Phylum Mollusca Ex: Chitons, Snails, Clams, Octopods, and Squid

General Characteristics It 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 Characteristics Body Plan Soft bodied (term Mollusca is Latin for “soft”) invertebrates with bilateral symmetry. Usually protected by a calcareous shell.

General Characteristics Body Plan Unsegmented body with a reduced coelum. The soft body parts are protected by a protective tissue called the mantle.

General Characteristics Body Plan They 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 Characteristics Body Plan Two major parts: Head-foot: contains head, mouth, sensory organs, and muscular foot.

General Characteristics Body Plan Visceral Mass: contains other organ systems, including circulatory, digestive, respiratory, excretory, and reproductive systems. (#4 is visceral mass)

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

General Characteristics The shell is secreted (made) by the mantle. The shell consists of 3 layers: Periostracum: outer Prismatic: middle Nacreous: inner

Class Polyplacophora “Many plates” Chitons Live 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 Polyplacophora They 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 Scaphopoda “Sheath foot” Tusk shells: shell resembles an elephant tusk with 2 openings. Foot protrudes from one end and is used for burrowing

Class Scaphopoda Water 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 foot” 75% of Mollusc species are Gastropods Great diversity Most are found on the benthos both on rocky and soft bottoms.

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

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

Class Gastropoda The Foot Broad and mucus covered. May function as an adhesive gland that works like a suction cup.

Class Pelecypoda or Bivalvia “Hatchet foot” Ex: Clams, Oysters, Mussels, Scallops. Typically sessile, living burrowed in sandy or muddy sediments.

Class Pelecypoda The Shell Consists 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 Pelecypoda The Foot Used primarily for burrowing and anchoring.

Class Pelecypoda Formation of Pearls An 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 Cephalopoda Ex: Squid, Octopus, Nautilus, Cuttlefish

Class Cephalopoda “Head foot” Most advanced class of Molluscs. Have a highly developed brain and sense organs.

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

Cephalopoda The Foot Modified 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.

Cephalopoda The Foot Part of the foot is modified into a siphon used in jet propulsion. Ink can be expelled from the siphon to distract and confuse predators.

Cephalopoda They are the largest of the invertebrates. Architeuthus: the giant squid can reach lengths of 60 ft.

Cephalopoda The Shell With 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.

Cephalopoda Color Change Cephalopods communicate through movements and color change. Color change involves specialized cells called chromatophores.

Cephalopoda Feeding They are active carnivores. The radula is reduced. They have a pair of jaws shaped like the beak of a parrot. tmb.edu/viddb/vidsrch 2.cfmhttp:// tmb.edu/viddb/vidsrch 2.cfm

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