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

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



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.


- 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


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


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)


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


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


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


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 –