Download LECTURES FOR ZOO 1010—CHAPTER 1

LECTURES FOR ZOO 1010—CHAPTER 10
MOLLUSCS
BASED ON HICKMAN ET AL. ANIMAL DIVERSITY
3rd Edition
Chapter Prologue—A Significant Space: Some descendents of the Precambrian
acoelomates evolved a space within mesodermal tissue, the coelom. This body cavity
allowed for more efficient burrowing than the condition in either acoelomates or
pseudocoelomates. The coelom allowed for a number of advances, including the
suspension of the internal organs and networks of blood vessels in mesenteries, the
functioning of this space as an efficient hydrostatic skeleton against which opposing
groups of circular and longitudinal muscles acted antagonistically, and the
development of a muscular, diversified digestive tract. Evolution of a coelom allowed
for the appearance of larger and more complex animals.
Mollusks comprise one of the largest animal phyla after the arthropods, with almost
50,000 living and 35,000 fossil species. The name Mollusca refers to the soft body
possessed by these animals. The group includes the snails, clams, chitons, octopuses,
and related animals. Size ranges from those that are almost microscopic to the giant
squids and clams. The largest molluscan classes are the Gastropoda, Bivalvia,
Polyplacophora, and Cephalopoda. The Monoplacophora, Scaphopoda,
Caudofoveata, and Solenogastres are much smaller classes.
Mollusks occurs in a broad range of habitats, from tropical regions to polar seas, from
mountains as high as 7000 m to the abyssal depths of the oceans of the world. Most
mollusks live in the sea, where they exhibit a variety of lifestyles.
Mollusks arose in the sea, based on fossil evidence, where most modern species still
live. Only gastropods and bivalves have entered freshwater habitats and only the
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gastropods have moved onto land, where they may have been the first animals to do
so.
Mollusks as a group have considerable economic importance. Many are used as food
by us. Pearls are produced by clams and oysters. Some are destructive, for example,
shipworms. Many snails serve as intermediate hosts for serious worm parasites.
Form and Function:
Body plan—The body plan of mollusks consists of a head-foot and a visceral mass.
The head is the location of the feeding apparatus and the major sensory structures,
such as photosensory receptors, including the complex eyes of cephalopods, and
tentacles. Within the mouth, there is often found a feeding structure termed a
radula, unique to mollusks. The foot is the chief locomotory structure in mollusks.
The visceral mass where the digestive, circulatory, respiratory, and reproductive
structures are located. The mantle, another unique feature of mollusks, is formed
of two folds of tissue that are outgrowths of the dorsal body wall that enclose a
space between the mantle and the body wall called the mantle cavity, within
which are found the gills or lungs. In addition, products from the digestive,
excretory, and reproductive systems empty into this cavity. In some mollusks, the
mantle secretes the shell. The shell consists of three layers, the periostracum, the
prismatic layer, and the nacreous layer, from outside to inside.
Internal structure and function—Gaseous exchange occurs through the use of gills or
lungs, as well as the body surface, particularly the mantle. Mollusks possess an
open circulatory system, consisting of a pumping heart, blood vessels, and blood
sinuses. Most cephalopods, however, have a closed circulatory system, with heart,
vessels, and capillaries. The digestive system is complex, consisting of a mouth,
radula, stomach, intestine, digestive gland, and anus. Most mollusks have a pair of
metanephridia. The nervous system consists of several pairs of ganglia with
connecting nerve cords. Most mollusks are dioecious, although some gastropods
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are monoecious. Many aquatic mollusks have a free-swimming trochophore and
veliger larval stages in the life cycle.
Classes Caudofoveata and Solenogastres:
Small groups of wormlike mollusks lacking a shell.
Class Monoplacophora:
Tiny, univalve mollusks showing pseudometamerism or vestiges of true metamerism.
Class Polyplacophora:
Mollusks with shells in the form of a series of eight plates; sedentary, lying strongly
attached to rocky substratum.
Class Scaphopoda:
Mollusks with a tubular shell, open at both ends, and the mantle wrapped around the
body.
Class Gastropoda:
Mollusks with an asymmetrical body exhibiting torsion and coiling of the shell; some
have an uncoiled shell or shell absent; largest molluscan group; head well
developed with radula; dioecious or monoecious, some with larvae, others
without; marine, freshwater, and terrestrial.
Class Bivalvia:
Marine and freshwater mollusks with a shell consisting of two valves joined by dorsal
ligament and held together by adductor muscles; most suspension feeders,
drawing water through gills and ciliary action; head greatly reduced; no radula;
usually dioecious, with trochophore and veliger larvae typically.
Class Cephalopoda:
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Most complex mollusks; all predatory, many fast swimming; shell often reduced or
absent; head well developed with complex eyes, a radula, and tentacles; foot
modified into a funnel, used for swimming by forcible expulsion of water;
nervous system complex, with ganglia centralized to form brain; dioecious, with
direct development.
Phylogeny and Adaptive Radiation:
Phylogeny— Mollusks are most reasonably thought to have branched from the
annelid line after the origin of the coelom but before the development of
metamerism.
Adaptive radiation—The cladistic relationships of the molluscan classes appears to be
as follows: Caudofoveata and Solenogastres closely related, sister groups to
remaining mollusks; Polyplacophora sister group to remaining mollusks;
Monoplacophora sister group to Gastropoda and Cephalopoda; Bivalvia and
Scaphopoda sister groups of one another.