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Lesson Note
Subject: Biology
Topic: Digestive System In Invertebrates.
Lesson Objectives: By the end of the lesson, the learners should be able to:

1. Feeding in Hydra
2. Digestion in Planaria
3. Digestion in Earthworm
4. Digestion in insects (Grasshopper)

Lesson Discussion

Hydra feeds on small aquatic organisms e.g. crustaceans. It captures its food through its structure called nematocysts found on its tentacles. The food is then passed into the mouth while the digestive enzymes are secreted into the enteron by the gland cells.                  Digestive Track Of Hydra

The digestion is extra cellular and the digested food is absorbed into the body while the undigested food is sent back into the enteron and released through the mouth. The digestion here is therefore intracellular and extracellular.

Tapeworm is a parasitic worm which has no alimentary canal; it feeds on digested food of the host. However, the planarian is a free living flatworm which feeds on small animals. Planaria has a simple alimentary canal with one opening i.e. the mouth present in the ventral part of its body cavity.                         Digestive Track Of Planaria

The pharynx of the planarian can be extended out of the mouth during feeding. The pumping action of the pharynx sucks in pieces of food which enter the pharynx and move into the small intestine. Digestion is partly extracellular and partly intracellular. The branched intestine enables the digested food to diffuse to all parts of the body while undigested food is egested through the mouth.

Compare feeding in hydra with feeding in planaria
Explain extracellular digestion.

The alimentary canal of the earthworm is a tube with two openings:
The mouth through which the food enters
The anus through which the undigested food leaves the body

The alimentary canal of the earthworm has the following parts:

  • Mouth and thin walled buccal cavity for food ingestion
  • A muscular thin walled pharynx which secretes mucus to lubricate food particles
  • A narrow tubular thin walled oesophagus
  • A thin walled crop for storing food
  • A thin walled muscular gizzard where food is ground against small stones to break it up.
  • A long straight intestine in which extracellular digestion occurs.

              Digestive Track of Earthworm

The prostomium is the mouth of the earthworm. Since it has no jaws or teeth, the earthworm uses its muscular pharynx to suck in soil containing food. The food particles and soil go through a long esophagus into a round organ called a crop. The crop stores the food temporarily. Then it is forced into a very muscular organ called the gizzard. The gizzard contracts and expands, causing grains of sand and food to rub together. In this way, the food is ground up. Food is digested in the intestine, which stretches from segment nineteen to the end of the worm. Here enzymes chemically break down the food. Then the digested food is absorbed by the blood circulating through the intestinal walls. The earthworm’s digestive system can be thought of as a tube within a tube.

The complicated organs of the digestive system take up most of the anterior half of the earthworm’s body. The earthworm ingests large amounts of soil, which has organic matter in it. The useless inorganic matter goes through the worm with no change. This is often left on the surface of the ground in the form of castings. As the earthworms feed, they loosen the soil, making it easier for air and water to enter. Their wastes also add to soil fertility. If there were no earthworms, the soil could not support crops, grasses, and other plants.

The alimentary canals of insects consist of three major parts:
The fore gut [mouth, pharynx, oesophagus crop, gizzard]
Mid gut [stomach]
Hind gut [intestine, ileum, colon, rectum, anus]

Insects such as grasshoppers feed on leaves. They use their mouth parts to cut and crush the leaves. Saliva is introduced or poured into the leaves from the salivary gland. The saliva helps to soften the leaves and the chewed food is in the crop, and broken up further into small pieces.
The foregut and the midgut secrete enzymes rich juice into the midgut where digestion and absorption occurs.

            Digestive Track of Insects (Grasshopper)

The hindgut is for water absorption. Only solid faeces pellets are egested from the anus after the food waste has been collected through the malpighian tubules joining the mid and hindgut.

Done studying? See all previous lessons in biology.
Take a quick test for this lesson.
1. State the three parts that make up the digestive system of insects.
2. Compare feeding in hydra with feeding in planaria.
3. Explain extracellular digestion.

Questions answered correctly? Bravo!!

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