Invertebrates: Phylum Mollusca

1. Invertebrates: Phylum Mollusca • Molluscs • Large and diverse group • Coelomates • Bilateral symmetry, three germ layers, organ level of organization, tube-within-a-tube body plan

2. Molluscacont’d • 3 body parts • Visceral mass • includes most organs • Head/Foot • Radula: rasping organ for feeding • muscular portion used for locomotion • Mantle • covering that envelopes visceral mass • Mantle cavity: space between the two folds of the mantle • Mantle may secrete a shell

3. Diversity of molluscs • Fig. 30.10

4. Molluscacont’d • Class Gastropoda • Gastro= stomach Poda = foot • Foot is flattened ventrally • Nudibranchs • sea slugs, lack shell • Conchs & snails • have a shell • Protection, prevents dessication in land snails • Aquatic gastropods: have gills, • Terrestrial gastropods:mantle has lung-like function

5. Molluscacont’d • Class Cephalopoda • Cephalo = brain poda = foot • Squid, octopus, Chambered nautilus • Body Structure • Foot forms tentacles • Powerful beak and radula for feeding • Extremely well-developed eyes; complex behaviors • Ink glands- secrete “ink”as defense mechanism

6. Gastropod and cephalopod anatomy • Fig. 30.11

7. Molluscscont’d • Class Bivalva • Bi = two valva= shells • Clams, oysters, scallops • Gills enclosed in mantle cavity • Filter-feeders- water enters through incurrent siphon • Visceral mass • coelom is reduced-pericardial cavity is the only remains of the coelom • Open circulatory system • blood not always in vessels • “sloshes” around in sinuses • Heart pumps blood to dorsal aorta and then to organs

8. Molluscscont’d • Class Bivalva cont’d • Lack cephalization • nervous system consists of ganglia connected by nerves • Digestive system • labial palps, esophagus, stomach, intestine • Excretion by kidneys • Sexes separate, fertilization produces a trochophore larva

9. Clam, Anodonta • Fig. 30.12

10. Comparison of clam to squid to land snail • Table 30.2