Question

28.2

1. Describe the germ layer and symmetry of cnidarians.

2. What is the defining characteristic of cnidarians?

3. Compare the two distinct body plans of cnidarians in terms of their structure, motility, and mode of reproduction.

4. Describe the gastrovascular cavity of cnidarians. Do they have a circulatory system? Explain.

5. Briefly describe the four classes of cnidarians.

6. Describe the life cycle of a typical Scyphozoan.

Answer 1

28.2.

1. Cnidarians are radially symmetric. This shows, they are the same on all sides.

It also means that they have two germ layers called the endoderm and ectoderm.

These germ layers form during gastrulation, which is the development of the young embryo.

2. Cnidarians are radially symmetrical, jelly-like and have a nerve net and one body opening.

They are Characterized by a digestive cavity that forms the main body.

3.There are two basic body plans in cnidarians. They are called the polyp and medus.

The polyp has a tubular body and is usually sessile. The medusa(plural, medusae) has a bell-shaped body and is typically motile. Some cnidarian species alternate between polyp and medusa forms.

Other species exist in just one form or the other

4.The gastrovascular cavity has only one opening that serves as both a mouth and an anus; this is termed an incomplete digestive system.

There is also no circulatory system, thus nutrients move from the cells which absorb them in the lining of the gastrovascular cavity through the mesoglea to other cells.

5. The phylum Cnidaria is made up of four classes: Hydrozoa (hydrozoans); Scyphozoa (scyphozoans); Anthozoa (anthozoans); and Cubozoa (cubozoans).

6. The larva develops into the hydroid stage of the lifecycle, a tiny sessile polyp called a scyphistoma.

The scyphistoma reproduces asexually, producing similar polyps by budding, and then either transforming into a medusa, or budding several medusae off from its upper surface via a process called strobilation.

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