Allows the Organs to Be Held in Place Better. Prevents Twisting

Molluscs

Coelomates

body cavity lined by:

allows the organs to be held in place better. prevents twisting

more room & flexibility is allowed for organ development

coelom provides hydrostatic skeleton

more organs are derived from mesoderm

Phylum Mollusca

 - diverse group (35,000 fossil sp.)

aquatic from tropic to polar waters, shallow to deepest of waters

some terr. up to 7000 m in altitude

General characteristics

1)triploblastic, bilateral, unsegmented protostome

2)body of two parts:

3)head-foot -

4)visceral mass - internal organs

5)coelom reduced to area around heart – pericardial cavity

mantle secretes calcareous shell

mantle - usu. covers visceral mass.

Classes

- lacks shell & head. wormlike, mantle secretes chitinous cuticle & calcareous scales

– lacks shell & head, wormlike, mantle covered with scales & spicules, no true gills

- single-arched shell

- chitons - 8 dorsal plates

- toothshells - open-ended tubular shell

- snails, slugs, limpets - 40,000 sp.

– bivalves – 30,000 sp - 2-lobes mantle producing 2 hinged shells

- octopus, squid, nautilus 550 sp.- shell often reduce or absent

Class Gastropoda

most diverse class – snails, slugs, limpets, whelks, conchs, nudibranchs

marine, freshwater, & terrestrial habitats

Distinguishing features

- gastropods w/shells are usu. coiled. some in one plane, others more 3-D. some assym., others symmetrical

Torsion

visceral mass twists 90-180 after larval stage so that gills, anus, & mantle cavities are facing forward. digestive tract is U-shaped. poss. advantages:

1) allows foot to recede last & thus protecting head, some sp. have on foot

2) allows clean water to enter mantle cavity, rather than water from behind snail that may be silted

3) more in anterior

Disadvantage

anus & excretory openings are above head & poss. pollute water near mouth.

some adaptations to avoid this (Ex: water in left side, waste out on right)

Feeding

diet - most scrape algae or detritus, others eat larger plants, are scavengers or predators. ectoparasites of bivalves or endoparasite of echinoderms

radula consists of chitinous belt & row of curved teeth covering a cartilaginous tongue-like structure ( ) which can protude from mouth.

predatory snails may have which extends into prey

some use radula as drill to bore thru mollusc prey. foot used to hold down prey. glandular secretions help soften shell – oyster drill

tropical cone shells kill prey by hurling a long, detached, harpoon-like radula tooth carrying a poison

Gas exchange

1-2 gills in mantle cavity

more advanced gastropods have 1 gill on the opposite side of the mantle cavity of the anus & excretory openings

gills reduced or absent in land snails but mantle cavity is highly vascularized for gas exchange

Nervous system

primitive sp. - six ganglia linked by nerves.

advanced sp. - fewer & larger ganglia, esp. in head

eyes may be located at base or end of tentacles, and may be simple photoreceptors or more complex w/lens and cornea. not great at object discrimination

 - chemoreceptors that detect prey & changes in water or soil chemistry in mantle cavity

tentacles are also sensitive to touch

statocysts are located in foot - provide orientation & equilibrium

Class Cephalopoda -

highly developed in many ways

anterior part of foot has been modified into arms or tentacles w/many functions: prey capture, mating, locomotion, attachment

head & visceral mass are dorsoventrally lengthened & body is reoriented so that tentacles are ventral & visceral mass in dorsal

mantle encloses body except head & tentacles in muscular sheath

Shell

only has shell but feature is found in ancestral forms

Nautilus shell is separated by septa into chambers. it occupies only the most recent chambers

cuttlefish have internal shell called

squid have chitinous , cartilaginous plates

octopus have no shells

Locomotion



muscles in mantle contract & water is forced out of posterior funnel in mantle

octopus usu. use tentacles for crawling, jet-propulsion in emergencies

Flying Squids shoot out of water & glide for short distances have been clocked at 20 mph

Feeding

diet - carnivorous - sm. inverts, mollusc, fish, baby whales

detect prey by sight, capture w/suction cup w/tentacles

can tear food w/horny beaklike jaws, radula works as tongue pulling food into GI tract

octopods have pair of salivary glands that have been modifies as poison glands that help to subdue prey

Nervous system

highly developed brain includes specialized regions for locomotion, capturing prey, etc.

eye is very similar to vertebrate eye - capable of image formation & color discrimination. lens is moved back & forth to focus. no blind spot

statocysts, osphradia (Nautilus) & many tactile & chemoreceptors throughout body, esp. arms & tentacles

Other features

 - alkaloid fluid w/melanin is discharged when predator approaches. smoke screen, confuses predator, also may slightly drug the predator

 - pigment cells which can expand when tiny muscles contract to change color of animal. may help camouflage animal, or confuse predators

 - bioluminescent cells - deep-sea sp.

Reproduction & development

dioecious, gonads in dorsal part of visceral mass

male releases sperm in encased packets -

eggs are large & yolky

copulation - male grabs female w/arms & uses specialized tentacle to transfer sperm are released & fertilize eggs as they leave oviduct

octopods tend eggs laid in substrate

direct development. miniadults emerge

Ecological, economic & medical importance

many used for food by humans

shells and pearls also used as jewelry, currency, decorations, making musical

pests –

intermediate hosts for many human parasites

ecological –

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