Dr. Muhammad Moosa Abro
Phylum Mollusca
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Class Polyplacophora
Class Scaphopoda
Class Monoplacophora
Class Solenogastres
Class Caudofoveata
Further Phylogenetic
Considerations

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CLASS POLYPLACOPHORA
The class Polyplacophora (Gr. polys, many plak, plate phoros,
to bear) contains the chitons.
Chiton are common inhabitants of hard substrates in shallow
marine water.
Early Native Americans ate chitons.
 Chitons have a fishy flavor but are tough to chew and difficult
to collect.
Chitons have a reduced head, a flattened foot, and a shell that
divides into eight articulating dorsal valves (figure 11.20a).

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CLASS POLYPLACOPHORA
A muscular mantle that extends beyond the margins of the
shell and foot covers the broad foot (figure 11.20b).
The mantle cavity is restricted to the space between the margin
of the mantle and the foot.
Chitons crawl over their substrate in a manner similar to
gastropods.
The muscular foot allows chitons to securely attach to a
substrate and withstand strong waves and tidal currents.

4 CLASS POLYPLACOPHORA
When chitons are disturbed, the edges of the mantle tightly grip
the substrate, and foot muscles contract to raise the middle of
the foot, creating a powerful vacuum that holds the chiton in
place.
Articulations in the shell allow chitons to roll into a ball when
dislodged from the substrate.
A linear series of gills is in the mantle cavity on each side of the
foot.
Cilia on the gills create water currents that enter below the
anterior mantle margins and exit posteriorly.
The digestive, excretory, and reproductive tracts open near the
exhalant area of the mantle cavity, and exhalant water carries
products of these systems away.

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Most chitons feed on attached algae.
A chemoreceptor, the subradular organ, extends from the mouth to
detect food, which the radula rasps from the substrate.
Mucus traps food, which then enters the esophagus by ciliary action.
Extracellular digestion and absorption occur in the stomach, and
wastes move on to the intestine (figure 11.20c).
The nervous system is ladderlike, with four anteroposterior nerve
cords and numerous transverse nerves.
A nerve ring encircles the esophagus.
 Sensory structures include osphradia, tactile receptors on the mantle
margin, chemoreceptors near the mouth, and statocysts in the foot.
 In some chitons, photoreceptors dot the surface of the shell.
Sexes are separate in chitons.
External fertilization and development result in a swimming
trochophore that settles and metamorphoses into an adult without
passing through a veliger stage.

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CLASS SCAPHOPODA
Members of the class Scaphopoda (Gr. skaphe, boat podos, foot) are called tooth
shells or tusk shells.
The over 300 species are all burrowing marine animals that inhabit moderate depths.
 Their most distinctive characteristic is a conical shell that is open at both ends.
The head and foot project from the wider end of the shell, and the rest of the body,
including the mantle, is greatly elongate and extends the length of the shell (figure
11.21).
Scaphopods live mostly buried in the substrate with head and foot oriented down and
with the apex of the shell projecting into the water above.
Incurrent and excurrent water enters and leaves the mantle cavity through the
opening at the apex of the shell.
Functional gills are absent, and gas exchange occurs across mantle folds.
Scaphopods have a radula and tentacles, which they use in feeding on foraminiferans.
Sexes are separate, and trochophore and veliger larvae are produced.

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CLASS MONOPLACOPHORA
 Members of the class Monoplacophora (Gr. monos, one plak, plate phoros, to
bear) have an undivided, arched shell; a broad, flat foot; a radula; and serially
repeated pairs of gills and foot-retractor muscles.
 They are dioecious; however, nothing is known of their embryology.
 This group of molluscs was known only from fossils until 1952, when a limpet like
monoplacophoran, named Neopilina, was dredged up from a depth of 3,520 m off
the Pacific coast of Costa Rica (figure 11.22).
 Approximately 25 species have been described since the discovery of Neopilina.

8 CLASS SOLENOGASTRES
 250 species, (Gk., solen, channel gaster, gut).
 These cylindrical molluscs lack a shell and crawl on their ventral
foot, which is modified into a pedal groove (figure 11.23).
 Solenogasters lack a shell.
 This condition is thought to represent the ancestral state for the
phylum.
 Instead of a shell, their bodies are covered by minute embedded
calcareous spicules.
 Some have secondarily lost the radula.
 True gills are absent; however, gill-like structures are usually
present.
 surface dwellers on corals and other marine substrates, and are
carnivores, frequently feeding on cnidarian polyps.
 are monoecious.

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CLASS CAUDOFOVEATA
 Members of the class Caudofoveata (L. cauda, tail, fovea, depression) are wormlike
molluscs that range in size from 2 mm to 14 cm and live in vertical burrows on the deep
sea floor.
 They feed on foraminiferans and are dioecious.
 They have scale like spicules on the body wall and feed using a radula.
 The absence of a shell, a muscular foot, and nephridia suggests that this group may
resemble the ancestral mollusc.
 120 species, but little is known of their ecology.

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FURTHER PHYLOGENETIC CONSIDERATIONS
 Fossil records of molluscan classes indicate that 500 million years old.
 fossils from later stages of the Ediacaran period and early Cambrian
 Molecular data and shared protostome characteristics are interpreted as placing the
Mollusca within the Lophotrochozoa along with the Annelida and other
 The segmental appearance of gills and other structures found in monoplacophorans,
such as Neopilina, is now considered a very different form of segmentation from that
found in Annelida or any other group of animals.
 The evolutionary ties between the Mollusca and Annelida are probably very distant.
 The shell and muscular foot that characterize most modern molluscs were probably
not present in the first molluscs.
 The mantle of solenogasters and caudofoveates is associated with a cuticle
containing embedded calcium carbonat spicules, and this may be similar to the
ancestral condition.

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FURTHER PHYLOGENETIC CONSIDERATIONS
 The “ girdle” surrounding the shell and covering the edge of the mantle of
polyplacophorans is considered by some to be a remnant of this cuticle
 The large muscular foot of most modern molluscs is first seen in the polyplacophora.
 The polyplacophoran shell, consisting of eight dorsal plates, is probably
intermediate between the calcareous spicules of caudofoveates and solenogasters
and the single shell of more derived molluscs.
 The diversity of body forms and lifestyles in the phylum Mollusca is an excellent
example of adaptive radiation.
 Molluscs began as slow-moving, marine bottom dwellers, but the evolution of
unique molluscan features allowed them to diversify relatively quickly.
 By the end of the Cambrian period, some were filter feeders, some were burrowers,
and others were swimming predators.
 Later, some molluscs became terrestrial and invaded many habitats, from tropical
rain forests to arid deserts.

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 Figure 11.24 shows one interpretation
of molluscan phylogeny.
 It reflects the idea that the muscular
foot and shell are not ancestral,
 and shows Caudofoveata and
Solenogastres as most closely
resembling the molluscan ancestor.
 All other molluscs have a shell or are
derived from shelled ancestors.
 The multipart shell distinguishes the
Polyplacophora from other classes.
 The extensive adaptive radiation of this
phylum has made higher taxonomic
relationships difficult to discern.

References
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1. Miller, A.S. and Harley, J.B. ; 1999 , 2002., 2007, 2009, 2012 & 2016 Zoology, 4th , 5th, 6th, 7th,
8th , 9th& 10th Edition (International), Singapore : McGraw Hill.
2. Hickman, C.P., Roberts, L.C/, AND Larson, A., 2018. INTEGRATED PRINCIPLES OF ZOOLOGY,
15th Edition (International), Singapore: McGRAW-Hill