Invertebrates 3
Phylum Annelida
and Phylum Mollusca
Branchpoints:
Pseudocoelomate
vs. coelomate
Protostomes vs.
deuterostomes
Aceolomates, Pseudocoelomates
and Coelomates
Pseudocoelom
Coelom
 Surface area for
nutrient absorption
and gas exchange
+
+
Larger organs
Fluid cushions/
protects organs
Hydrostatic skeleton
is possible
+
+
+
+
+
+
Completely lined by
mesoderm
-
+
Mesenteries suspend
internal organs
Independent
movement of gut?
-
+
-
+
Protostomes vs. deuterostomes
Fig 32.7
Major Protostome Phyla
(traditional phylogeny)
 Phylum Annelida
 Phylum Mollusca
 Phylum Arthropoda
Phylum Annelida:
The segmented worms
Basic Annelida features
 Have all features of coelomates and
protostomes (know!)
 Have body segmentation (new feature)




Repetition of body parts
Similar structures found in each region
Some specialization of regions
Some continuous parts (digestive system)
Body Plan (earthworm)
 Digestive system
 Complete
 Specialized regions
 Circulation
 Closed system
 Multiple hearts
 Gas exchange
 Body surface
 Coelom
 Excretory
 Metanephridia
Body Plan (cont.)
 Nervous system
 Cerebral ganglia
(brain)
 Ventral nerve cord
(fused pair of cords)
 One ganglion per
segment
 Movement
 Peristalsis
 Setae
Annelid diversity
 Earthworms
 Marine segmented
worms
 Leeches
See previous diagrams
Feeding in different classes

Earthworms




Earth suckers (create vacuum by expanding muscular
pharnyx); lots left undigested
Process 40 tons of soil/acre of earth/year!
improve aeration/drainage
Leeches




Create vacuum by expanding pharynx, digestive tract
Blood suckers hang on with suckers, feed with jaws or
have pharynx they stick into prey, suck out blood.
Secrete anticoagulants, vasodilators and numbing
agents
Marine segmented worms

Wide variety of feeding modes
Phylum Mollusca:
The “soft-bodied” animals
Evolutionary relationships
 Protostomes, same
branch as annelids.
Molluscan body plan (HAM)




Shell
Mantle
Visceral mass
Mantle cavity




With ctenidia
Head and assocated
sensory structures
Radula
Foot and epipodial
tentacles
Molluscan body plan (HAM)

Digestive system



complete
note position of anus
Circulation/Respiration



ctenidium
open circulatory system
coelom=pericardial cavity.


True coelom, but reduced.
Water/ion balance and
ammonia excretion

Has paired nephridia:
kidney-like in function
Molluscan body plan (HAM)

Nervous system



few ganglia at key locations, connected by
paired ventral nerve cords.
have brain
several sensory structures (see description
of HAM)
Major Molluscan Classes
 LEARN!: How is HAM modified for
each of the major classes?
?
Class Polyplacophora:
“many plates”
 The chitons
 Differences from HAM
 8 shell plates
 Expanded mantle cav.
 Multiple ctenidia
 Mantle grows over shell
 Reduced head
Class Gastropoda:
“stomach-foot”
 Representative members: Snails,
limpets, abalone, nudibranchs, sea
and land slugs
 Differences from HAM
 Undergo torsion (sketch)
 What is torsion
 Advantages of torsion
 Disadvantages of torsion
Class Bivalvia:
“two valves”
 Representative members: Clams,
mussels and scallops
 Differences from HAM
 Two shells
 Expanded mantle cavity
 Enlarged ctenidia
 Respiration AND feeding
 Reduced head
 Foot more elongate
Class Cephalopoda:
“Head-footed”
 Representative members: Nautilus, squid, octopus
 Differences from HAM
 Shell reduced (pen in squid)
 Foot gives rise to
 tentacles
 funnel
 Mantle
 Thickened and protective
 Functions in movement
 Nervous system highly developed
 Great brain!
 Eyes similar to vertebrates
 Circulatory system closed!
 Other features