📚 Summary: Phylum Mollusca and Phylum Annelida
In this lesson, you will study two important animal phyla — Mollusca and Annelida. These groups include a wide variety of organisms, from soft-bodied aquatic creatures to segmented earthworms.
Phylum Mollusca – The Soft-bodied Animals
Molluscs are soft, unsegmented animals found in aquatic and moist environments. Examples include snails, mussels, octopuses, and oysters.
Key Characteristics:
- They are triploblastic, bilaterally symmetrical, and coelomate animals.
- Their body has three main parts: head, visceral mass (contains organs), and a muscular foot for movement.
- The mantle is a special covering that may secrete a calcareous shell.
- Most molluscs have a radula, a tongue-like feeding organ (except in bivalves).
- They have an open circulatory system (except in cephalopods) and a tube-like digestive system.
- Their nephridia excrete waste into the mantle cavity.
- Gills are used for respiration.
- Their nervous system includes ganglia in the head, foot, and visceral mass.
- Most molluscs are unisexual.
Phylum Annelida – The Segmented Worms
Annelids are segmented worms found in marine, freshwater, and moist terrestrial habitats. Examples include earthworms, leeches, and nereis.
Key Characteristics:
- They are triploblastic, bilaterally symmetrical, coelomates, and protostomes.
- Their bodies are divided into segments, internally separated by septa — a feature called metamerism.
- They possess setae (bristles) in most segments for movement, but leeches lack setae.
- They have a moist cuticle, a tube-like digestive system, and closed circulatory system with pulsating vessels.
- Haemoglobin is dissolved in their blood plasma for respiration.
- They use their skin for gaseous exchange.
- Their nervous system includes a brain, nerve cords, and segmental ganglia.
- They have sensory receptors like touch, chemical, balance, and light detectors. Some even have simple eyes.
- Most are hermaphrodites, but some (like nereis) are unisexual.
What You Will Learn
By the end of this lesson, you’ll be able to:
- Identify and describe the body structures and systems of molluscs and annelids.
- Compare their modes of reproduction, movement, and habitats.
- Understand the significance of segmentation, radula, setae, and nephridia.