Mollusca

Definition

Mollusca is a diverse phylum of invertebrate animals known for their soft bodies, which are often protected by a hard shell. This group includes well-known organisms like snails, clams, and octopuses. Molluscs display a range of body plans and adaptations that have allowed them to thrive in various environments, showcasing their significance within the superphylum Lophotrochozoa, alongside annelids.

5 Must Know Facts For Your Next Test

1. Mollusca is the second-largest phylum in the animal kingdom, with over 85,000 recognized species, showing incredible diversity in form and function.
2. Most molluscs have a three-part body plan consisting of a foot for locomotion, a visceral mass containing internal organs, and a mantle covering.
3. Many molluscs possess a hard external shell made of calcium carbonate, though some, like octopuses and slugs, either lack shells or have reduced ones.
4. Mollusca are important ecologically as they play roles as herbivores, predators, and prey within their ecosystems, contributing to nutrient cycling.
5. The nervous system of molluscs varies greatly among classes, from the simple nerve nets in bivalves to the complex brains found in cephalopods like squids and octopuses.

Review Questions

• Compare and contrast the body structure of molluscs with annelids and discuss how these differences reflect their ecological roles.
  • Molluscs possess a distinct three-part body plan consisting of a foot, visceral mass, and mantle, while annelids have segmented bodies with repeated units. This structural difference allows molluscs to adapt to various ecological niches; for example, the muscular foot enables snails to crawl effectively on land or substrate. In contrast, annelids use their segments for burrowing and locomotion in soil or water. These adaptations reflect how each group has evolved to exploit different resources and habitats.
• Analyze the evolutionary significance of the radula in molluscs and its impact on their feeding strategies compared to other invertebrates.
  • The radula is a key evolutionary adaptation that has enabled molluscs to diversify their feeding strategies. It allows for scraping or cutting food sources such as algae or other organic material, giving them an advantage over many other invertebrates that may rely solely on filter feeding or absorption. This adaptability has led to the success of various mollusc classes in different environments, as they can exploit a wider range of food resources than many of their invertebrate counterparts.
• Evaluate how the diversity of molluscan forms and functions relates to their survival in various environments and the implications for ecological balance.
  • The incredible diversity among molluscs—from the terrestrial adaptations of land snails to the complex behaviors of cephalopods—illustrates their ability to occupy various ecological niches. This diversity allows molluscs to fulfill vital roles within ecosystems, such as herbivores grazing on plants, predators controlling populations of other species, and prey for larger animals. Their presence helps maintain ecological balance; for example, bivalves filter water and improve water quality. Thus, understanding molluscan diversity not only highlights their evolutionary success but also underscores their importance in ecosystem health.

Related terms

Mantle: The mantle is a significant part of a mollusc's anatomy, responsible for secreting the shell and forming a protective layer over the body.

Radula: The radula is a specialized feeding structure found in many molluscs, functioning like a tongue covered with tiny teeth to scrape food from surfaces.

Gastropoda:

Gastropoda is a large class within Mollusca that includes snails and slugs, characterized by their unique torsion process and often coiled shells.