PHYLUM MOLLUSCA (Molle= soft body)

PHYLUM MOLLUSCA(Molle= soft body)

PHYLUM MOLLUSCA • OVER 100,OOO LIVING SPECIES; MANY 1000’S OF FOSSIL SPECIES • FOUND IN ALMOST ALL ENVIRONMENTS: MARINE, FRESHWATER, TERRESTRIAL • INCLUDES SNAILS, SLUGS, CLAMS, SQUIDS, OCTOPUSES,

CHARACTERISTICS • Body Plan: three main parts- head, foot, visceral mass • body is unsegmented

CHARACTERISTICS • Mantle - specialized tissue, secretes shell, may be involved in feeding, reproduction and respiration • Radula – for rasping; in all but one class Radula closeup

ALL ORGAN SYSTEMS ARE PRESENT • Circulation - Open (Hemocoel) - Hemocyanin • Respiratory System - gills (Ctenidia), lungs, mantle, epidermis • Osmoregulation - Metanephridia also excretion

ALL ORGAN SYSTEMS ARE PRESENT • Feeding - Radula (scraping) • Bivalves - Filter feed • Nervous System - Complex system of Ganglia

ALL ORGAN SYSTEMS ARE PRESENT • Reproductive System - usually dioecious, monoecious in some • Marine forms have free living trochophore larva • Other larval stages may be present

MOLLUSCAN CLASSES

Class Monoplacophora“one plate” • all known from deep marine waters (>12,000 feet) • have a single circular shell • with radula

ClassPolyplacophora“many plates”- (chitons) • have 8 overlapping plates or shells • reduced head, no eyes or tentacles • all marine; usually live on rocky shores • have radula, gills • external fertilization • scrape algae

Polyplacophora • Respiration - Gills in pallial groove • Circulation - Heart at posterior end - single Aorta • Excretion - Nephridia • Nervous System and sense organs - Ganglia and nerves • Sub Radular organ - Feeding • Esthetes - light sensing • Osphradia - Sampling water

Polyplacophora • Reproduction - Sexes Separate • Usually external fertilization • Trochophore larva

ClassAplacophora“without plates” • Rare; lack shell; wormlike • deep marine; burrowing species; predators • About 320 species, not much is known of them • Your book separates them into two separate classes: • ! Caudofoveata • ! Solenogastres

Class Gastropoda • Largest class of Molluscs • Most important feature is Torsion

Torsion • Twisting of body into loop • Perhaps adaptation of veliger larva • Disadvantages: • Reduction of organs on right side • Excretion - Mouth moves near anus • Adavntages • Allows water to be drawn from the front

Gastropod - Locomotion • Foot • Ciliary action below the foot • Operculum • Swimming

Gastropod - Nutrition • Herbivores - Radula • Carnivores • Filter feeders • NERVOUS SYSTEM • Similar to Chiton - but in the shape of “8”

Class - Bivalvia • Includes Calms, Oysters, Mussels • Has two Shells (Valves) • Shell held together by Adductor muscle • Head is lost • Filter feeders • Mantle secretes the shell

MOLLUSCAN SHELL STRUCTURE • Periostracum; outer layer • Prismatic; middle layer • Nacre; inside layer- mother of pearl

BIVALVIA • Locomotion • Foot (burrowers) • Active swimmers (Scallops) • Circulation • Open system • Excretion • Nephridia

BIVALVIA • Nervous System • 2 pairs of ganglia • Sense organs (statocysts, ocelli, ospharidia) • Reproduction • Sexes separate • Internal fertilization • Larva - Glochidium

Class - Cephalopoda • Includes Squid, octopus, Nautilus • Most advanced of Molluscs • Well developed head and eyes • Head has tentacles with suckers • Squids are largest of invertebrates • Locomotion by jet propulsion

Class Scaphopoda(tusk shells) • slender, tubular shells open at both ends • all marine • have tentacles around mouth • no defined head • no gills, respiration across mantle

PHYLUM MOLLUSCA (Molle= soft body)