Phylum MolluscaEx: Chitons, Snails, Clams, Octopods, and Squid

General CharacteristicsIt is believed that the molluscs evolved from annelids, but some scientists argue that they may have evolved from flatworms.50% of the species are marine.

General CharacteristicsBody PlanSoft bodied (term Mollusca is Latin for soft) invertebrates with bilateral symmetry.Usually protected by a calcareous shell.

General CharacteristicsBody PlanUnsegmented body with a reduced coelum.The soft body parts are protected by a protective tissue called the mantle.

General CharacteristicsBody PlanThey possess a mantle cavity where the gills are located, into which the anus and kidneys release excretia, and into which eggs and sperm are released.

General CharacteristicsBody PlanTwo major parts:Head-foot: contains head, mouth, sensory organs, and muscular foot.

General CharacteristicsBody PlanVisceral Mass: contains other organ systems, including circulatory, digestive, respiratory, excretory, and reproductive systems. (#4 is visceral mass)

General CharacteristicsMost possess a radula. A ribbon of tissue that contains teeth.The radula is used for scraping, tearing, piercing, or cutting food.

General CharacteristicsThe shell is secreted (made) by the mantle.The shell consists of 3 layers:Periostracum: outerPrismatic: middleNacreous: inner

Class PolyplacophoraMany platesChitonsLive in the rocky intertidal zone.Retain the greatest number of ancestral characteristics.

Class Polyplacophora

Shell: 8 overlapping plates held together by a tough girdle formed from the mantle.Foot used for attaching tightly to rocks.

Class PolyplacophoraThey possess a radula with teeth that are mineralized with magnetite.Magnetite may become magnetized and the chitons may use the magnetic field of the earth to navigate (like a compass).

Class ScaphopodaSheath footTusk shells: shell resembles an elephant tusk with 2 openings.Foot protrudes from one end and is used for burrowing

Class ScaphopodaWater enters and exits small end to exchange gases and remove waste.The foot or special tentacles emerge from the other end to feed on foramniferans.

Class Gastropoda

Stomach foot75% of Mollusc species are GastropodsGreat diversityMost are found on the benthos both on rocky and soft bottoms.

Class GastropodaThe ShellTypically a pointed tube or cone into which the animal can contract.The opening can be closed with a cover called the operculum.Operculum----

Class GastropodaNudibranchs

A subgroup of Molluscs that have lost all traces of a shell.

Class GastropodaThe FootBroad and mucus covered.May function as an adhesive gland that works like a suction cup.

Class Pelecypoda or BivalviaHatchet footEx: Clams, Oysters, Mussels, Scallops.Typically sessile, living burrowed in sandy or muddy sediments.

Class PelecypodaThe ShellConsists of 2 valves (plates) that generally completely cover the body, and are connected at a hinge by ligaments.They possess no head or radula. Filter feeders.

Class PelecypodaThe Foot

Used primarily for burrowing and anchoring.

Class PelecypodaFormation of PearlsAn irritant (Usu. a grain of sand) comes between the shell and the mantle tissue.The response is to cover it with layers of nacreous material.If the irritant is spherical and embeds in the mantle tissue it becomes a pearl.

Class CephalopodaEx: Squid, Octopus, Nautilus, Cuttlefish

Class CephalopodaHead foot Most advanced class of Molluscs.Have a highly developed brain and sense organs.

CephalopodaThe eyes are very much like vertebrate eyes and are the dominant sense organ.

CephalopodaThe FootModified into a head-like structure with a ring of tentacles surrounding the mouth.The tentacles are used for capturing prey, defense, reproduction, and in some cases locomotion.

CephalopodaThe FootPart of the foot is modified into a siphon used in jet propulsion.Ink can be expelled from the siphon to distract and confuse predators.

CephalopodaThey are the largest of the invertebrates.

Architeuthus: the giant squid can reach lengths of 60 ft.

CephalopodaThe ShellWith the exception of Nautilus they all have lost the heavy external shell.Squid have a small internal shell called the Pen, and octopi have no shell.

CephalopodaColor ChangeCephalopods communicate through movements and color change.Color change involves specialized cells called chromatophores.

CephalopodaFeedingThey are active carnivores.The radula is reduced.They have a pair of jaws shaped like the beak of a parrot.http://www.cephbase.utmb.edu/viddb/vidsrch2.cfm

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