Phylum Mollusca

1
Phylum Mollusca Phylum and Class
Characteristics

• Phylum Characteristics mantle secretes shell
  muscular foot radula (belt of rasping teeth)
• Overview of Diversity and Class Characteristics
• Class Polyplacophora chitons shell composed of
  7-8 limy plates common herbivores in tide pools
• Class Gastropoda snails most diverse of marine
  animals (approx. 70,000 extant species) most
  with single shell some terrestrial species
  (pulmonates)
• Class Cephalopoda octopuses, squids, and
  relatives shell reduced or absent largest
  brain among invertebrates beak-like jaws
  exhibit complex behaviors varied diet includes
  other molluscs
• Class Bivalvia clams, scallops, mussels,
  oysters shell with two valves lack radula and
  head most marine foot reduced or used for
  digging include shipworms (cause damage to
  wooden piers)
• Class Scaphopoda tusk/tooth shells burrowers
  feed on detritus and meiofauna with ciliated
  tentacles

2
Fig. 16.1
3
Fig. 16.24
4
Fig. 16.33
5
Fig. 16.32
6
Fig. 16.12
7
Fig. 16.2
8
Fig. 16.42
9
Body Structures in Molluscs

• Head-Foot Portion
• Head with sense organs (eyes, antennae), mouth,
  and tentacles
• Foot muscular, provides suction and movement
  modified (bivalves ? digging cephalopods ? arms
  and tentacles) or vestigial (ex. mussels attach
  to substratum with byssal threads)
• Visceral Mass internal organs typically covered
  by shell
• Ciliary tracts include gills (some w/ lungs) and
  mantle (produces shell)
• Open circulatory system (excl. cephalopods)
  metanephridia nervous system with multiple
  ganglia and interconnecting nerve cords
• Shell one or two valves reduced in
  opisthobranchs (ex. Aplysia) nudibranchs and
  octopuses lack (often with defensive chemicals)
• Three layers outer periostracum (organic),
  prismatic layer, and inner nacreous layer (with
  mother-of pearl)
• Radula a protrusible belt of rasping teeth
• Used for grazing in chitons and many gastropods
  used for drilling in predatory snails and
  octopuses modified tooth ? harpoon in cone
  snails bivalves filter-feed via siphons (radula
  absent)

10
Fig. 16.3
11
Fig. 16.5
12
Fig. 16.6
13
Fig. 16.13
14
Fig. 16.16
15
Fig. 16.23
16
Fig. 16.22
17
Fig. 16.4
18
Fig. 16.17
19
Fig. 16.28
20
Fig. 16.34
21
Larval Development in Molluscs

• Larval Stages (some molluscs with direct
  development, ex. Cephalopods hatch from egg with
  adult form)
• Trochophore shared larval stage with annelids
  free-swimming stage emerges from egg develops
  into adult stage (chitons) or into veliger stage
  (gastropods and bivalves)
• Veliger free-swimming larval stage with foot,
  shell, and mantle
• Glochidium specialized parasitic larvae of
  freshwater clams attach to specific fish hosts
  for weeks
• Gastropod Development
• Torsion process where gastropod larval body
  twists such that anus and mantle cavity opens
  near head (causes some fouling of gills)
• Spiral coiling process where gastropod shell
  coils during larval development (occurs
  concurrently with torsion)
• Evolutionary questions regarding functions of
  torsion and coiling
• Two mantle cavities (ex. chitons) ? one cavity
  (torsion)
• Weight distribution of shell modified by coiling
• Ancestral molluscs with bilateral symmetry,
  anterior/posterior ends

22
Fig. 16.35
23
Fig. 16.7a, 16.8,
16.36
24
Fig. 16.14
25
Fig. 16.15
26
Diversity and Behavior of Cephalopods

• Diversity and Characteristics siphon allows jet
  propulsion brain with multiple lobes all
  members are active, marine predators
• Ammonites predominate ocean predators in
  Mesozoic (extinction at end of the Cretaceous)
  chambered shells are common fossils
• Nautiloids five extant species (Nautilus spp.)
  two pairs of gills 60-90 tentacles without
  suckers gas-filled chambers in shell allow
  neutral buoyancy simple eye with constricted
  pupil
• Coleoids one pair of gills eight arms squids
  and cuttlefish with reduced shell (pen), fins,
  and two tentacles octopuses lack shell complex
  eyes with lens and retina diversity includes
  Humboldt and giant squids (Architeuthis), mimic
  octopus, and many bizarre deep-sea species
• Behaviors chromatophores control color changes
  (courtship, crypsis) ink released as defense
  many toxic (blue-ringed octopus deadly) squids
  with giant neurons (control rapid escape
  response) octopuses capable of observational
  learning

27
Fig. 16.37
28
Fig. 16.38
29
Fig. 16.39
30
Fig. 16.40
31
(No Transcript)