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

Class Polyplacophora “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

Class Aplacophora “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 Circulation Excretion Foot (burrowers) Active swimmers (Scallops) Circulation Open system Excretion Nephridia

BIVALVIA Nervous System Reproduction 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