Animal Phyla Porifera to Mollusca: PECTAA Class 11 Biology, Punjab Board

A sponge, a jellyfish, a tapeworm, a roundworm and a snail look nothing alike. Yet the book puts all of them in one kingdom. Body layers, body cavity and symmetry tell the phyla apart.

Section 1.4 of Chapter 1 in the PECTAA Biology 11 book divides kingdom Animalia into phyla. This note covers the first five. It is written for FSc Part 1 (1st year, Class 11) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.

What to know

The book describes kingdom Animalia as eukaryotic, multicellular and heterotrophic. Animals have no cell wall, and they take in food and digest it inside the body.

1. Phylum Porifera (sponges).

• Most are marine. Some live in fresh water.
• Book examples: Leucosolenia and Euplectella (Venus' flower basket) are marine. Spongilla is a freshwater sponge.
• No tissue level of organization. Most are asymmetrical.
• Numerous pores called ostia let water into the body. The inside cavity is the spongocoel.
• Three layers: thin flat pinacocytes outside, a jelly-like mesohyle with amoeboid cells in the middle, and choanocytes (collar cells) lining the spongocoel.
• The skeleton is made of tiny needles of calcium carbonate or silica.
• Most reproduce asexually by budding or regeneration. Some form gemmules, which are resistant capsules of amoeboid cells.

2. Phylum Cnidaria.

• Almost all are marine. Book examples: hydra, jellyfish, obelia, corals, sea fans.
• Radially symmetrical and diploblastic. The two layers are the epidermis and the gastrodermis, with a jelly-like mesoglea between them.
• Cnidocytes hold a stinging organelle, the nematocyst. It defends the body and captures prey.
• A gastrovascular cavity (enteron) has a single opening. The book calls this a sac-like digestive system.
• Two body forms: polyps (cylindrical, attached, reproduce asexually) and medusae (umbrella-like, swimming, reproduce sexually).
• Corals are colonial. Their calcium carbonate skeleton makes coral reefs and coral islands.

3. Phylum Platyhelminthes (flatworms).

• Soft, mostly unsegmented and flattened from top to bottom (dorsoventrally compressed).
• Some live on their own, for example planaria. Some are endoparasites, for example liver fluke, tapeworm and blood-fluke.
• Triploblastic and bilaterally symmetrical.
• Acoelomate: parenchyma fills the space between body wall and organs.
• Excretion by protonephridia with flame cells.
• Asexual reproduction by fission. Sexually reproducing flatworms are hermaphrodites.
• The book notes that tapeworms are segmented.

4. Phylum Nematoda (roundworms).

• Elongated worm-like round body with pointed ends.
• Examples: Caenorhabditis elegans (lives on its own in water and soil) and the parasites ascaris, hookworm, pinworm and whipworm.
• Triploblastic, bilaterally symmetrical, unsegmented.
• Pseudocoelomate: a false body cavity, the pseudocoelom, filled with fluid.
• A tube-like gut with two openings, a mouth and an anus.
• A nerve ring around the pharynx joins four longitudinal nerve cords.
• Unisexual: males have testes and females have ovaries.
• The book says the pseudocoelomates fall into seven phyla, grouped as Aschelminths, and Nematoda is the representative phylum.

5. Phylum Mollusca.

• Soft, unsegmented bodies. Examples: mussels, octopus, oyster (aquatic) and the land snail (moist places).
• Triploblastic, bilaterally symmetrical, with a true coelom. They are protostomes.
• Body in three parts: head, visceral mass and foot.
• The mantle wraps the visceral mass. Its outer surface usually secretes a calcareous shell.
• A radula is present in all except bivalves.
• Heart with a single ventricle and two auricles. Excretion by nephridia. Gills for gas exchange.
• Three pairs of ganglia, in the head, visceral mass and foot.
• Most are unisexual. Book classes: Gastropoda, Bivalvia and Cephalopoda.

Key points

Use the table as a revision map. Read it down one column to compare phyla on a single feature.

Three layers or two. Diploblastic means two layers. Triploblastic means three, from ectoderm, mesoderm and endoderm.

Worked examples

Example 1. Name the phylum from these clues: radial symmetry, nematocysts, one opening to the gut.

1. Nematocysts sit inside cnidocytes.
2. A single-opening gastrovascular cavity is a sac-like gut.
3. The phylum is Cnidaria.

Example 2. A worm has a false body cavity filled with fluid and a gut with mouth and anus. Which phylum?

1. A false cavity is a pseudocoelom.
2. The book uses the pseudocoelom for Nematoda.
3. The phylum is Nematoda.

Example 3. Why is a tapeworm different from the other flatworms?

1. Most flatworms are unsegmented.
2. The book notes that the tapeworm is segmented.
3. It is also an endoparasite.

Example 4. Name the three body parts of a mollusc.

1. The head.
2. The visceral mass, which holds digestive, excretory and reproductive organs.
3. The foot, attached to the visceral mass.

Common mistakes in the exam

1. Calling sponges diploblastic. The book says sponges have no tissue level of organization.
2. Mixing ostia with the mouth. Ostia are the many small pores. Water enters through them.
3. Writing "coelomate" for flatworms. They are acoelomates.
4. Mixing pseudocoelom with true coelom. Roundworms have the false cavity. Molluscs have a true coelom.
5. Giving a radula to every mollusc. Bivalves do not have one.
6. Saying all flatworms are parasites. Planaria lives on its own.

Exam technique

• For a "compare" question, draw a small table with symmetry, layers and cavity.
• Always add one book example for each phylum.
• Use exact terms: gemmules, cnidocytes, flame cells, mantle.
• Check your board's notice for the sections in your paper.

Practice questions

1. Name the three layers of a sponge from outside to inside. Answer: Pinacocytes, the mesohyle with amoeboid cells, and the choanocytes lining the spongocoel.
2. What are the two body forms of cnidarians? Answer: Polyps and medusae.
3. Which cells remove waste in flatworms? Answer: Flame cells of the protonephridia.
4. Give the symmetry and the sex condition of nematodes. Answer: Bilateral symmetry, and they are unisexual.
5. Name the three classes of molluscs shown in the book. Answer: Gastropoda, Bivalvia and Cephalopoda.

Ready to practise this? Continue in 1st Year Biology.