Phylum Mollusca

Question 1

What is the Phylum Mollusca?

Answer 1

• Soft body
• Largest animal phylum after Arthropods (90,000 named living species)
• Snails, clams, mussels, octopuses, chitons, nudibranchs, squid, etc
• Lophotrochozoan Protostomes
• Triploblastic
• Coelomate (coelom is confined to small area around the heart)
• Bilateral symmetry
• Hydrostatic skeleton, and in some, a shell
• Complete gut
• No metamerism
• Reproduction: sexual (monoecious or dioecious), no asexual
• Organ system grade of organization
• Mostly open circulatory system (closed in cephalopods)

Question 2

What are the classes to Phylum Mollusca?

Answer 2

Class Gastropoda
Class Bivalvia (two valves)
Class Cephalopoda

Question 3

Explain Class Gastropoda

Answer 3

• Very diverse: snails, limpets, slugs, whelks, conchs, periwinkles, sea slugs, sea hares, sea butterflies
• more than 70,000 living species
• Many but not all, have shells
• Terrestrial or aquatic (only molluscs to exploit terrestrial environments)
• Bilateral symmetry (however, because of torsion, the visceral mass is asymmetrical)

Question 4

Explain Class Bivalvia

Answer 4

• Mussels, clams, scallops, oysters and shipworms
• Mostly sedimentary filter feeders (draw water through their gills by ciliary action)
• Marine and freshwater forms
• No head, no radula
• Very little cephalization (concentration of sensory and neural organs)

Question 5

Explain Class Cephalopoda

Answer 5

• Squids, octopuses, nautiluses, devilfish and cuttlefish
• Marine active predators (chiefly eat small fishes, other molluscs, crustaceans, and worms)
• Most complex molluscs
• Highly mobile
• Vary in size
  2cm-18m
  Giant squid - largest invertebrate known
• All are predators - rapid swimmers
• Tentacles and arms capture prey by adhesive secretions or by suckers
  Octopuses and cuttlefishes have salivary glands that secrete a
  venom for immobilizing prey
• Swim by expelling a jet of water from their mantle cavity through funnel (derived from the foot)

Question 6

Where do they live?

Answer 6

• Aquatic: mostly marine
  Cephalopods are exclusively marine
  Bivalves and gastropods can live in brackish or freshwater
  habitats
• Brackish = more salinity than freshwater, but not as much as seawater
• Terrestrial
  Some gastropods are terrestrial
  May have been among the first animals to leave the oceans
  and become terrestrial

Question 7

What is the form and function of Mollusca?

Answer 7

• Mollusca body consists of two basic parts:
  Head-foot (feeding, cephalic, sensory, locomotor organs)
  Visceral mass (digestive, circulatory, respiratory and
  reproductive organs)
• Many molluscs have a protective shell secreted by the mantle

Question 8

Explain head-foot

Answer 8

• Well developed head with mouth and sensory organs (exception: bivalves)
• Mouth contains a structure unique to molluscs; the radula
• Radula is a rasping, protrusible, tonguelike organ found in most molluscs (with the exception of some bivalves and some gastropods)

Question 9

Explain the foot

Answer 9

• Adapted for locomotion and/or attachment
• Usually a ventral, sole-like structure
• Modifications
  Laterally compressed foot (bivalves)
  Funnel (siphon) for jet propulsion in cephalopods

Question 10

What is the mantle?

Answer 10

• Mantle is a sheath of skin, extending dorsally from the visceral mass, that wraps around each side of the body
  Protects the soft parts
  Outer surface of the mantel secretes the shell

Question 11

What is the mantle cavity?

Answer 11

• The mantle cavity houses respiratory organs (gills or a lung)
• The mantle’s own exposed surface also participates in gas exchange
• Products from the digestive, excretory and reproductive systems empty into the mantle cavity

Question 12

What is their reproduction?

Answer 12

• Sexual reproduction only (mostly dioecious)
  Some gastropods are monoecious
• Most pass through free swimming trochophore and veliger larval stages

Question 13

What is a trochophore?

Answer 13

minute, translucent with circlets of cilia

Question 14

What is a veliger?

Answer 14

Free-swimming larva of most marine snails, and bivalves. It develops from a trochophore and has the beginning of a foot, shell and mantle.

Question 15

What is the trochophore larvae?

Answer 15

• Molluscs and annelids have a trochophore larvae

- Trochophore larvae: a free swimming ciliated marine larva characteristic of members of the Lophotrochozoa clade

Question 16

What is the veliger larvae?

Answer 16

-

Question 17

What does the circulatory system transport?

Answer 17

Gases
Nutrients
Waster 
Hormones 
Heat

Question 18

What’s simple circulation?

Answer 18

• The simplest circulation is diffusion
  Slow
  Inefficient over long distances
• Works for single celled or small organisms

Question 19

Examples of circulation by diffusion

Answer 19

• Protozoans
• Porifera
• Platyhelminthes
  Large but very thin
  Internal branching of digestive tract brings materials close to
  all cells
• Cnidaria
• Nematoda

Question 20

What are the types of circulatory systems?

Answer 20

• Closed circulatory system
• Open circulatory system
• Most molluscs have an open circulatory system
  Hemolymph is not confined to vessels; it flows through vessels
  in some parts of the body and into open sinuses in others
  Not as efficient as a closed system and so is common in slow
  moving animals
• Cephalopods have a closed circulatory system
  Closed system more common for active predators

Question 21

Explain closed circulatory system

Answer 21

• Blood is contained within vessels

- Vertebrates and some invertebrates

Question 22

Explain open circulatory system

Answer 22

• Blood is confined to vessels in only a portion of circuit through body
• Blood mixes with interstitial fluids in the hemocoel
• Because it is mixed with fluid, it is properly called hemolymph
• Arthropods and some molluscs

Question 23

What are the class Gastropoda adaptations to avoid fouling?

Answer 23

• All but a few living gastropods exhibit bilateral asymmetry
  Because of coiling, the gill, and kidney on the right side have
  been lost
• The loss of the right gill also reduces the effects of fouling
  Water flows one-way: in the left side, over the gill, and out the
  right side, clearing waste from the rectum

Question 24

What is the class bivalvia sensory organs?

Answer 24

• Eyes but no head
  Some bivalves have simple eyes
• Ocelli: a simple eye or eyespot in many types of invertebrates

Question 25

What is the locomotion for the class bivalvia?

Answer 25

• Some bivalves are sedentary (mussels) or sessile (oysters)
• Others can move slowly
• A few bivalves move by clapping their valves together

Question 26

What is the form and function of the class cephalopods?

Answer 26

• Ancestral Cephalopods had shells
  Only remaining shelled Cephalopods are the Nautilus species
  Cuttlefish and squid have an internal shell called a pen
  Octopuses have no shell

Question 27

What are the unique adaptations for the class Cephalopods?

Answer 27

• Special pigment cells: chromatophores
  Produce colour changes in skin
  Used as camouflage
  Associated with alarm or courtship
• Cephalopods (other than Nautilus) have an ink sac
• When animal is alarmed, it releases a cloud of ink through the anus to confuse an enemy