Trochozoa: Molluscs

Question 1

Conchifera (5 classes)

Answer 1

1. Gastropoda
2. Bivalvia
3. Scaphopoda
4. Cephalopoda
5. Monoplacophora

Question 2

Aculifera (2 classes)

Answer 2

Aplacophora and Polyplacophora

Question 3

Polyplacophora (Chitons)

Answer 3

Look like Conchifera, but Chitons have 8 separate plates (not homologous with Conchifera shells)

Question 4

Generally, Molluscs have (9)

Answer 4

1. Secretions: Calcium carbonate spicules
2. Outer structure: Mantle and mantle cavity from body wall
3. Ctenidium (respiratory surface)
4. Reduced coelom (pericardium, gonads)
5. Open BVS and 3 chambered heart
6. Radula (pharyngeal tongue)
7. Large muscular foot and large pedal retractors
8. Orientation: internal organs dorsal in visceral mass (gut excretory and gonad)
9. Multiple paired nerve cords (*most conserved part); pedal and visceral

Question 5

3 Synapomorphies of Mollusca

Answer 5

Ctenidium, Radula, Multiple paired nerve cords

Question 6

What is Ctenidium?

Answer 6

Ciliated series of filaments; rods or plates where the edges are ciliated and motion of cilia generates a water current from side of the mantle cavity to other side, upstream and downstream

Question 7

What is the Osphradium?

Answer 7

Sensory organism often on the upstream side of ctenidium, anus on downstream with nephridiopore and gametes

Question 8

What is Conchiolin?

Answer 8

Cuticular layer of collagen which forms organic matrix of shell

Question 9

Spicules of Gastropods (snails)

Answer 9

Can produce calcium carbonate spicules like Porifera

Question 10

What is the Periostracum?

Answer 10

When conchiolin cuticle is underlain by carbonate layers, it becomes the periostracum

Question 11

3 Types of Shell

Answer 11

Prismatic shell: Tall carbonate crystals arranged in columnar series perpendicular (often aragonite)

Nacreous shell: single carbonate crystals arranged in plates parallel to plane of shell (often calcite)

Crossed lamellar shell

Question 12

Calcite vs Aragonite

Answer 12

Calcite: if magnesium ions are actively excluded from calcium carbonate mixtures in shells (energetically expensive)

Aragonite: if magnesium ions are mixed with calcium carbonate mixtures in shells (less strong)

Question 13

Ctenidium

Answer 13

A comb like structure (respiratory structure/gill with projecting filaments) - can assist in water flow, feeding and brood space

Question 14

Radula

Answer 14

Specialized patch of scleratized cuticle; contains odontophore (cartilaginous protonaceous skeletal structure under radula)

2 opposing muscles drag epithelium of radula over odontophore, abrades materials to ingest

Question 15

Trochophore of Gastropods, Bivalves and Scaphopods

Answer 15

Trochophore develops pair of ciliated bands (veliger) that form a disc (velum)
Capable of long distance migration, some called teleplanic

Question 16

3 Notable Characteristics of Trochophores

Answer 16

High gene flow, Reduced local adaptation, Few opportunities for speciation

Question 17

Polyplacophora (Chitons) Shell

Answer 17

Shell has exactly 8 plates secreted by dorsal lobe of mantle that are separate and appear in early larval stage

Question 18

Aplacophora Reductions and 2 Types

Answer 18

Reduction of foot, mantle and shell

1. Bright to camo with corals (prey on soft coral)
2. Dull in sediments (graze on microorganisms)

Question 19

Why are Aplacophora and Polyplacophora sister groups?

Answer 19

The Burgess Shale fossils: Wiwaxia and Halkeria show that Aplacophora shells are similar to Chiton scales and plates

Question 20

Scaphopoda (tusk shells) Characteristics

Answer 20

Benthic, soft sediments, anterior end down
Captacula
Mantle cavity is an elongate space along posterior edge
No ctenidia

Question 21

What is the Captacula?

Answer 21

Modified foot structure to make tentacles that capture microorganisms

Question 22

3 Derived Features of the Bivalves

Answer 22

1. Modified posterior edge of mantle to form siphons (restricted opening into mantle cavity for controlled directional flow of water)
2. Organization of single calcified shell into a pair of valves with uncalcified hinge alone dorsal margin between two shells (movement relative to each other)
3. Loss of cephalization; lost anterior sensory organs and no radula

Question 23

4 General features of the Bivalves

Answer 23

1. Enormous foot with extension of digestive tract and gonad
2. Specialization of musculature from patches of epiderms; muscular pull shell together, mechanical to open
3. Protobranch deposit feeders; ctenidium is suspension feeding structure with pair of big palps
4. Most bivalves feed from water flow of ctenidial filaments and eat phytoplankton

Question 24

Gastropoda 2 Features:

Answer 24

1. Twisted orientation of visceral structures to head and foot - visceral nerve cord is twisted
2. Spiral shape of gastropod

Question 25

Gastropoda Shell Features

Answer 25

Protoconch: shell apex

Siphonal canal: extended folds in mantle that function as siphon

Operculum: Sclerotized on posterior end of foot

Columella: central column resists crushing and remodels for living space

Question 26

Cephalopoda Features:

Answer 26

Mantle cavity posterior, no shells except in Nautilus (siphuncle pumps water and ions through chambers in spiral shells)

Question 27

Complex reproductive tracts that are made from ____________ become __________ capable of ____________

Answer 27

Complex reproductive tracs that are made from modifies nephridial tubules become veligers capable of long distance migration