MOLLUSKS Chapter 16

Molluscs are triploblastic which means three tissue layers.   II.             Molluscs are coelomates:  a fluid filled body cavity lined by mesoderm.  Have a complete digestive system.  

2 GROUPS OF COELOMATES:

1.  Protostomes:  Molluscs and Annelids  Spiral, determinate cleavage The blastopore forms the mouth  

2.       Deuterostomes: echinoderms, chordates v  Radial, indeterminate cleavage. v  The blastopore forms the anus.  

The coelom may have formed from splitting of the mesoderm or from out-pocketing of the gut.

PHYLUM MOLLUSCA SHARES CHARACTERISTICS III. 8 different classes  Body composed of a head-foot and visceral mass (organ systems)A mantle – covers visceral mass and may secrete a calcareous shell

OTHER COMMON CHARACTERISTICSBilateral symmetry

Protostome characteristics

A trochophore larva

A reduced coelom

Open circulatory system

    (exception: cephalopods which have a closed      

     circulatory system)

Many molluscs feed via a radula

Class GastropodaExamples Snails and SlugsFeeding/digestion

Herbivores, feeding with a radula

Respiratory Gills; mantle contractions circulate air; have a siphon which serves as an inhalant tube.

Circulatory Open (blood flows from heart leaves the vessels and goes directly into sinuses.)

Skeletory Hydraulic (blood confined to tissue spaces for support.)

Nervous system 6 ganglia in the head foot and a visceral massa.    eyes on tentaclesb.   statocysts on the foot for

equilibriumc.    ophradia in the anterior wall

of the mantle that helps detect prey.

Excretory Nephridium – converts ammonia to uric acid

Reproduction

External fertilization with some being monoecious and others being dioecious.

Locomotion A ciliated, flattened foot covered with glands used to creep across the substrate.

Special Features

Torsion – gives shell the coiled appearance

Class BivalviaExamples Clams, oysters, mussels, and scallopsFeeding/digestion Filter-feeders; loss of head and radulaRespiratory Cilia covered gills forming folded

sheets called lamellae; cilia moves water over an incurrent siphon

Circulatory Open Skeletory External shellNervous system 3 pairs of ganglia, sensory cells on the

margin of the mantle, photoreceptors, statocysts and osphradium

Excretory NephridiumReproduction Most dioecious; external fertilizationLocomotion SedentarySpecial Features Two convex halves of shell called

valves

Class CephalopodaExamples Octopuses, squids, cuttlefish, and nautili

Feeding/digestion Locate prey by sight and capture prey with tentacles that have adhesive cups; radula and beak for tearing food; peristalsis replaces ciliary action

Respiratory GillsCirculatory Closed – blood confined to vessels;

more efficientSkeletory Shell is reduced or absent; except

nautilusNervous system Large brain – memory and decision

making; can form images, distinguish shape & some color; statocysts by brain; osphradia only in nautilus

Excretory Have a kidney which allows waste to filter directly from the blood stream which is very efficient

Reproduction Dioecious; males have a hectocotylus for spermatophore transfer into the female near the opening of the oviduct; eggs deposited onto a substrate and hatchlings are miniature adults.

Locomotion Jet propulsion (water forced through funnel)

Special Features Modified foot into tentacles or arms.  They have chromatophores (pigment cells) which allow animal to change color and discharge ink for defense; confuses predator allowing the cephalopod to escape.