This unit will be used to teach sixth grade students about the phylum Mollusca. I suggest teaching this unit during the spring for approximately three weeks. The primary objective will be to concentrate on marine mollusks in the New Haven area. I will use my personal collection as a teaching aid. Through this unit I hope to spark an interest in shell collecting among my students. I will try to guide students in an organized manner on how to begin to collect and to enhance their powers of observation by identification of the many different varieties of shells that can be found in our area.
Introduction—Background
Mollusks are an important group of invertebrates in the animal kingdom. They are classified in the phylum Mollusca, and are divided into 6 classes and subdivided based on differences in anatomical features such as the foot, mouth, and breathing organs. The classes are gastropoda, pelecypoda (bivalves), amphineura (chitons), cephalopoda, scaphopoda (tusk shells), and monoplacophora. The major concentration in this unit will be on the classes gastropoda and pelecypoda because of the occurrence in the New Haven region.
To increase students’ awareness of the shells and the living organisms that once inhabited them I have provided information on growth, feeding, locomotion, and reproduction. Body parts will be studied through diagrams and explanations of the physiology of these animals. To aid in this study I have included a separate glossary* for uncommon terms such as
radula,
byssus, and
carnivorous.
*Terms underlined will be found in the glossary.
The name “mollusk” means soft body which is the reason they have a shell for protection. Most of the mollusks live in the water, this is an advantage for food gathering since most do not move easily. They have a similar anatomy and physiology to humans that is each animal has a head, heart, blood, and nervous system and as with most animals oxygen is essential for life. In mollusks
gills
are the structure that extract oxygen from the water. Four interesting areas of the mollusks’ life include growth, feeding, locomotion, and reproduction.
Growth of Shell
—The shape or structure of a shell is predestined by inheritance, that is, the mineralized outer structure is species specific. The
mantle
which covers the body of the mollusc with a thin sheet of tissue is the specialized part of the body which builds the shell. The
mantle
contains cells which produce an external organic matrix that is rapidly mineralized with calcium carbonate. It is the presence of calcium carbonate that makes the shells hard. There is a variation in degree of mineralization. Daily increments of matrix with mineral are formed, the thickness and rate is controlled by a variety of physiological and environmental factors. The shells can grow and repair fairly quickly. A shell with a crack can be repaired within a few days. Shells reach their adult size within 1 to 6 years, other species grow throughout their lifetime. Each shell species continues to add shell depending on the nutrients in the environment. Some shells grow with a steady process while others grow at intervals. The end product is a unique and beautiful structure often with external patterns and different colors.
Feeding
—Mollusks eat a variety of minute animals and plants. Most bivalves and chitons are
herbivorous.
All cephalopods and more than half of the gastropods are
carnivorous.
The
gills
of bivalves are important filters in the feeding process. Small food particles stick to the mucous film that covers the gills and slowly passes to the mouth of the clam. Single shelled molluscs, gastropods for example, have teeth or
radulae
which enable the animal to rasp or grind algae from rock surfaces and digest food other than that from the sea water surrounding the animal.
Radulae
are present in all types of mollusks except in the bivalves. There can be from 20 to more than 300 rows of teeth.
Locomotion—
There are a variety of ways that mollusk move about such as crawling, swimming, or digging. A snail moves about by using its
foot
. The motion is accomplished through a series of muscular waves that begin at the forward end and move towards the opposite end. Clams use their muscular
foot
to dig through mud or sand. Some bivalves can change their location by rapidly opening and closing the
valves
; pushing out a jet of water while others are immobile or sessile and attach themselves to rocks. The anchor is by means of
byssus
threads that extend from the animal to the substrate with a form of glue.
Reproduction
—Many mollusks such as the whelk and the octopus lay eggs in protective capsules but all produce a great number of egg cells to offset losses of progeny through dips in water temperature, currents, or predators. An octopus may lay 200,000 eggs with only 1 or 2 surviving into adulthood. The eggs may be fertilized in the ocean or in the female depending on the species. Some eggs are laid freely into the sea water depending on the water temperature, moon position, and season. The eggs may be attached to the surface of the ocean floor, rocks, seaweed, or have buoyancy and float to the surface. Some eggs are laid in the sand and have sand collars for protection. Eggs are either abandoned by the parent or shepherded carefully as in the case of the octopus. There is a variation in the size of the eggs: larger eggs contain more nutrients permitting more complete development of the hatchling inside the capsule, and smaller eggs hatch sooner and become free-swimming larvae. The whelks eat their way out of the capsule by eating the other eggs until only one large whelk is left.
Most species have separate sexes but some like the scallop and cockle are
hermaphroditic
, containing both female and male reproduction organs. The slipper shells sex is determined by a hormone produced constantly by the female. When the female becomes too old or dies and stops producing the hormone, the neighboring males will develop ovaries and become female within a week. The octopus and snail have a period of courtship, an octopus may even change color during this time. The packets of male sperm is placed into the female oviduct by the tip of the third arm.
(Univalves)
Gastropoda
(Fig. I A,B)
This class is the largest containing 3/4 of all the living species of mollusks. It includes snails, whelks, limpets, and slipper shells. It is the only class which contains species living on land. Most gastropods have shells usually in a single coiled structure which are composed of calcium carbonate. Exceptions are the limpets and slipper shell which are flat and flared. Females usually have a slightly larger shell than the males.
Eyes and
tentacles
are contained in the head region to aid in guiding movements. (Fig. IA) Gastropoda means “stomach-footed”, because they eat through their
foot
. They have a single
foot
attached to their body. They move on their foot slowly over rocks and plants. The
spire
faces backward as the snail moves in a forward direction. The
operculum
is a hard disc attached to a portion of the foot that fits tightly into the
aperture
of the shell after the animal retracts itself. It is used for protection from enemies and the sun.
The
radula
is used with a pulling action to scrape and shred food. Some gastropods use their
radulae
to drill through the shells of clams and other mollusks and may also excrete acid to dissolve the calcium carbonate. They may be either
carnivorous
or
herbivorous
.
A. Animal in shell
(figure available in print form)
B. View of exterior of a univalve
(figure available in print form)
(Bivalves)
Pelecypoda
(Fig. II A,B,C)
Bivalves always have 2 shells, they may be equivalent in size, equilateral, or not equilateral. Quahogs have equilateral
valves
while oysters and jingle shells have distinctly different or inequilateral
valves
. The
valves
are held together by two internal strong muscles and flexible
ligaments
which occur at the hinge or head. (Fig. IIA) The
valves
must be open for feeding or moving. Muscle action is used in closing the
valves
, when
valves
are closed they are in a relaxed position. The
cardinal
and
lateral
teeth make sure that the
valves
are properly aligned when closed. The
pallial line
is the point of attachment to the mantle
.
The
pallial sinus
is where the muscle operating the siphons was attached, it is an identation showing the size and strength of the siphonal muscle. (See Below) Unlike the univalves the head region is not well defined and they do not contain a
radula
. Sometimes they have rows of tiny eyes along the edge of the
mantle
, as in the scallop.
Most pelecypods move slowly through the sand and water with the aid of a hatchet-shaped
foot
. Some such as scallops move by quickly opening and closing their
valves
. When the
valve
is open the space fills with water, when it closes the water shoots out behind and advances the animal. Others such as oysters and mussels permanently attach themselves to rocks or other objects by secreting strands of
byssus
by a gland in the
foot
.
To obtain food,
siphons
or tubular extensions are used to bring water inside the body. The
gills
strain the water to remove food particles as well as function in respiration. Another
siphon
gets rid of water and wastes. Bivalves generally are herbivorous, feeding on plankton and debris.
A. View of interior of one of the bivalves
(figure available in print form)
B. View of bivalve from umbo
(figure available in print form)
C. Dorsal view Axis of maximum growth
(figure available in print form)
(Chitons)
Amphieura
(Fig. III A,B)
These animal shells are a series of eight arched hard overlapping valves inside the mantle of the organism. There are 3 types of valves, the anterior valve near the head end, 6 intermediate valves, and the posterior valve at the hind end. The pieces are held together with a band, or
girdle
, around the outside edge. The valves give them the ability to attach to rocks or to irregular surfaces. When they are removed from the surface with a sharp knife they curl into a ball to protect their fleshy undersurface. Chitons wander at night and return to their hiding place during daylight hours.
Most chitons have a simple head in the anterior position, which lacks eyes and
tentacles
and an
anus
posteriorly. A strong set of
radulae
teeth rasps off food. Chitons have a wide, flat
foot
which is used for slowly creeping along and as a sucking unit. Most chitons are
herbivorous
and eat algae, but a few eat shrimp.
Fig. IIIA. Side view
(figure available in print form)
Fig. IIIB. Ventral view
(figure available in print form)
Cephalopoda
(Fig. IV)
The squid and octopus belong to this group which do not have an outside shell. The squid is the fastest swimming and the largest invertebrate ranging from 1 inch to 65 feet in length. The
tentacles
in some species may be 5 to 10 times the length of the body. As example, of a giant squid species, the body of the giant could be 20 feet with
tentacles
35 feet long. “Cephalopoda” means “head foot” because the
tentacles
are attached to their heads. They have 1 or 2 rows of
suction cups
on their
tentacles
or arms that function for grabbing and holding onto other animals as well as locomotion. The octopus has strong
radula
teeth, well-developed eyes, and 8 long arms. The squid is similar except it has small
radula
teeth. The nervous system is so well-developed that they have great strength, speed, and mobility.
They are
carnivorous
mollusks which prey on fish and crustacea, the octopus concentrates on snails and clams. For defense they secrete an inky substance or they may change color.
(figure available in print form)
Fig. IV. Dorsal view
(Tusk Shells)
Scaphopoda
(Fig. V)
The tusk shell is a gently curved, elongated, and tublar shell which is open at both ends, the
mantle
completely surrounds the animal. The shell grows from the larger anterior end. They live partially buried in the mud from shallow to very deep sea waters. Their location is worldwide with approximately 200 species found in the United States.
The body does not have a defined head, heart, eyes, or
gills
present.
Gills
are not needed because the circulatory system is so efficient. A muscular, cylindrical, and pointed
foot
at the larger end aids in locomotion, feeding, and keeps water flowing through the
mantle
cavity. A set of
radular
teeth is used to obtain the food and pass it to the intestines.
(figure available in print form)
Fig. V. Side view
Monoplacophora
(Fig. VI)
This class today is represented by only 5 living deep-sea species, all have single, oval, spoon-shaped shells. They were thought to be extinct until a discovery in 1952 off the Pacific Coast of Costa Rica. They resemble the chiton because they have a row of
radular
teeth, and lack eyes or
tentacles
. Also they have a thin, flattened, disc shaped
foot
used for moving slowly but not for sucking.
Gills
and 5-6 pairs of muscle scars are present on the inside of the shell.
(figure available in print form)
Fig. VI. Ventral view
