[Photo credit: PlanMine]
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 enter 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.
Digestion 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 SYSTEM OF TAPEWORM
The tapeworm is found in the intestine of some animals as endoparasite. It has no alimentary canal as it feed on digested food of the host. It absorbs the digested food from the host by simple diffusion.
DIGESTIVE SYSTEM OF AMOEBA
Amoeba is found in muddy water at the bottom of ponds and ditches. It feeds on microscopic organisms smaller than itself or decayed food substances found in the water. It also has no alimentary canal. When it comes across food particles, it immediately puts out a pseudopodium.
This flows around the particles and becomes cut-shaped. Finally, it engulfs the particles. This happens when the two ends of the pseudopodium meet and fuse. Some water is also enclosed and the small cavity formed is called a food vacuole. The process is termed ingestion.
Enzymes from the cytoplasm of the cell are secreted into the food vacuole to digest the food. The soluble products formed diffuse from the vacuole into the cytoplasm. Undigested material is left behind and egested as waste
DIGESTIVE SYSTEM OF EARTHWORM
In the earthworm and most other animals, the alimentary canal is a tube with two openings: the mouth through which food enters and the anus through which undigested food leaves the body. With food moving only in one direction, it is possible for different parts of the alimentary canal to become specialized and perform different functions in succession.
The alimentary canal of earthworm includes the mouth, pharynx, oesophagus, crop, gizzard, intestine, caecum, rectum and anus.
- The mouth is where the food is ingested.
- The pharynx secretes mucus to lubricate food particles.
- The oesophagus transfers the ingested food to the crop.
- The crop is where the food is temporarily stored.
- The gizzard is where the food is ground against small stones to break it up.
- The intestine is where digestion and absorption of food takes place.
The earthworm feeds mainly on dry vegetables 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.
DIGESTIVE SYSTEM OF INSECTS
Insects have variously modified mouthparts according to the food they eat. The grasshopper feeds on green vegetables and the cockroach feeds on household materials like foods, e.t.c.
The digestive system or alimentary canal of a grasshopper or cockroach include the mouth(mandibles), salivary gland, oesophagus, crop, gizzard, mid-gut, caeceum, ileum, hind gut, malphigian tubules and anus.
The mouth is surrounded by mouth parts such as maxillae, mandibles and labium with sharp cutting edges for the 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 a 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 the pellets and absorbs water and mineral salts. The faeces are later passed out through the anus.
DIGESTIVE SYSTEM OF BIRDS
In birds, the jaws are drawn out to form a horny break which does not have teeth. Birds use their beaks for feeding. The beaks show adaptations according to the types of food the birds eat. In man birds, the feet also show adaptations for feeding.
The digestive system or alimentary canal of the bird includes: the beak, mouth, pharynx, oesophagus, crop, proventiculus, gizzard, intestine, caecum, rectum and anus(cloaca).
The bird has no teeth but the food such as fruits, grains, e.t.c are picked up by the beak. The food then moves from the mouth through the oesophagus to the crop. 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 proventiculus where digestive enzymes are secreted on the food. The proventiculus is often regarded as glandular stomach because it has glands which secret digestive enzymes, e.g pepsin, amylase, e.t.c. on food.
From the proventiculus, the food moves to the gizzard where the actual grinding of food takes place. With the aid of small stones or grit, the food is ground 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 than other animals. The reason for this long alimentary canal is to provide a large surface area for the digestion and absorption of food.
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