Sample Comparison Question

“Explain how the digestives structures of a Hydra and an earthworm differ in structure and function.”

It helps sometimes to quickly draw up a table of comparison before beginning to write. It can be helpful in planning your answer and writing a good introduction.

Characteristic	Hydra	Earthworm
Level of organization	Tissue	Organ system
Regional specialization	none	Pharynx, crop, gizzard, intestine
Number of openings	One	Two, mouth and anus
Type of digestive system	Incomplete	Complete
Direction of food movement	Two way	One way
Serial processing	No	yes
Name of system	GVC	Digestive tract

Sample Answer

Introductory Paragraph

The simple gastrovascular cavity of the Hydra differs in many ways from the complex digestive tract of the earthworm. The earthworm has a much more complex system than the Hydra because of the level of its level of organization, its regional specialization and its serial processing of food.

Body

The gastrovascular cavity of the Hydra is a simple bag-like structure with one opening; whereas, the digestive tract of the earth is essentially tube-shaped with two openings. Since the Hydra has only one opening (the so called mouth) it serves two functions: ingestion (feeding) and egestion (removal of waste products of digestion). The earthworm, on the other hand has both a mouth for ingestion and an anus for egestion. Systems with only one opening , like the Hydra are called incomplete digestive system; whereas, systems like the earthworm with both mouth and anus are called complete digestive systems.

The significance of the earthworm having both a mouth and an anus is there is a one way flow of food through the earthworm’s system; whereas, the hydra has a two way flow of food since the food and wastes must enter and leave through the same opening.

In the earthworm this one way flow has lead to the serial processing of food and regional specialization. The Hydra has neither regional specialization nor serial processing of food. Serial processing is something like an assembly line for building cars. As the car passes along the conveyor belt more and more parts are added until it is completely built. As the food moves down the earthworm’s digestive tract the various steps of digestion are performed one after another in different chambers. Since the Hydra consists of only one chamber all the processes of digestion are performed in the one chamber. No serial processing of food in different areas can occur.

Serial processing of food by the earthworm is made possible by the evolution of different chambers for different functions. Since the Hydra has neither one-way movement of food nor more than one chamber, serial processing is not possible. The food is sucked into (ingested) the earthworm by a muscular pharynx; then it is passed onto the next chamber, the thin walled crop, for temporary storage; from there is moves into the muscular thick walled gizzard where the food is mechanically digested by the alternate contraction and relaxation of the gizzard wall. The presence of small grains of sand here helps grind the food into smaller pieces. The final chamber, the tubular and thick walled intestine, makes up approximately 80% of the digestive system of the earthworm. Here chemical digestion and absorption of food molecules occurs.

Since the earthworm has regional specialization in its digestive system it is said to be at the organ system level of organization. The Hydra is at the tissue level of organization because its tissues have not become organized into larger functioning units (organs). The lining of the GVC consists of only one tissue, called the gastrodermis. In the earthworm the pharynx, crop, gizzard and intestine are all composed of different tissues and each carries out a discrete function.

Concluding paragraph

Even though the digestive structures of Hydra are much less complex than the earthworm and the Hydra lacks many of the structures possessed by the earthworm such as a separate anus and mouth, organs and regional specialization, the Hydra is as capable at digesting and absorbing its food as the earthworm.