gullet, stomach, small intestine or ileum, caecum, appendix, large intestine or colon, rectum and anus. All these parts can be found in most vertebrates. However, there are modification of different parts of the alimentary tracts in various animals.In some animals, some parts may be reduced or enlarged while in others, some parts may be absent as shown in the table below

Animals have different types alimentary tracts which vary in size, complexity, mode of feeding or the types of diet. The nature of alimentary tract has some evolutionary trend as the complexity tends to increase from simple unicellular animals like protozoa to complex multicellular animals like mammals.


PLANARIA The planaria is a free living flatworm that feeds on aquatic animals called zooplanktons. It has a simple alimentary canal with one opening—the mouth. The mouth opens into a short buccal cavity which leads into the pharynx. The pumping action of the pharynx sucks in pieces of food which enters the pharynx and pass into the small intestine. The small intestine has three branches—one leading to the head and the other two towards the tail. The branches give off numerous side branches.
Digestive in planaria is intracellular. The branched intestine enables digested food to diffuse to all parts of the body. The undigested food is egested from the mouth.

DIGESTIVE ORGANS OF A TAPEWORM The tapeworm is found in the intestine of some mammals as endoparasites. It has no alimentary canal as it feeds on digested food of the host. It absorbs the digested food from the host by simple diffusion. DIGESTIVE ORGANS OF AN EARTHWORM The alimentary tract of the earthworm is a tube with two opening The mouth—through which food enters and the anus—through which undigested food leaves the body. The alimentary canal of the earthworm includes these parts: mouth, pharynx, Oesophagus, crop, gizzard, intestine, caecum, rectum and anus.

  1. The mouth is where the food is ingested.
  2. The pharynx secretes mucus to lubricate food particles.
  3. The oesophagus transfers the ingested food to the crop.
  4. The crop is where the food is temporarily stored.
  5. The gizzard is where the food is grinded against small stones to break it up.
  1. The intestine as a critical part of the digestive system, is where digestion and absorption of food takes place.
    The earthworm feeds mainly on dry vegetable matter found in the soil. The digested food then diffuses through the intestinal wall into the blood and finally to the various parts of the body. The undigested food materials are then removed through the caecum, the rectum and finally to the anus.

The grasshopper feeds on green vegetables while the cockroach feeds on household materials like books, sugar and food. The alimentary canal of a grasshopper or cockroach includes the mouth (mandibles)(biting and chewing insects), salivary gland, oesophagus, crop, gizzard, mid-gut, caecum, ileum, rectum, hind gut, Malphighian tubules and anus. The mouth is surrounded by mouth parts such as: maxillae, mandibles and labium with sharp cutting and grinding of food. A pair of salivary glands opens into the mouth which secretes enzymes for digestion of food.

The food is then swallowed through the muscular oesophagus into the crop. The crop is where the food is stored. The food is later taken to muscular gizzard for grinding.

The short mid-gut has six fleshy vascularized diverticulum or mid-gut caecum which increases the surface area for digestion and absorption. The hind gut consisting of ileum, colon, and rectum shapes the faeces into pellets and absorbs water and mineral salts. The faeces are later passed out through the anus.

DIGESTIVE SYSTEM OF BIRDS The digestive system of the bird includes the beak, mouth, pharynx, oesophagus, crop, proventriculus, gizzard, intestine, caecum, rectum and anus (cloaca).
The bird has no teeth but the food such as fruits, grains etc. are picked up by the beak. The food then moves from the mouth through the oesophagus to the crop which is the key function of digestive system.

The food is stored temporarily in the crop where it is moistened and fermented by some bacteria. From the crop, the food now pass on to the proventriculus where digestive enzymes are secreted on the food.

The proventriculus is often regarded as glandular stomach because it has glands which secret digestive enzymes, e.g. pepsin and amylase on the food. From the proventriculus, the food moves to the gizzard where the actual grinding of the food takes place. (The gizzard is often regarded or called grindular stomach) with the aid of small stones or grit the food is grinded by the gizzard. From the gizzard, the food now moves to the duodenum and the intestine where further digestion and absorption of food take place. The undigested food particles are moved through the caeca to the rectum and finally to the anus (cloaca) where the faeces and urine are removed from the body at the same time. It should be noted that birds just like other herbivores have a relatively long alimentary canal is to provide a large surface area for the digestion and absorption of food.

Similarities and Differences Between the digestive system – ALIMENTARY canals of a Bird and That of Grasshopper

1. Both have narrow esophagus
2. Crop is present in both organisms
3. Muscular gizzard is present in both
4. There is the presence of caecum in both
5. Mid-gut is present in both organisms

1. There is presence of tongue in the mouth
2. Mouth is modified into beak
3. There is presence of duodenum
4. It has a long alimentary canal
5. The hind gut ends in cloaca
6. Malphighian tubules are absent
7. There is presence of pancreas

grasshopper digestive system
1. Tongue is absent
2. Mouth is modified into mandible and maxillae for chewing and grinding
3. Duodenum is not present
4. It has relatively short alimentary canal
5. Hind gut ends in anus distinct from the reproductive tract
6. Malphighian tubules are attached to alimentary canal.
7. Pancreas is absent

Alimentary Canal and Digestion of Food in Man

The alimentary canal of man includes: the mouth, oesophagus, stomach, duodenum, small intestine or ileum, caecum, appendix, large intestine or colon, rectum and anus. read more of these here

The description and importance of the parts of digestive system are as follows:
1. Mouth: the alimentary canal of man starts from the mouth. The mouth contains the teeth, salivary gland and tongue
2. The teeth: (i) the teeth are used to cut, grind or chew food into tiny particles. (ii) they expose large area of food for the action of enzymes

  1. The tongue: (i) the tongue rolls the food into bolus. (ii) it aids movement of food in the mouth (iii) it allows mixing of food with saliva or ptyalin (iv) it aids swallowing of food into the gullet or oesophagus.
  1. The salivary gland: (i) the salivary gland secretes saliva which contains an enzyme called ptyalin. The ptyalin breaks down starch into maltose which is later swallowed into the gullet in form of bolus. (ii) It allows easy chewing or movement of food in the mouth for swallowing. (iii) it also serves as solvent for food. The saliva is slightly alkaline.
  1. Oesophagus or Gullet: the oesophagus connects the mouth to the stomach. The food swallowed is passed down through the oesophagus by a peristaltic movement into the stomach.
  1. Stomach: in the stomach, the food is temporarily stored for few hours and it is released at regular intervals by the opening of the pylotic sphincter.
  1. In the stomach, the gastric gland secretes gastric juice which contains two enzymes—renin and pepsin.
    The renin acts on milk (or it helps to curdle milk) while the pepsin breaks down proteins to peptones.
  1. The gastric gland also secretes hydrochloric acid (Hcl) which creates an acid medium for two enzymes to act. The Hcl also helps to kill some bacteria in the stomach. The food is churned by muscular contraction of the stomach wall (churning movement) which enables the mixing of food with digestive juice. The churning movement then converts the food into a semi-liquid state called chyme.
functions of the Duodenum

digestion of food also takes place in this region of the alimentary canal. The duodenum contains pancreas which secrets pancreatic juice that contains three enzymes

These enzymes are:

the conversion of food by enzymes

i. Amylase: this converts starch to maltose
ii. Lipase: lipase converts fats and oil to fatty acids and glycerol.
iii. Trypsin: it converts protein and peptones to polypeptides.
The pancreatic juice is alkaline and provides that medium for enzymes.

The digestion of fat and oil is aided by a green alkaline liquid called bile which is secreted by the liver and stored in the gall bladder. The bile helps in the emulsification of fats, i.e. breaking down fats into tiny droplets.

At the end of digestion in the duodenum the food now in liquid form called chyle passes into the ileum or small intestine.

digestion of food in the Small intestine

: the small intestine or ileum is found between the duodenum and the large intestine. Two major events take place in the small intestine. These events are: (a) digestion and (b) absorption of the digested food.

  1. Digestion: the digestion of food also takes place in the small intestine or ileum.The intestinal wall secrets intestinal juice which contains the following enzymes: lipase, erepsin, maltase, sucrase and lactase. The lipase converts fats and oil to fatty acids and glycerol, erepsin converts polypeptides to amino acids, maltase converts maltose to two units of glucose, sucrase converts sucrose to glucose and fructose while lactase converts lactose to glucose and galactose.

In man, the final digestion of food ends in the small intestine. The end products is the digestion of protein are amino acids, fats and oil are fatty acids and glycerol, while that of starch is glucose.

  1. Absorption of digested food: the end products of digestion of food (amino acids, glucose, fatty acids and glycerol) are absorbed in the small intestine by tiny finger-like structures called villi (singular villus).

The folding of the small intestine combined with presence of numerous villi creates a large surface area for the absorption of digested food.
The inner surface layer or epithelium of each villus is thin. This allow the absorption of the end product by either diffusion or active transport through it.

The glucose and amino acids are easily absorbed by blood capillaries through the epithelium while the fatty acids and glycerol enter the lacteal where they are carried through the lymph vessels which eventually empty their contents into the blood vessels near the heart. The blood then carries the fats and other food materials to various parts of the body where they are needed.

  1. Caecum and appendix: in man, the functions of caecum and appendix are not well known but the caecum usually contains some bacteria which aid minor digestion of cellulose. Some vitamins such as K and B-complex are partially synthesized in this region.
  2. Large intestine: the undigested food passes into the colon or large intestine. Here, water is absorbed. This absorption of water concentrates the waste products and turn them into faeces.

The faeces is passed into the rectum and finally out of the body through the anus. For summary, see the table below showing enzyme, source, location, substrate and effects HERE YOU WILL FIND EVERY Reasons why an athlete in a race would be given a glucose drink rather than a pieces of bread Glucose is the simplest form of carbohydrate/soluble form of carbohydrate, an energy giving food substance which does not require any form of digestion. Hence it is absorbed in the ileum/small intestine where it diffuses/enter into the blood stream/circulatory system. It is then assimilated and oxidized within the shortest time/quickly/immediately to give energy to the athlete.

what is bread? Bread is a complex carbohydrate/polysaccharide/starch-which will require a series of digestive process/hydrolysis which starts in the mouth and ends in small intestine/ileum. This takes a longer time to produce energy for use and cannot give immediate energy.


There are five modifications and mechanisms of feeding associated with some organisms. These feeding mechanisms include:

Absorbing mechanisms, e.g. tapeworm:

the tapeworm is an endoparasites which carries out parasitic feeding on its host i.e. man. It has no mouth but absorbs digested food from the intestine of its host. The body of the tapeworm is modified and adapted for parasitic feeding as follows:

i. The alimentary canal is absent, hence food is absorbed through its entire body surface.
ii. The tapeworm has hooks and suckers which are used for attachment to the intestine of the host to avoid dislodgement.
iii. The body has thick cuticle which resists digestive enzymes of the host.
iv. The flat body surface of the tapeworm provides a large surface area for the absorption of already digested food.
v. The entire body is used also for the absorption of food.

Biting and chewing mechanism, e.g. grasshopper or cockroach:

the grasshopper or cockroach has mouth parts adapted for biting and chewing. These insects has four different mouth parts which are modified and adapted for biting or chewing food. These mouth parts are:

i. Labrum or upper lip: the grasshopper has labrum or upper lip which prevents the food from falling off the mouth.
ii. Mandibles: it possesses a pair of mandibles which are heavy, toothed and jaw-like structure used for cutting and chewing food materials.

iii. Maxillae: the grasshopper also has a pair of maxillae which is also a biting blade. This breaks down the food which the mandible has chewed into smaller particles.
iv. Labium: the labium (lower lip) prevents the wastage of food from the mouth.

  1. Sucking mechanism: there are three popular organisms which exhibit sucking mechanism. These are: mosquito, butterfly and housefly. These insects have different modifications of mouth parts adapted for feeding on food through the mechanism of sucking.


i. The mosquito possesses a piercing mouth parts called the proboscis used for sucking the blood of man and animals.
ii. The mouth parts altogether form a strong stylet capable of penetrating the skin to suck blood.
iii. The stylet except the labium is sharp and pointed to ease penetration into the skin.

iv. The mouth can produce saliva to prevent clotting of blood while it is being sucked in.
v. It has the ability to fold back the labium easily so as to allow the stylet to perform its work of penetration of the skin to suck blood.


The butterfly feeds on liquid food like nectars of flowers. It has its mouth parts modified for sucking in the following ways:
i. It possesses a long, coiled proboscis used for sucking nectars of flowers.

ii. The insect is capable of recoiling the long proboscis under its head when not in use.
iii. There is the non-functioning of the other mouth parts due to the type of food taken by the insect.


The housefly feeds on liquid food materials. It has mouth parts modified for sucking in the following ways:
i. The housefly possesses enlarged enlarged labella which are sucking structures for liquid food.

ii. The housefly has the ability to feed on solidtate food, e.g. sugar by pouring out its saliva to change the food to a liquid state

iii. It has sucking mechanism called sponging in which the mouth is placed on the liquid food and it will start to rush into the mouth.
iv. The labella have fine channels which aid rapid absorption of liquid food into the mouth.

Grinding mechanism:

grinding mechanism is common among mammals, e.g. man, cattle, sheep, and goat. These animals are capable of grinding the food before swallowing. This grinding is aided by the presence of hard and strong teeth made of enamel and dentine. The animals are adapted to the grinding mechanism by the following features:

i. They possess different sets of teeth to grind food.

ii. The teeth are hard and strong to withstand biting, chewing or grinding and cracking of food.

iii. They possess incisors which are sharp with flat edges used for cutting off bits of food.

iv. Animals have pointed canine teeth which are used for tearing food.

v. There is presence of premolars and molars with undulating and wide surfaces used for grinding of food.

vi. The absence of front teeth [incisors] in sheep [a herbivore] is a special adaption as it helps to grip the grasses during feeding by the animal.

Trapping and absorbing mechanism:

the trapping and absorbing mechanism are common among the insectivorous or carnivorous plants such as bladder wort and sundew. Bladderwort and sundew have structures which enable them to adapt to this mode of feeding.

I. Sundew, for example, traps insects by undergoing nastic movements in response to touch from the body of the insects.
II. The sundew leaf has long hairs which carry digestive glands.
III. Insects on landing on these hairs causes other hairs to curl over the insect and cover it.

IV. The sundew is capable of secreting a fluid rich in enzymes to digest the insect externally.
V. The protein so synthesized is easily absorbed by the carnivorous plant into its body.


Organisms exhibit different feeding habits.
These are:

Filter feeding

Filter feeders which are also called microphagous feeders feed on very tiny organisms which cannot be easily picked to the satisfaction of the feeder.

Filter feeders are mainly aquatic animals and they have to wallow in water through sieve-like structure into their body in order to collect reasonable quantity of their prey or food. Examples of filter feeders are mosquito larva, mussel, ducks and prawns.

filter feeding in mosquito larva

The mouth of mosquito larva, for example, has horny jaws and two small ciliated appendages or mouth brushes. These are used to create a current of water. The small colloidal particles or planktons, small plants and animals are filtered from water and ingested into the mouth as food. fluid feeding

Animals which feed on any fluid materials are classified as fluid feeders. These are two major groups of fluid feeders in relation to the digestive system. These are:
i. Wallowers: these organisms rest within or wallow in their food, e.g. the tapeworm in intestine of man. The tapeworm lives within the digested food of its host and absorb already digested food of the host.
ii. Suckers: suckers are organisms, mainly insects, which feed by sucking fluid from plants and animals. Examples of suckers are bug, mosquito, butterfly, aphid, tse-tse fly and housefly. The fluid feeding mechanism of mosquito, housefly and butterfly has been discussed earlier.saprophytic feeding

Saprophytes are mainly non-green plants which do not have chloroplasts and therefore cannot manufacture their own food but they have a very complex digestive system. They then feed on dead and decaying organic matter from which they derive their food.

Examples of saprophytes are Rhizopus, mushroom and mucor. Their body is adapted in the following ways:
i. They have hyphae instead of roots through which they pour out enzymes for digestion.
ii. They are capable of carrying out extracellular digestion, i.e., digestion of food outside the body cells of the plant.
iii. The digested portion of the organic matter is later reabsorbed into the body.

digestive system
function of the digestive system

parasitic feeding

Parasitic feeding is found in both plants and animals. Animals parasites are tapeworm, roundworm, liver fluke, louse, tick and guinea worm while plant parasites are Cassytha, dodder and mistletoe.

Parasites are structurally modified organisms that depend wholly or partially on other living organisms for their food and survival. The structural adaptation of tapeworm for example, demonstrating parasitic feeding has been discussed earlier in this chapter


feeding in protozoa

Protozoa like amoeba exhibits holozoic mode of nutrition. Amoeba feeds on micro-organisms like diatoms, demids or other organic particles.

It engulfs the food particles by p

utting out its pseudopodia which surround the food particle by forming a cup-shape when the two ends of the pseudopodia touch. The pseudopodia on meeting or touching, fused and enclosed the food into the body with little drop of water; this forms a food vacuole. The food is digested inside the food vacuole.

feeding in hydra

Hydra feeds mainly on small crustaceans or other smaller aquatic organisms. The food is captured by the action of nematocysts borne on the tentacle. Enzymes are secreted into the enteron by the gland cells. Here digestion takes place extracellularly, i.e., inside
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