BLG316- Zoology- Mollusca

Phylum Mollusca def'n

soft body contained within shell (lost in 3 classes)

- all are coelomate

How many classes are in phylum Mollusca?

8

Molluscan form and function:

Body plan

head-foot portion- feeding, sensory and locomotion

viceral mass portion- digestion, circulation, respiration, reproduction

- all housed within the mantle cavity

Mantle def'n

tissue fold draping over visceral mass

- secretes shell

Nephridium def'n

excretory pores

Mantle cavity def'n

mantle extension beyond visceral mass

- water-filled chamber with gills, anus, excretory pores

What is the name of the circulatory fluid?

Hemolymph

What kind of circulator system do phylum mollusca have?

open circulatory system

Radula def'n

backwards curved teeth

- scrapping action for feeding (structure inverts to do this)

Phylum Mollusca :

Shell def'n

not always present

- secreted by the mantle

- typically 3 layers

What is the inner most layer of the shell?

Nacre

What does the nacre continually excrete?

CaCO3

Phylum Mollusca :

Reproduction

Most are dioecious

- aquatic members usually have ciliated free swimming larval stage

What are the distinct characteristics of the two clades that divide phylum Mollusca?

1 has univalve shell and the other has multiple shell plates

Class Caudofoveata & Solenogastres def'n

m.c.: worm like and shell-less

- calcareous scales/ spicules in integument

- reduced head and no nephridia

Class Polyplacophora (chitons) def'n

m.c.: flat with convex shell containing 8 plates

- mantle forms protective girdle around plates

- shell sometimes with eyes osphradia

Osphradia def'n

chemosensory organ that samples water

Class Monoplacophora def'n

m.c.: small low rounded shell

- many serially repeated segments

- univalve shell

Class Scaphopoda (tusk/tooth shell) def'n

m.c.: sedentary and slender

- tubular shell opened at both ends due to mantle being wrapped around viscera

Class Gastropoda def'n

m.c.: extreme variety

- torsion and coiling

members: snails, limpets, slugs, periwinkles, whelks

Class Gastropoda:

form and function:

Torsion

development usually as ciliated larval stage with gradual shell formation

- mouth anterior and anus posterior initially (relative position of body parts change)

Why might digestive tracts reposition to have anus above head within mantle cavity?

Osphradia located in mantle structure

- advantage to have them located upfront

-

Class Gastropoda:

form and function:

Coiling

spiral winding of shell

- shell becomes whorled and weight is unevenly distributed

- shell is shifted to oblique position but there is a loss of gill and kidney on right side

- bilateral asymmetry

Class Gastropoda:

How do they avoid fouling?

-loss of right gill

- water flows one way from left to right in modern species)

- those with 2 gills vent water through dorsal clef/ hole

- some undergo detorsion as adults

Class Gastropoda:

Feeding

- all use some form of radula

- many herbaceous

- some are predatory (radula for boring holes and tearing tissue)

- Cone snail shoots modified radula harpoon with neurotoxin

Class Gastropoda:

RESPIRATION

- many aquatic members use gills

- vascularized mantle acts as lung

- anus and nephridiophore open near lung opening to outside (pneumostome) to expel waste with air

Class Gastropoda:

Sensory and nervous

3 pairs of ganglia connected to nerves

- eyes, statocysts and tactile organs

Class Gastropoda:

Reproduction

Monoecious and dioecious

- exchange spermatophores

- lay eggs on land

- ciliated larvae for marine

Class Bivalvia def'n

2 shelled (bivalve)

- mostly suspension feeders

- no head

- radula and little cephalization

members: mussels, clams, scallops

Class Bivalvia

Shell

Laterally compressed

2 halves (valves) held together dorsally by hinge ligaments

- abductor muscles draw halves together to close

Class Bivalvia :

What is the name of the raised bump on the shell?

umbo

- oldest part of the shell

Class Bivalvia :

Body and mantle

visceral mass suspended dorsally

- gills on each side covered by mantle folds

- folds form incurrent and excurrent openings

- cilia on gills direct water over gills (food and oxygen)

What kind of heart does Class Bivalvia have?

3 chambered heart

- pumps blood through gills to kidneys to excrete waste

Class Bivalvia :

Feeding

Suspension feeder

- glands on gills and palps- secrete mucus

- cilia move particles to mouth

- crystalline rod in stomach rotates to aid digestion

Class Bivalvia:

Locomotion

- most move by extending foot between valve

- pump fluid into foot

- some sessile and attach shell to surface (anchoring threads or cement like substances secreted)

Class Bivalvia :

Reproduction

dioecious

- usually separate sexes and external fertilization

- marine often 3 larval stages

- internal fertilization in FW clams (bring sperm in to shell)

- hitchhiking larvae to disperse

Class Cephalopoda def'n

"Head foot"

- shell reduced and internal or lost

- still have mantle

- active predators

- only mollusc with closed circulatory system

- Well developed sense organs and complex brain

Squid anatomy

have a main heart and accessory heart

- radula (aka beak)

- 8 arms and 2 tentacles are highly modified foot

- siphon expels water for propulsion

- mantle cavity draws in water

Cuttle fish anatomy

8 arms and 2 tentacles

- excellent at camouflage- chromatophores

- eye is equally complex as human eye (convergent evolution)

Nautiluses def'n

only living cephalopod with external shell

- numerous internal gas chambers

- non-suckered tentacles extrude from shell to grasp prey

What are the chambers of the Nautilus shell connected by?

chord of living tissue called Siphuncle