Transcript Phylum Porifera – Diversity and Structure

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
Fig. 16.1
Fig. 16.24
Fig. 16.33
Fig. 16.32
Fig. 16.12
Fig. 16.2
Fig. 16.42
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)
Fig. 16.3
Fig. 16.5
Fig. 16.6
Fig. 16.13
Fig. 16.16
Fig. 16.23
Fig. 16.22
Fig. 16.4
Fig. 16.17
Fig. 16.28
Fig. 16.34
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
Fig. 16.35
Fig. 16.7a, 16.8, & 16.36
Fig. 16.14
Fig. 16.15
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
Fig. 16.37
Fig. 16.38
Fig. 16.39
Fig. 16.40