33.1.6: Phylum Mollusca
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Learning Objectives
- Describe the unique anatomical and morphological features of molluscs
Phylum Mollusca
Phylum Mollusca is the predominant phylum in marine environments. It is estimated that 23 percent of all known marine species are mollusks; there are around 85,000 described species, making them the second most diverse phylum of animals. The name “mollusca” signifies a soft body; the earliest descriptions of mollusks came from observations of unshelled cuttlefish. Mollusks are predominantly a marine group of animals; however, they are known to inhabit freshwater as well as terrestrial habitats. Mollusks display a wide range of morphologies in each class and subclass. They range from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately-sculpted and colored shells. In spite of their tremendous diversity, however, they also share a few key characteristics, including a muscular foot, a visceral mass containing internal organs, and a mantle that may or may not secrete a shell of calcium carbonate.
Mollusks have a muscular foot used for locomotion and anchorage that varies in shape and function, depending on the type of mollusk under study. In shelled mollusks, this foot is usually the same size as the opening of the shell. The foot is a retractable as well as an extendable organ. It is the ventral-most organ, whereas the mantle is the limiting dorsal organ. Mollusks are eucoelomate, but the cavity is restricted to a region around the heart in adult animals. The mantle cavity develops independently of the coelomic cavity.
The visceral mass is present above the foot in the visceral hump. This includes digestive, nervous, excretory, reproductive, and respiratory systems. Mollusk species that are exclusively aquatic have gills for respiration, whereas some terrestrial species have lungs for respiration. Additionally, a tongue-like organ called a radula, which bears chitinous tooth-like ornamentation, is present in many species, serving to shred or scrape food. The mantle (also known as the pallium) is the dorsal epidermis in mollusks; shelled mollusks are specialized to secrete a chitinous and hard calcareous shell.
Most mollusks are dioecious animals where fertilization occurs externally, although this is not the case in terrestrial mollusks, such as snails and slugs, or in cephalopods. In some mollusks, the zygote hatches and undergoes two larval stages, trochophore and veliger, before becoming a young adult; bivalves may exhibit a third larval stage, glochidia.
Key Points
- A mollusk’s muscular foot is used for locomotion and anchorage, varies in shape and function, and can both extend and retract.
- The visceral mass inside the mollusk includes digestive, nervous, excretory, reproductive, and respiratory systems.
- Most mollusks possess a radula, which is similar to a tongue with teeth-like projections, serving to shred or scrape food.
- The mantle is the dorsal epidermis in mollusks; in some mollusks it secretes a chitinous and hard calcareous shell.
Key Terms
- visceral mass : the soft, non-muscular metabolic region of the mollusc that contains the body organs
- mantle : the body wall of a mollusc, from which the shell is secreted
- radula : the rasping tongue of snails and most other mollusks