MOLLUSCS

1. Molluscs belong to the phylum Mollusca.

There are more than 100,000 species known. They live everywhere from the deepest oceans trenches to the tops of trees.

• 2. They can be as small as a grain of sand or as large as a school bus.

• 3. All molluscs have a similar developmental pattern. They develop from a trochophore larva.

• 4. these larva float around in the water and feed on plankton. All mollusc (except one) have an internal or external shell.

• 5. Most mollusc have 4 main body parts. These are the foot, mantle, shell, and visceral mass.

• 6. The muscular foot usually contains the mouth and feeding structures.

• 7. It may be adapted for crawling or may be like tentacles.

• 8. The mantle is a thin tissue that covers most of the body.

• 9. The shell is made by glands in the mantle that secrete calcium carbonate.

• Beneath the mantle is the visceral mass. The visceral mass contains the organs.

• 10. These basic body parts have evolved to adapt to different habitats. The type of foot and kind of shell they have are used to group molluscs into classes.

• 11. Molluscs feed on about everything. Most are herbivores, carnivores or filter feeders. A few feed on detritus or are parasites.

• 12. Many molluscs like snails and slugs feed by using a structure called a radula. A radula is a tongue-like structure consisting of a layer of skin covered with hundreds of tiny teeth.

• 13. It acts a feels like sandpaper. It uses the radula to scrape food particles into its mouth.

• Carnivorous molluscs can drill holes through the shells of other animals using the radula.

• 14. Once they create a hole they will extend their mouth and radula through it and rip apart and swallow the soft tissues of the animal.

• 15. Some carnivorous molluscs like the octopus and sea slugs have jaws with a beak-like structure they use to tear their prey apart.

• 16. Some produce poison used to kill their prey. Some are fatal to humans.

• 17. Molluscs like clams, oysters, and scallops are filter feeders. They use feathery gills to strain food from the water.

• 18. They pass water over the gills and tiny plankton gets gets caught in the sticky mucus covering the gills. Cilia move the mixture of mucus and food into the mouth.

• 19. The gills are used in respiration (to breath). Terrestrial molluscs breath by using specially adapted mantle cavities that are lined with blood vessels. This must be kept moist in order for gas exchange to occur.

• 20. The oxygen taken in travels in the blood to various parts of the mollusc. The blood is pumped around with a simple heart. Most molluscs have an open circulatory system.

• 21. This means the blood is NOT contained within vessels. The blood works its way into open spaces in the body called sinusus.

• 22. Squid and octopus have a closed circulatory system where the blood stays inside blood vessels all the time. This is a more efficient way to circulate blood. Why do you think they need a closed system?

• 23. Undigested food is removed in the form of feces. Cellular waste has a lot of ammonia in it. Because ammonia is toxic it has to be removed from the body.

• 24. Molluscs get rid of ammonia by using simple tube-shaped organs called nephridia which remove it from the blood and release it to the outside.

• 25. There is a wide variety of nervous systems found in molluscs. The simplest may include a ganglia, ocelli, and statocysts.

• 26. The more active molluscs like squid and octopus have very highly developed nervous systems. They are capable of remembering and learning.

• 27. Reproduction – most molluscs have two separate sexes and fertilization is external. The eggs and sperm are released into the open ocean and find each other by chance.

• 28. This results in a free swimming larva.

• In molluscs with tentacles fertilization is internal.

• Some snails are hermaphroditic. They will get together and fertilize the other ones eggs.

• 29. Class Gastropoda

• The name means stomach foot. They got the name because of the broad muscular foot attached to the stomach.

• 30. Gastropods include snails, land slugs, abalones, sea butterflies, sea hares and nudibranchs.

• abalone

• Sea butterfly

• Sea hare

• Nudibranch

• 31. Many gastropods have a single shell for protection. They are able to retreat inside when threatened. They may also have a rigid part of the foot called an operculum that closes like a door and seals them inside.

• 32. Some gastropods have no shell (slug). For defense they only emerge at night when predators are not around. Sea hares have ink producing glands. The can squirt ink into the water which confuses the predator

• 33. Sea butterflies are capable of swimming very fast in order to escape predators.

• Nudibranchs have special chemicals in their bodies that taste bad or are poisonous.

• 34. Nudibranchs may also be brightly colored. This serves as an indication that they are dangerous and not good to eat.

• 35. Members of class Bivalvia include clams, oysters, scallops, and mussels. They have two shells that are hinged together in the back and are held together with one or two very strong muscles.

• 36. Bivalve larva are free swimming but as they grow the become bottom dwellers. Some live in the sand or mud, other attach themselves to rocks.

• 37. Some are sessile (oysters) while others can swim around (scallops).

• The shells are secreted by glands in the mantle. These glands also keep the inside of the shell smooth by secreting a substance known as mother-of-pearl.

• 39. If an object gets caught inside the mantle gland will cover it with the mother-of-pearl and it will develop into a pearl.

• 40. Class Cephalopoda (head-foot)

• Members include octopi, squid, cuttlefish, and nautiluses. They range in size from a 2 cm cuttlefish to the colossal squids larger than a school bus.

• 41. Most have eight flexible tentacles each with round sucking disks that are used to grab and hold onto their prey.

• 42. Cuttlefish and squid also have two long slender arms with suckers on the end called feeding tentacles.

• 43. Nautiluses may have up to 90 tentacles. These do not have suckers but are covered with sticky mucus.

• 44. The only living cephalopod with an external shell is the nautilus.

• Cuttlefish have a small shell found inside the body called a cuttlebone. (find one in the pet store).

• 45. The squids internal shell has been reduced over time into a thin, flexible supporting rod called a pen.

• Octopus have lost the shell completely.

• 47. For protection most cephalopods are capable of swimming very quickly. They may also use jet propulsion by squirting water out of the siphon which they are able to direct in any direction.

• 49. Many cephalopods are capable of changing color rapidly by using chromatophores in the skin.

• They are also capable of producing an ink that clouds the water allowing them time to escape.

• 50. They also use bioluminescence to attract mates and scare off predators.