Mollusca Flashcards
What is the body plan of the Mollusca?
No segmented
Muscular foot/head
Shell
Mantle
Visceral mass (non muscular)
Mantle cavity with gills
Radula
What is the dorsal surface of the Mollusca covered by?
Covered by a mantle of tissue
Mantle secretes a dorsal shell
What is the posterior and anterior part of the mollusca?
The dorsal and posterior part is non-muscular and never leaves the shell (visceral mass)
The anterior and lower part is firm and muscular and may extend beyond shell (head-foot)
How is the ancestral mollusc described?
Probably small (1-2cm)
Poorly defined head
Ventral surface broad and flat
Thin shell
Posterior mantle cavity
What is one of the leading features of the molluscs?
Mantle cavity
The posterior part of the mantle cavity is enlarged and contains the ctenidia (gills)
What is the radula?
The mouth is carried on a snout close to the ground, rasping food particles off the surface of the rock by mean of a radula
How is the radula structured?
A radula is a ribbon-like structure covered with extremely hard mineralised teeth
It runs over a cartilaginous ‘odontophore’ which is extruded from the mouth and the radula acts as a rasp tearing off food particles
Is the visceral mass muscular?
It is non-muscular
The oesophagus and stomach operate without the use of muscles
What is process in collecting food in the molluscs?
Salivary glands mucus to lubricate the radula and to collect food particles
Cilia in the stomach create a rotary action forming a ball of rotating mucus that acts like a windlass
Material scraped off by the radula is pulled up the oesophagus by a mucus rope
How is the circulatory system in molluscs described?
Open circuit circulatory system
How is the primitive heart of the mollusca thought to be structured?
Thought to have consisted of a median ventricle and two lateral auricles
How does blood flow in the molluscs?
Blood flows from the ctenidia through the auricles and is pumped forward by the ventricle
Primitively there is no complex system of blood vessels, the blood passing into a series a haemocoelic sinuses
How do pearls form in the mollusca?
Nacreous layer- foreign body enters through it
Foreign body is surrounded by layers of nacreous shell eventually to form a pearl
What classes exist within the phylum mollusca?
Class Monoplacopjora
Class Polyplacophora (chitons)
Class Aplacophora
Class Aplacophora
Class Gastropoda (snails)
Class Bivalvia (clams)
Class Scaphopoda (tusk shells)
Class Cephalopoda (squid)
How are the class Monoplacophora described?
Known from fossils dating back to the cambrian- thought to be ancestral to gastropods, bivalves and cephalopods
What is an example of a Monoplacophora?
Neopilina
A limpet liked molluscan was dredged up from deep sea
The shell looked normal, but it was turned over there were signs that the animal was segmented
How are the class Polyplacophora described (chitons)
1,000 species
8 articulating dorsal plates and a marginal girdle with tiny scales
Poorly developed head
Where do the class Polyplacophora live?
Live on hard surfaces in the intertidal and shallow subtidal
The articulating plates allow them to adjust to fit irregular surfaces
Feed by scraping algae and other encrusting organisms off rock surfaces with radula
Thought to have diverged early from main lines of molluscan evolution
How is the class Aplacophora described?
320 species
Small worm-like molluscs
Some are infaunal others crawl on seabed and feed on cnidarians
Body covered with fine scales
How is the class Scaphopoda described?
500 species
Cylindrical tusk shaped shell open at each end
Burrow in sediment with narrow end of shell projecting
How do the class Scaphopoda feed?
Feed on intersitial organisms captured by small tentacles (captacula) and ingested with radula
How are the class Gastropoda described?
70,000 species the largest and most diverse of the molluscan groups
What sub classes exist within the class Gastropoda?
Sub class Prosobranchia- includes most of the marine snails
Sub class Opisthobranchia- This group consists mainly of sea slugs
Sub class Pulmonata- These are mainly the fresh water and land snails and slugs
How are the Opisthobranchia structured?
Loss of shell but chemical defenses and often store nematocysts in cerata
Can release defensive ink and can swim to escape
What does loss of shell increase in the Opisthobranchia?
Loss of shell increases mobility and allows adaptation to planktonic life
What orders exist in the Opisthobranchia?
Order Gymnosomata
Order Nudibranchia
How are the pulmonata structured?
Gill is lost- wall of mantle cavity highly vascularised and functions as a lung
What is the torsion in the molluscs?
Adaptation to enable rapid retraction of head in larvae
How do the class Gastropoda feed?
The majority of gastropods are grazing animals rasping algae and other encrusting organisms off hard surfaces
There are also many predatory species, some of which may drill into the shells of other molluscs by means of the radula
What are some species of the Mollusca able to feed on?
Some species are even able to feed on fast moving prey such as fish. The most notorious belonging to the family Coniidae. The fish are speared by the radula, quickly killed by a fast acting poison and collected up by the mollusc after the fish is dead
How are the class Bivalvia described?
20,000 species
Vast majority feed by filtering water using modified gills
Most are burrowers in sediments but also fill a range of other niches
How do the class Bivalvia feed?
Most are burrowers in sediment with tubular siphons extending to the surface to gather water for filtering
What are some examples of the class Bivalvia?
Queenie- epibenthic and unattached
Horse mussel- epibenthic, attached by bysuss threads
Oyster- epibenthic, shell cemented to rock
How are the class Bivalvia structured?
Bivalves have completely lost the head, buccal mass and radula
The mantle cavity is much larger than in other Mollusca and has dominated bivalve evolution
What sub classes exist in the class Cephalopoda?
Sub class Nautiloidea
Sub class Coleoidea
What orders exist in the sub class Coleoidea?
Order Sepiodea
Order Teuthoidea
Order Octopoda
What functions exist in squids?
Camouflage
Mimic
