Lecture 3 Mollusc

8/13/2019 Lecture 3 Mollusc

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Class Gastropoda and Class Bivalvia

Dr. Wan Fatma Zuharah Wan Musthapa


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Torsion

Torsion is a phenomena where veliger larvaundergo a 180 degress torsion of visceral mass

Due to the imbalance growth in retractormuscle system connecting head foot region withshell. This creates a pull of the visceral mass

As a result, the mantle cavity moves from

original posterior to anterior region, directlyabove the head.


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Torsion


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Advantage and disadvantage of torsion

Without torsion, thefoot will be

withdrawn to themantle cavity first

whihe the head islast

Cleaner water enter

the mantle cavity ifits in the anteriorposition.

Anus is situated nearhead


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Coiling

For better weightdistribution

Compactness by

conispiral shape

Right side- loss of gill,

atrium, and kidney

Adaptation to carrying

coiled shell


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Internal form and function


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Respiration- ctenidium located in mantle, serves aslung to small opening called pneumostome

Single nephridium circulatory and nervous system Sense organ- osphradium

Reproduction dioecious and monoecious

Courtship ceremony exchange of spermatozoa

After copulation deposits eggs in shallow burrow


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CLASS BIVALVIA

Also known as hatched-footed animals

Presence of bi-shell structure

Most of them sedentary filter feeder

Body compressed laterally.

Dorsal part of the organism is characterized bypresence of hinge while ventral is at the opening ofthe shell

Umbo: situated near hinge, where shell formingmaterial for secretion is stored.


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Form and function

Shell- laterally compressed, held together by hingeligament

Umbo-oldest part of shell, growth occurs in concentricline

Pearl- when foreign object lodged between shell andmantle, secrete nacre

Locomotion- slender muscular foot, pump blood swell and work as anchor

Longitudinal muscle contract to shorten foot Scallops-clapping valves together, create jet propulsion

Gas exchange- through mantle and gills


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Bivalve gills is made of arranged

layers of lamella. Movement of

water through tiny pores in lamella

allow exchange of gas between

water and blood.


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Reproduction and development

Fertilization is external

Embryo develops into tracophore,

veliger and spat stages

Freshwater clams:

-Fertilization is internal

- sperm entering with incurrent water

flow-Glochidium larvae (specialized

veliger)

- attached to specific fish host (gills

or skin)

-Parasitic for several week

- detach and sink to the bottom


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Boring

Burrow in mud or sand Evolved mechanism burrowing into harder substances

(wooden ship)

Teredoslender posterior siphon, water flowing overthe gills, a pair of small globular valves on the anteriorend

Have microscopic teeth- wood rasp, cellulose produce

by symbiotic bacteria

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Lecture 3 Mollusc