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

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.

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

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

  2. 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.

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

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

  5. General CharacteristicsBody Plan • They possess a mantlecavity where the gills are located, into which the anus and kidneys release excretia, and into which eggs and sperm are released.

  6. General CharacteristicsBody Plan • Two major parts: • Head-foot: contains head, mouth, sensory organs, and muscular foot.

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

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

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

  10. Class Polyplacophora • “Many plates” • Chitons • Live in the rockyintertidal zone. • Retain the greatest number of ancestralcharacteristics.

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

  12. 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).

  13. Class Scaphopoda • “Sheath foot” • Tusk shells: shell resembles an elephant tusk with 2 openings. • Foot protrudes from one end and is used for burrowing

  14. 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.

  15. Class Gastropoda • “Stomach foot” • 75% of Mollusc species are Gastropods • Great diversity • Most are found on the benthos both on rocky and soft bottoms.

  16. Class GastropodaThe Shell • Typically a pointedtube or cone into which the animal can contract. • The opening can be closed with a cover called the operculum. Operculum----

  17. Class GastropodaNudibranchs • A subgroup of Molluscs that have lost all traces of a shell.

  18. Class GastropodaThe Foot • Broad and mucus covered. • May function as an adhesive gland that works like a suction cup.

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

  20. Class PelecypodaThe 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.

  21. Class PelecypodaThe Foot • Used primarily for burrowing and anchoring.

  22. Class PelecypodaFormation 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.

  23. Class CephalopodaEx: Squid, Octopus, Nautilus, Cuttlefish

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

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

  26. CephalopodaThe 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.

  27. CephalopodaThe 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.

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

  29. CephalopodaThe 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.

  30. CephalopodaColor Change • Cephalopods communicate through movements and color change. • Color change involves specialized cells called chromatophores.

  31. CephalopodaFeeding • They 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

  32. ****The End****

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