Molluscs Flashcards
What is a mollusc ?
Mollusca (from latin word molluscus meaning soft) one if the largest phyla second to arthropods
3 major classes
-Gastropoda(slugs,snails, limpets, sea slugs, conchs)
-Bivalvia(mussels, clams, oysters)
-Cephalopoda (squids, octopuses, cuttlefish, devilfish)
Where do molluscs occur ?
Vast majority live in marine environments
Two groups, bivalves and gastropods also contain freshwater species
Gastropods are the only ones with land representatives
Name the anatomy of a snail
Explain the foot of a snail ?
-Most snails have a noticeable muscular crawling foot with a flat sole.
-This “foot” is used to move the snail(slow)
-some use it for digging
- certain sea slugs species use it to swim
Explain the head of a snail
-At the foots front is a head with eyes and variable amount of tentacles
- most terrestrial snails are equipped with four tentacles
- remaining species only have 2
Explain the operculum of a snail
-At the rear end of the foot (many water living and some terrestrial) snails carry a calcareous lid(operculum) that closes the shell aperture when the snail withdraws.
- Using this sabre- shaped lid, conches are able to not only defend themselves but are also able to move in jumps by pushing their operculum to the ground and jerking themselves forward
Explain the movement(locomotion) of snails
-The foot of the snail is ventral, sole like ciliated structure in which waves of muscular contraction cause a creeping locomotion.
- The foot is extended from the body hydraulically by engorgement with blood
- mucus secreted from glad at the front of the foot to aid in adhesion or as a slime tract
- mucus left behind also acts as a form of communication through a pheromone trail
- slime well visible after it dried
- snail slime trail patchy / slug slime trail constant
Explain the shell of a snail and its three consisting layers
-Shell of a snail secreted by the mantel
-periostacum outermost horny layer secreted by glands in the mantle rim, acts as protection for underlying calcareous layers from erosion by boring organisms.
- made of organic material called conchin which is a mix of organic compounds, mostly protein(not made of calcium carbonate)
- gives colour to the shell
- snail can repair its shell and can easily see repaired areas because it does not possess all the layers
hypostacum - (middle prismatic layer) composed of calcium carbonate laid down in a protein matrix
-provides rigidity adding strength to the shell
inner nacreous layer is calcareous and secreted constantly in thin layers by the mantle surface
- increases in thickness during life of a snail
The shell of a snail partially develops during embryonic stage, freshly hatched snail has a glassy shell called protoconch
- by eating its own egg shell it gains calcium carbonate which strengthens the shell, shell then becomes opaque.
- shell of a land snail is in one piece and coiled
- apex of the shell contains oldest and smallest whorl
-whorl become successively larger and spiral about the central axis( columella)
- shell may become genetically right handed( dextral- more common) or left handed( sinistral) depending on direction of the coiling
- shell connected to main body at only one point, where the main rector muscle(columellar muscle) is attached to the spindle of the shell
Explain shell coiling
Snail shells are asymmetrical for this reason snail shells have a high point know as the tip or apex
Can be orientated either right or left
Mostly right(dextral)
Explain what shell torsion is and how it happens
-Change from bilateral to asymmetrical
-Phenomenon occurs during veliger stage in snail development( land snails spend veliger stage in case or capsule and emerge as young snails) that moves the mantle cavity, originally at the back to the front of the body
-happens due to uneven growth of right and left muscles that attach the shell to the head-foot.
- before torsion anus and mantle cavity at back and after they are at the front
- takes place for protection, due to the shift the head- end can hide within the shell below the mantle cavity and foot remain outside
Explain the feeding behaviour of snails and how they feed including the anatomy thereof
-Snails eat green plants, decaying vegetable matter and some species eat other snails.
- feed by means of a radula(rasping tongue like organ) a ribbon like membrane on which rows of tiny teeth that are pointed backwards are mounted
- complex muscles move the radula and supporting cartilages (odontophore) in and out while the membrane is partly rotated over the tips of the cartilage.
- rasps of fine particles of food and acts as a conveyer belt for carrying the particles in a continuous stream towards the digestive track
- as radula wears away from the front, new rows of teeth are continuously replaced by secretion at the back
Briefly explain excretion of snails
The anus and nephridiopore opens near the pneumostome(lung opening) and waste is expelled forcibly with air from the lungs to avoid fouling the head area
Explain the reproduction of snails
-Land snails are Monoecious, simultaneous hermaphrodites.
- each snail has both male and female reproductive organs
- during mating each partner inserts its penis into the others vagina transferring spermatophore.
- the eggs are laid by each individual in a moist area and development takes up to a month
- identical miniature replicas if the adults emerge
What predators do snails and slugs have ?
-Many predators, insects, other snails, spiders, amphibians, birds, mammals
-Given that parents do not protect the eggs they are extremely vulnerable to predation
- snails protect themselves from potential predators smaler than themselves by secreting foamy mucus which sticks the predators together and stops them from harming the snail
What is ment by snails as parasite hosts ?
-Snails act as intermediate hosts to a number of other animal such as liver flukes or flat worms
- the parasitic disease known as Schistosomiasis or Bilharzia is caused by flat worms
- acute type of bilharzia known as snails fever and cutaneous bilharzia sometimes called swimmers itch
- affects people in developing countries low mortality but very debilitating
