Invertebrates: Phylum MolluscaMolluscs Large and diverse groupCoelomatesBilateral symmetry, three germ

layers, organ level of organization, tube-within-a-tube body plan

30-1

Mollusca cont’d3 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

Diversity of molluscs

Fig. 30.10

Mollusca cont’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

30-4

Mollusca cont’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

Gastropod and cephalopod anatomy

Fig. 30.11

Molluscs cont’dClass 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

Molluscs cont’dClass 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

Clam, Anodonta

Fig. 30.12

Comparison of clam to squid to land snail

Table 30.230-10