Characteristics of Mollusca: Study Notes

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General Characteristics of Mollusca: The Soft-Bodied Animals

The phylum Mollusca includes animals commonly known as mollusks, characterized by their soft, unsegmented bodies and diverse habitats. Below are the defining features of mollusks and their unique adaptations.

1. Name Meaning

• The term "mollusc" is derived from Latin, meaning soft-bodied, which reflects the typical body structure of animals in this phylum.

2. Body Type

• Mollusks have a soft and unsegmented body, which allows them flexibility and adaptability across various environments.

3. Phylum Size

• Mollusca is the second-largest phylum in the animal kingdom, boasting 50,000 living species and 35,000 fossil species. This brings the total known species to around 85,000.

4. Body Organization

• Mollusks have a multicellular, triploblastic body structure with bilateral symmetry. They are coelomate (possessing a true body cavity) and protostomes (mouth develops before the anus during embryonic development), making them relatively advanced among invertebrates.

5. Body Division

• The body of a mollusk is divided into three main regions:
  • Head: For sensory functions and feeding.
  • Foot: Primarily for locomotion.
  • Visceral Mass: Contains the majority of the internal organs.

6. Mantle & Shell

• The mantle is a protective layer that covers the visceral mass and plays a crucial role in secreting the shell. The shell can be external, internal, or completely absent, depending on the species.

7. Feeding Organ

• Mollusks possess a specialized rasping organ called the radula in their mouth, which helps them scrape or cut food, facilitating feeding.

8. Circulatory System

• Mollusks generally have an open circulatory system, where blood flows freely within body cavities. An exception to this is the class Cephalopoda (e.g., octopuses and squids), which have a closed circulatory system to support their active lifestyle.

9. Skeleton

• Mollusks use a hydrostatic skeleton, which relies on fluid pressure within their body cavities to maintain structure and support.

10. Respiration

• Gills are used for respiration in aquatic mollusks, while rudimentary lungs may be present in terrestrial species. Some mollusks have adaptations to use both gills and lungs, depending on their habitat.

11. Larval Stage

• The larval stage of a mollusk is called a trochophore. This free-swimming larval form is a key developmental stage for many aquatic mollusks, aiding in dispersal.

12. Basic Features

• Mollusks have three essential body features:
  • Head-foot: Used for feeding, sensory activities, and locomotion.
  • Visceral Mass: Houses vital organs, including the digestive, reproductive, and excretory systems.
  • Mantle: A thin, protective membrane that may produce a shell.

13. Habitat

• While the majority of mollusks are marine, many have adapted to freshwater and terrestrial habitats, showcasing their adaptability to different environments.

14. Reproduction

• Mollusks typically have separate sexes and reproduce through external fertilization, where fertilization occurs outside the body.

Mollusks exhibit remarkable diversity in form and function, from simple clams to complex cephalopods like octopuses. Their unique body structure, specialized feeding mechanisms, and adaptation to various habitats have enabled them to become one of the most widespread and successful animal groups on Earth.

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