Phylum Mollusca - Class Polyplacophora Flashcards

Class Polyplacophora Overview

• Belongs to Phylum Mollusca, meaning "many plate-bearing."
• Typically features shells made up of 7-8 separate overlapping plates.
• Approximately 800 species identified, commonly known as "chitons."
• Size ranges from 3-10 cm in length.

Habitat and Environment

• Generally inhabits the intertidal zone, close to shore.
• Primarily lives on hard substrates, especially rocks.

Shell Composition

• The shell consists of 7-8 articulating plates located on the dorsal surface.
• Plates are partially to mostly embedded in the mantle tissue called the girdle.
• Multi-sectioned design allows adaptation to various substrate shapes.

Girdle and Mantle Cavity

• Girdle is the thick lateral mantle that embeds the shell plates.
• In many species, the girdle bears spicules that are secreted independently.
• The mantle cavity features two lateral grooves on each side of the body.

Bipectinate Ctenidia

• Each groove can hold up to 80 bipectinate ctenidia, acting as gills.
• Ctenidia divide the mantle cavity into incurrent and excurrent chambers for water flow.
• Water enters through gill cilia, while waste is expelled posteriorly.
• Blood flows through gill lamellae via countercurrent exchange for efficient respiration.

Locomotion and Suction

• The foot extends along the entire ventral surface, covered by the shell and girdle.
• Movement occurs through pedal waves of muscular activity.
• Capable of creating suction by pressing the girdle against a substrate and lifting the foot, an adaptation for high-wave areas.

Nervous and Sensory Systems

• Nervous system is simple and ladder-like; ganglia may be lacking or poorly developed in some species.
• Sensory systems are minimal, lacking tentacles and eyes, with the presence of Aesthetes.
• Aesthetes are sensory organs derived from mantle tissue that can function as light or chemoreceptors.

Digestive System

• Digestive tract is linear, with the mouth and anus at opposite ends.
• Primarily herbivorous, scraping food like algae from substrates; some species are carnivores.
• Sugar glands, a pair of pharyngeal glands, secrete amylase into the stomach for digestion.

Fossil Records and Evolution

• Fossil records date back approximately 500 million years.
• Polyplacophorans likely diverged early in molluscan evolution.

Explore the fascinating world of Class Polyplacophora, part of Phylum Mollusca, through these informative flashcards. Discover their unique characteristics, living environments, and classification. Perfect for students and enthusiasts of marine biology!