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Phylum Mollusca
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Lecture notes on Phylum Mollusca
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Phylum MolluscaClass Bivalvia – has two equivalve pair of shells, similar to brachiopods; shells are
equivalve, which means it is symmetrical; ex. clam (siphons, foot)
Class Polyplacophora – from the term, it means multiple shell plates; ex. chiton
(valves or plates, girdle)
Class Scaphopoda – from the term, it means boat foot(?); tusk shell or scaphopod
(shell, foot, tentacles (captacula))
Class Cephalopoda – from the term, it means head foot; it looks as though its
tentacles are going out of its head; nautilus (shell, funnel, tentacles, eye, hood) and
squid (tentacles, suckers, arm, eye, fin)
Class Gastropoda – from the term, it means stomach foot; the stomach and the foot
are contained in the same body part; whelk (siphon, tentacles, mouth, eye, foot,
operculum)
General Characteristics
• soft-bodied (means soft in Latin)
• with or without shell
• bilaterally symmetrical for Class Bivalvia (compared to brachiopods)
• mantle tissues secrete shell
• aquatic or terrestrial
• visceral organs
• complete digestive tract
• [some] are filter feeders
• mostly epifaunal, which means they live above the sea floor
Distinct Characteristics
• cephalization: brain, eyes, mouth, radula (tiny teeth used to scrape food particles
off a surface and drawing them to the mouth), sensory organs
• cephalization means a trend in evolution where the nervous system and the
sensory organs of the organism is positioned near the head region or the anterior
region of your organism
• univalve or bivalve (equivalve pair of shells) or none
• adapted to a wide variety of habitats
, BRACHIOPODA SIMILARITIES MOLLUSCA
• lophophore • filter feeders • gills for respiration
• inequivalved pair of • bilaterally • equivalved pair of
shells symmetrical shells (bivalves)
• no distinct • with shells • cephalization
cephalization • body cavity and • some are terrestrial
• mostly aquatic visceral organs
• separate sexes
Mode of Life
• Habitat
o have adapted to a wide variety of habitats
▪ marine
▪ freshwater
▪ terrestrial (gastropods)
o aquatic mollusks
▪ planktonic (Limacina inflata)
▪ benthic (Acanthopleura spinosa)
▪ nektonic (Ammonites)
• Movement
o terrestrial mollusks (mostly gastropods)
▪ crawlers (Helminthoglypta walkeriana)
• Feeding Mechanism
o virtually every feeding habit is carried out by the mollusks
o may be herbivores, carnivores, scavengers, deposit feeders, filter feeders,
or parasites
o these strategies are carried by some members of almost all the major
groups
• Reproduction
o monoecious (can have both male and female reproductive organs,
hermaphroditic) or dioecious (the organism can produce either male or
female reproductive organ, but never both at the same time)
o external and internal fertilization
o larval stages
▪ trochophore larva: free-swimming, ciliated, shell formation begins;
the first part of larval stages of mollusks
▪ veliger larva: free-swimming, ciliated velum forms, shell/body
torsion occurs
▪ spat: metamorphic form between veliger and juvenile, shell
elaborates
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