bio 205 chapter 12: the mollusks

phylum mollusca

2nd largest phylum in kingdom animalia
120,000 living species
represented in fossil record
most are aquatic, 2 are terrestrial
bilaterally symmetrical, triploblastic, coelomate, protostomes, complete gut

muscular foot

highly modified for a variety of functions
gastropods → locomotion
cephalopods → divided into arms and/or tentacles
bivalves → shovel or hatchet shaped for burrowing

mantle

body covering or lines the inside of the shell
secretes calcium carbonate to enlarge the shell and is enzymatically controlled
creates annual whorls → spring, summer, fall for most growth, restricted winter growth

visceral mass

connected mass of body organs, contains some of the digestive, excretory, and reproductive organs

shell of calcium carbonate

slugs and octopuses → none
chitons and snails → 1
bivalves → 2
nautiluses → chambered external
squid and cuttlefish → internal, dorsally through bodies

radula

teeth-like structures made of calcium carbonate used for scraping or grinding of food that is arranged in a belt attached to an odontophore

odontophore

a mass of cartilage sitting within the mouth

radula in various mollusks

gastropods - pushed out of the mouth to scrape surface, controlled by protractor and retractor muscles (radula sac)
bivalves - N/A
cephalopods - within a buccal mass which food is delivered to

gills

aka ctenidia, respiratory organs possessed by all aquatic mollusks and possess osphradia

gill movement

water bathes gills allowing for extraction of oxygen and release of CO2
countercurrent mechanism - water flows over gills in opposite direction to the blood

osphradium

olfactory organ that tests incoming water for silt or food particles, 2 types

bipectinate gills

gills with two rows of gill filaments extending from the gill axis, rows face opposite direction, typical of more primitive mollusks

monopectinate gils

gills with one row of gill filaments extending from the gill axis (resembles a comb), typical of more contemporary mollusks

lungs

present in terrestrial slugs and snails
oxygen enters from a pneumostome

pneumostome

found along one side of the body, an opening in the lung through which oxygen enters

open circulatory system

blood leaves heart through vessels and empties into a hemocoel, re-enters circulation through ostia

hemocoel

space in the heart through which blood empties

ostia

holes in the heart of an open circulatory system

closed circulatory system

blood travels through heart and vessels, no ostia, cephalopods

Class Polyplacophora

chitons
shell consists of 8 overlapping, articulating plates
broad muscular foot with numerous sets of bipectinate gills on each side
sucker-like mouth with radula attached to an odontophore
pharyngeal gland
open circulatory system

pharyngeal glands

AKA sugar glands, produce and secrete amylase into the stomach

chiton circulatory system

open circulatory system
3 chambered heart, 2 auricles, 1 ventricle
each auricle receives blood from the left or right arrangement of gills

Class Monoplacophora

monoplacophorans
fairly flattened dome-like shell
3-6 pairs of ctenidia that run along broad muscular foot
sucker-like mouth with radula attached to an odontophore

Class Gastropoda

snails, slugs and sea angels (pteropods)
contains terrestrial species
dome-like or coiled chambered shell
radula attached to an odontophore
exhibit cephalization and possess antennae
broad muscular foot

Subclass Prosobranchia

gastropods with bipectinate gills

Subclass Opisthobranchia

gastropods with monopectinate gills

Subclass Pulmonata

gastropods with lungs, only terrestrial species

pneumostome

opening below the shell that connects to the lung on a snail

operculum

folds up to seal the aperture of the shell when a snail retracts its shell, on the dorsal surface of the posterior foot

snail circulatory system

open circulatory sytem
water enters and bathes the gills for gas exchange
oxygenated blood is carried from the gills to the heart

columellar muscle

connects the snail body to the columella of the shell

columella

runs through the center of the shell and connects the chambers

protandric hemaphrodites

exist as males first then mature into females, gonads first produce sperm then produce eggs

simultaneous hermaphrodites

have both sets of gonads which are active at the same time

sea slugs

nudibrachs, lack a shell
branched cerata and rhinophores

branched cerata

functions as an external gill taking oxygen in from the water

rhinophores

function as chemosensory structures

sea angels

pteropods, lack a shell completely
foot is divided into 2 flap-like wings
considered planktonic and live in deeper ocean waters

land slugs

lack a shell entirely or have small remnants on their dorsal surface

gastropods with 2 pairs of antennae

upper pair is used for photoreceptive sight and smell, lower pair is used for taste and touch

Class Bivalvia

clams, mussels, scallops and oysters
bodies enclosed within 2 shells
filter-feeders and lack radula → incurrent and excurrent siphon
no obvious cephalization
monopectinate gills
1 or 2 adductor muscles to open and close shells

clams

burrow with a shovel, hatchet-like foot
anterior and posterior adductor muscles
shells hinged together with teeth-like projections on inside of shells near umbo

mussels

sessile and attach using byssal threads to anchor to rocks or hard substrates
one adductor muscle
shells are connected by wing-like extensions from shells

scallops

benthic, sitting on sea floor sediment
1 adductor muscle
move by quickly opening and closing shells
frilly edge to shell
photoreceptive eyespots

oysters

in intertidal zones and attach to hard substrates or each other forming reefs
shell that attaches is slightly flattened compared to second shell
some produce pearls

Class Cephalopoda

nautiluses, cuttlefish, squid, octopuses
most cephalized mollusks with eyes
foot divided into arms or tentacles with suckers
buccal mass with radula
closed circulatory system
monopectinate gills

nautilus

external shell
siphuncle
foot is divided into tentacles without suckers

siphuncle

channel that runs through the chambers of the shell and can fill with gas allowing for buoyancy

cuttlefish

internal shell (pen)
foot divided into 8 shorter arms and 2 longer tentacles with suckers
fins extend completely along mantle on each side of the body

squid

internal shell (pen)
foot is divided into 8 shorter arms and 2 longer tentacles all with suckers
fins only extending from posterior portion of mantle

pen

internal shell in cuttlefish and squid that runs below the dorsal mantle

octopus

lack a shell
foot is divided into 8 arms with suckers
highly intelligent, one of the largest invertebrate brains

cephalopod capabilities

changing color, camouflaging, bioluminescece, excreting ink to deter predator or unwanted mate

chromatophores

pigmented cells in the skin that allows for the skin color to change, controlled by muscles

iridophores

AKA iridocytes, sit below the chromatophores and reflect light like mirrors

photophores

cells that can emit light

open circulatory system of Littorina

1) deoxygenated blood moves through an afferent branchial vessel to the gills
2) blood becomes oxygenated and leaves the gills through an efferent branchial vessel
3) efferent branchial vessel carries blood to the auricle
4) auricle pumps blood to ventricle
5) ventricle pumps blood to anterior and posterior aorta
6) anterior → head and foot, posterior → visceral mass and nephridium = oxygen is delivered to body cells
7) deoxygenated blood leaves head and foot through cephalopedal vein
8) deoxygenated blood leaves visceral mass through visceral vein
9) veins merge and blood is carried to space around nephridium and moved to gills through afferent branchial vessel to be re-oxygenated

closed circulatory system of Loligo

1) deoxygenated blood is carried to gills by afferent branchial vessel
2) blood becomes oxygenated and leaves gills through efferent branchial vessel
3) efferent branchial vessel carries blood to systemic heart
4) systemic heart pumps oxygenated blood to body through an aorta
5) oxygen is delivered to body cells
6) deoxygenated blood leaves head through anterior cephalic vena cava
7) deoxygenated blood leaves posterior region through a right and left posterior vena cava
8) right posterior vena cava carries blood to right branchial heart and right gill through afferent branchial vessel
9) left posterior vena cava carries blood to left branchial heart and left gill through afferent branchial vessel