Description
Read
pages 16 – 25 in Chapter 16 Phylum Annelida of Unit III of the Study
Guide. Watch the video on the Dissection of the Earthworm.The video:https://ny.pbslearningmedia.org/resource/41b655c3-…
Unformatted Attachment Preview
Earthworm Dissection Questions
Detailed Clam (bivalve, Earthworm Dissection (Jr. High, High School and College
Review)
Steven Rokusek
Dr. Dale Droge
Dakota State University
1. The earthworm’s skin is moist. What is the importance of this in regard to the life
functions of the earthworm?
2. The body of the earthworm is segmented. What does this term mean? What is the
advantage of segmentation?
3. Describe how the earthworm moves. Be sure to include the action of the muscles in
your answer.
4. Is the earthworm a he or a she? Explain.
5. What allows the earthworm to grab onto a slippery surface so that it can move?
6. What is the band that is closer to the anterior end of the earthworm? What is its
function?
7. The reproductive openings are pointed out quickly on the video, but use the study
guide to name the openings and tell what they are used for.
8. Explain how the earthworm feeds. What is the ecological importance of the feeding
activity of earthworms?
9. Can earthworms regenerate?
10. Is the earthworm segmented inside as well as outside? What are the partitions
inside the body called?
11. What type of digestive tract does the earthworm have?
12. What is the name of the fluid-filled skeleton of the earthworm?
13. Trace the path of the food as it passes through the digestive tract of the earthworm.
Name each part of the digestive tract that the food passes through and give its
function. What is the advantage of this type of digestive tract in which the food
passes through in one direction?
14. What type of circulatory system does the earthworm have? Name the parts of the
circulatory system. Two main parts are shown on the video, but use the study guide
to more fully describe the parts.
15. Where (on what surface?) do we find the nerve cord of the earthworm?
16. Where is the brain of the earthworm located? What is it called? Is the earthworm
cephalized?
17. What is the large whitish reproductive structure identified on the video? What is its
function?
18. What are the smaller, rounded reproductive structures seen in the dissection?
What is their function?
19. Explain how earthworms mate. Where does fertilization take place?
20. What important evolutionary trends have we seen as we have looked at the
structure of the earthworm?
Unit III
Diversity of Invertebrates
Phylum Mollusca to Phylum Echinodermata
and Phylum Hemichordata
Introduction
Unit III comprises a survey of the phyla Mollusca, Annelida, Arthropoda, the Rotifers, the
Nematodes, Phylum Echinodermata, and Phylum Hemichordata. They belong to the clade
Protostomia. Protostomes are organisms in which the blastopore opening of the embryo
develops into the mouth. The clade Protostomia is divided into the taxa Lophotrochozoa and
Ecdysozoa. The separation of Ecdysozoa and Lophotrochozoa into two monophyletic clades
was based on molecular genetic evidence and the possession different morphological
characteristics.
Lophotrochozoans have either a feeding structure known as a lophophore as or a
trochophore as a larval form. The lophophore is a characteristic feeding organ found in the
Brachiopoda, the Bryozoa, and the Phoronida. A lophophore is a horseshoe shaped, ciliated
loop of hollow tentacles that surround the mouth. A lophophore can be extended while feeding
and respiring and withdrawn for protection. It is a feeding device that captures food particles in
the water, traps them in mucus, and transports them to the mouth. The lophophore contains a
fluid-filled coelomic cavity. The ciliated walls enclosing this cavity function as a respiratory
surface for gaseous exchange.
A trochophore is a free-swimming, top-shaped, feeding, larval stage. The trochophore is
found in the groups Nemertea, Mollusca, Sipuncula, Echiura, Entoprocta, Pogonophora, and
Annelida. The larva has two bands of cilia around the middle that are used for
swimming and for gathering food, and at the “top” is a cluster of longer flagella that is
sensory. The trochophore has a ring of ciliated cells in front of the mouth called a prototroch
that is used for swimming. It also aids in feeding by drawing the food particles closer to the
mouth. The mouth opens up into to the stomach, which leads to the anus.
Ecdysozoa are characterized by a possess known as ecdysis. This is a process in which
their outer body covering or cuticle is shed, or molted as they grow larger. Ecdysozoa consists
of the phyla Kinorhyncha, Nematoda, Nematomorpha, Priapulida, Arthropoda, Tardigrada,
Onychophora, and Loricifera.
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Lophophore
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CHAPTER 15
PHYLUM MOLLUSCA
Objectives:
1) To learn the significance of the coelom, or true body cavity found for the first time in the
Molluscs.
2) To learn the characteristics of the Molluscs, including their distinctive features: the foot, the
mantle, and the radula.
3) To survey the diversity of Molluscs.
4) To outline distinctive features of gastropods including torsion and a coiled shell.
5) To examine the structure and physiology of Cephalopods especially their advanced nervous
system.
6) To learn how Bivalves produce pearls as a protective response.
7) To understand the body organization of the clam by examining a cross sectional view of its
body.
8) To understand how the clam carries out locomotion.
9) To understand how the clam carries out respiration, digestion, and circulation.
10) To study the structure of the nervous system of the clam.
11) To study reproduction in the clam and to note the role of the ciliated larval stages known as
glochidia in the dispersal of freshwater clams.
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Phylum Mollusca
The phylum mollusca includes the clams, oysters, mussels, snails, slugs, squid, chitons, tooth
shells, nudibranchs, sea butterflies, and nautiluses. Phylum mollusca is the second largest animal
phylum. Estimates of the number of species range from 50,000 to 150,000. Only phylum
arthropoda is larger. The name mollusca refers to their soft body. They are the first group of
animals to have a true coelom. The coelom provides room for the organs, which are supported
by mesenteries. The alimentary canal can become more muscular, and specialized. It can move
more freely. The coelom also serves as a hydrostatic skeleton, and aids in producing body
movements. Although the coelom appears for the first time in Phylum Mollusca, it is not
extensive. The coelom in molluscs is limited to a space around the heart, and perhaps around the
gonads and part of the kidneys.
Characteristics
1) Body bilaterally symmetrical, unsegmented; most have a definite head. Surface epithelium
usually ciliated.
2) A true coelom is present but limited mainly to area around heart, and perhaps lumen of gonads
and part of kidneys. A coelom is a body cavity. A body cavity is a space that is located
between the body wall and the wall of the digestive tract. The coelom is a true body cavity
because it is lined by a membrane that is known as the peritoneum.
3) They have a ventral, muscular foot that is usually used for locomotion, but may be modified
for other purposes.
4) The dorsal body wall forms a pair of folds called the mantle, which encloses the mantle
cavity, is modified into gills or lungs, and secretes the shell.
5) All the organ systems are present and well developed.
6) Complete digestive system; rasping organ (radula) usually present.
7) Open circulatory system in most consisting of heart (usually three-chambered), blood
vessels, and sinuses; respiratory pigments in blood. Cephalopods have a closed circulatory
system. (In an open circulatory system, the blood leaves the arteries and enters open spaces,
or sinuses in the body where the blood bathes the organs, supplying them with oxygen and
removing wastes. In a closed circulatory system, the blood remains confined within blood
vessels as it travels through the body.)
8) Gaseous exchange by gills, lungs, mantle, or body surface.
9) Excretion by one or two kidneys (metanephridia) opening into the pericardial cavity and
usually emptying into the mantle cavity.
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10) Nervous system of paired ganglia and connecting nerves, nerve ring in gastropods and
cephalopods.
11) Sensory organs for touch, smell, taste, equilibrium, and detecting light; image forming eyes
in cephalopods.
12) Sexes usually separate (dioecious), some monoecious.
13) Spiral cleavage; protostome development; trochophore or veliger larva, some with direct
development.
Diversity of Molluscs
Gastropods
Gastropoda includes the snails and slugs and is the largest and most diverse molluscan class.
They are common in marine and freshwater environments. The pulmonate snails and slugs, as
well as several other groups, have conquered land by eliminating the gills and converting the
mantle cavity into a lung. They are the only Molluscs found in terrestrial habitats.
Gastropods are unique among Molluscs in undergoing torsion, a 180° counterclockwise
rotation of the visceral mass on top of the foot. Many gastropods have a shell, which is typically
a conical spiral wound around a central axis or columella. The turns of the spire form whorls,
demarcated by lines called sutures (Brusca and Brusca, 2003). In some species the shell spirals
in a clockwise (right-handed or dextral) direction, in others a counterclockwise (left-handed or
sinistral) direction. Some species can spiral in either direction. They glide on a broad flat foot.
The head is distinct and bears one or two pairs of sensory tentacles and a pair of eyes. A radula,
a rasping tonguelike structure is used for feeding. The nervous system is cephalized.
Cephalopoda
Class cephalopoda includes approximately 700 species including the squid, octopuses, cuttlefish,
and the chambered nautilus. Cephalopods are marine and typically pelagic. They are active
swimmers and carnivorous predators. This class includes the largest of all living invertebrates,
the giant squid, with body and tentacle lengths exceeding 20 m. Among living cephalopods,
only the nautilus has retained an external shell.
Body Regions
Early in their evolution, cephalopods elongated the dorsoventral axis to make it the major body
axis, replacing the traditional anterior-posterior axis. The end of the body bearing the head and
foot is the ventral end. The opposite end is the dorsal end. The back surface is the anterior end.
The opposite end is posterior. This is a potential source of confusion for those who, reasonably
enough, expect an animal’s head to be at its anterior (rather than ventral) end. To minimize
confusion, it is customary to rename the major axes on the basis of function, rather than
morphology. So, for example, the head can be called the functional anterior end.
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The body of cephalopods consists of a foot, head, and visceral mass. The head includes the
mouth that is surrounded by prehensile appendages (arms and tentacles) that are provided with
suckers. The tentacles are derived from the foot, which has been transformed into these flexible
appendages. This close association of head and foot is responsible for the name cephalopod
(head-foot). The head also contains the brain, sense organs including large eyes, radula, and
anterior gut. The mantle of squid and octopuses is the fleshy body surface. The mantle walls are
heavily muscularized. They surround the mantle cavity that contains a pair of gills. A ventral
region of the foot forms a tubular siphon, or funnel, exiting the mantle cavity. In squid and
octopuses, the mantle expands and contracts to draw water into the mantle cavity and then forces
it out through the siphon. The funnel is highly mobile and can be manipulated to point in nearly
any direction, thus allowing the animal to turn and maneuver. The ejection of this stream of
water produces the “jet propulsion” responsible for the rapid locomotion of cephalopods. Most
extant cephalopods have a reduced shell or are shell-less. In the squid, support of the body is
provided by an internal structure composed of chitin, known as a pen. Unlike all other molluscs,
cephalopods have a functionally closed circulatory system. The nervous system of cephalopods
is the most sophisticated of all molluscs, if not all invertebrates. Most of these modifications are
associated with the adoption of an active predatory lifestyle by these remarkable creatures
(Brusca and Brusca, 2003).
Feeding and Digestion
Squid are voracious predators. They are rapid swimmers, being able to overtake their prey. Prey
is located with the image-forming eyes. Squid capture their prey using their appendages, which
surround the mouth. These include eight arms and two long retractile tentacles. The inner
surface of each arm is provided with cup-shaped suckers. The mouth is equipped with a pair of
large dorsal and ventral jaws forming a beak. The mouth leads into the buccal cavity that
includes a radula with its pouch, and receives secretions from two pairs of salivary glands and a
submandibular gland. The radula is a strip that contains rows of transverse and longitudinal
teeth. The buccal cavity opens into a long esophagus that carries food to the stomach. The
stomach communicates with the cecum, a large, coiled, thin-walled absorptive pouch. The
digestive gland and the pancreas empty into the cecum via the digestive ducts
(hepatopancreatic ducts).
The circulatory system of cephalopods has become adapted to support an active, predatory
lifestyle. Cephalopods have a closed circulatory system capable of providing blood at high
pressure. There is a systemic heart that pumps blood to the body and two auxiliary branchial
hearts that pump blood to the gills. The three-chambered systemic heart consists of a central
ventricle and a pair of atria. Arteries and veins are connected by capillary beds (and some
sinuses), where exchange with the tissues occurs. Oxygenated cephalopod blood is blue due to
the presence of a copper-containing pigment known as hemocyanin.
Gas exchange is accomplished by a pair of gills.
Excretion is carried out by a pair of kidneys that lie between and slightly anterior to the branchial
hearts.
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The nervous system is more highly developed in Cephalopods than in any other invertebrates.
The nervous system has become more cephalized. Most of the ganglia have shifted forward and
are concentrated as lobes of a large brain encircling the anterior gut. In most cephalopods, much
of the brain is enclosed in a cartilaginous cranium. The octopuses may be the most intelligent
invertebrates, as they can be taught to perform memory-dependent tasks.
When threatened, cephalopods respond by performing a rapid escape movement. This action is
facilitated by giant motor axons that carry impulses to the circular muscle fibers of the mantle,
leading to their contraction. Study of the squid giant axon by neurophysiologists has been
instrumental in elucidating the mechanism responsible for transmission of the nerve impulse.
The cephalopod eye forms an image. The eye of the squid is similar in structure to that of the
vertebrate eye; it consists of a cornea, diaphragm, lens, and retina. However, there are several
differences between the squid eye and the vertebrate eye. The lens in the squid eye has a fixed
shape and focal length. In the squid eye, the lens is not focused by changing its shape as is done
in the vertebrate eye; rather, the ciliary muscles can accomplish limited focusing by changing the
position of the lens. The structure of the retina also differs in the squid eye and the vertebrate
eye. Both consist of three layers of cells. However, in the squid eye the light-sensitive cells, the
photoreceptors, form the outermost layer of cells closest to where the light strikes first. In the
vertebrate eye, the light must pass through the two other layers before striking the
photoreceptors. Although the squid eye and the vertebrate eye are similar, it is not believed that
the squid eye directly evolved into the vertebrate eye. Instead the evolution of both eyes is an
example of convergent evolution. Both eyes evolved independently as adaptations to similar
environmental demands.
Cephalopod Coloration and Ink
Some species of squid exhibit color change when alarmed. The color changes are produced by
pigment cells known as chromatophores, which are under nervous system and hormonal
control. Such chromatophores can be individually expanded or contracted by means of tiny
muscles attached to the periphery of each cell. Contraction of these muscles pulls out the cell
and its internal pigment into a flat plate, thereby displaying the color; relaxation of the muscles
causes the cell and pigment to concentrate into a small, inconspicuous dot.
Ink
When alarmed, the squid or octopus can release a cloud of black or brown “ink”. The ink is
produced in a large ink sac located near the intestine. The sac contains an ink-producing gland
lying in its wall. The ink is ejected through a duct that runs to a pore into the rectum. The cloud
of ink may distract predators while the squid performs evasive maneuvers.
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Reproduction
Cephalopods are dioecious. They have a single gonad located in the posterior region of the
visceral mass. The gonads release sperm into the coelom. The testis releases sperm to a coiled
vas deferens, which leads anteriorly to a seminal vesicle (spermatophoric gland). Here various
glands assist in packaging the sperm into elaborate spematophores (sperm packets), which are
stored in a large reservoir called Needham’s sac (spermatophoric sac). From here the
spermatophores are released into the mantle cavity via a sperm duct.
Mating in Cephalopods is often preceded by elaborate courtship behaviors and displays. These
include striking color displays by the male intended to attract the females and discourage other
males. Males often seize the female with their tentacles, and copulation occurs as the two swim
head-to-head. During mating, the male uses a specialized arm called the hectocotylus to transfer
spermatophores to the female. It is the right or left fourth arm in squid and has several rows of
small suckers that serve as an adhesive area for handling spermatophores. During copulation the
hectocotylus picks up spermatophores from the spermatophoric sac, is inserted into the female’s
mantle cavity, and deposits the spermatophores in the female’s seminal receptacle.
In females the ovary is located in the posterior region of the visceral mass. The eggs
pass through the oviduct to the oviducal gland. As the eggs pass through the oviduct, they are
covered with a protective membrane produced by the oviducal gland. The eggs are released
through the gonopore into the mantle cavity. As the eggs enter the mantle cavity, the
nidamental glands add a gelatinous coating, forming an oval mass containing approximately
100 eggs. The female holds these egg cases in her arms and fertilizes them with sperm ejected
from her seminal receptacle. The egg masses harden as they react with sea water and are then
attached to the substratum. The adults die shortly after mating and egg laying.
Class Bivalvia (Pelecypoda)
Class Bivalvia includes the mussels, clams, scallops, oysters, and shipworms. The members of
this class are called “bivalves” because they have two valves or shells that are hinged dorsally
and cover and protect their soft bodies. They are also known as Pelecypoda or “hatchet-footed”
animals because they possess a wedge-shaped or hatchet-shaped muscular foot, an organ that is
used for locomotion. Bivalves occur in marine environments as well as brackish and fresh water.
There are approximately 8,000 total species of Bivalves, including about 1300 that live in fresh
water.
The body has bilateral symmetry. Clams differ from other Molluscs in that they have no head,
no radula, and no brain. The nervous system is not cephalized. It consists of three pairs of
separated ganglia connected by nerves. Most bivalves are sedentary filter feeders. This is a
feeding process in which particulate food is filtered from water in which it is suspended. The
radula has been lost and the bivalves rely on the gills to filter food from the water. Water
currents are drawn into the sedentary animal through an incurrent siphon or aperture and exit
after circulating through the clam, through an excurrent opening. Using filter feeding and
siphons, the bivalve can feed, respire, and excrete while remaining buried in the sediment.
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Structure and Function of the Clam
The clam is covered by a protective shell, which is composed of two valves held together by a
hinge ligament. The shells enclose the body and spacious mantle cavity. The umbo is an
elevated hump located dorsally and anterior. It is the oldest part of the shell. The shell is
composed of three layers: 1) the external periostracum, which is made of chonchiolin, a protein,
2) a middle prismatic layer of crystalline calcium carbonate, and 3) the innermost layer, the
nacreous layer or mother of pearl, formed of many thin layers of calcium carbonate and having a
slight iridescence. The mantle secretes proteins and minerals to form the shell. A thickened rim
of mantle tissue at the mantle edge deposits new shell material at the shell edge. The clam
extracts calcium from the water to build its shell. As the clam shell grows, concentric lines of
growth beginning at the umbo are formed. Bivalves secrete their shells at varying rates
throughout the year, often forming white bands during optimal growth conditions and dark
translucent growth lines or bands during adverse conditions. Temperature is the dominant
control on bivalve growth at most latitudes. During the winter cold temperatures lead to
cessation of growth and the formation of a dark band. An increase in the frequency of growth
cessation is found as bivalves are examined at higher and higher latitudes moving toward the
poles. The annual growth rings on the exterior of the valves can be used to estimate the age of a
clam by counting each ring as one year of growth. However, an extra ring may form during
spurts of growth and may cause confusion. Also as the clam ages, rings may get worn away near
the margin making it difficult to interpret the age in fully grown individuals. A more accurate
way to determine the age of a clam is to examine microscopic sections of the outer prismatic
layer of the shell.
Pearls
Pearl production is a protective device used by the animal when a foreign object (grain of sand,
parasite etc.) becomes lodged between the shell and mantle. The mantle secretes many layers of
nacre around the irritating object, resulting in the formation of a pearl. Because the surface of
the pearl is very smooth, it is far less irritating to the clam than the original particle. Cultured
pearls are produced by artificially inserting small particles into pearl oysters.
Opening and Closing of the Shell
On the dorsal surface of the shell is a protuberance called the umbo, which is the oldest part of
the shell. Concentric growth lines radiate from the umbo. The two valves are attached by an
elastic, proteinaceous hinge ligament. The valves are closed by contraction of the adductor
muscles. Slippage of the valves is prevented by hinge teeth that interdigitate with sockets
opposite them. The closing of the valves by the adductor muscles is not opposed by an opposite
action performed by abductor muscles to open the shell. The contraction of the adductor muscles
produces tension in the hinge ligament. When the adductor muscles relax, the ligaments spring
back to their original shape, opening the valves.
Scars on the inner surface of each valve mark the attachment point of muscles. These are the
large anterior and posterior adductors, which draw the valves together, the anterior and posterior
retractors, which draw the foot into the shell, and the anterior protractor, which helps to extend
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the foot. The mantle lines the inside of the shell. The line of mantle attachment appears on the
inner surface of each valve as a scar called the pallial line.
The mantle covers the visceral mass dorsally and encloses the mantle cavity. In most marine
bivalves the mantle forms tubular siphons that extend outward from between the shell. In the
freshwater clam the incurrent and excurrent openings are slitlike openings between the two
mantle layers. Water flows into the clam by way of the incurrent siphon and out by way of the
excurrent siphon.
Organization of the Body
The overall organization of the body can be seen clearly if one examines a cross-sectional view
through the clam. Using this view, we will identify the layers of the body from the outside to the
inside. The first and outermost layer is composed of the shell that is made up of two valves, held
together by a dorsal hinge ligament. Next is the mantle, a sheet of muscle that lines the inside of
the shell. Next, there are a pair of gills that hang down on either side of the visceral mass. The
central part of the body, the visceral mass, hangs down from the dorsal midline. The visceral
mass continues ventrally into the muscular foot, which is used for locomotion. Between the
mantle and the visceral mass is a space, the mantle cavity. Above the visceral mass dorsally is
the pericardial cavity containing the heart.
In dissection, with one valve removed, the soft body can be seen contained within the shell. The
thin muscular mantle can be seen adhering to the inside of the shell. Posteriorly the mantle
forms an incurrent aperture, through which water enters the clam, and an excurrent aperture,
through which water leaves. In the freshwater clam, the portals are simply spaces between the
two layers of the mantle. In marine clams they are usually tubular siphons that can be extended
from the body through the shell. The beating of cilia on the gills draws water into the clam
through the incurrent siphon. The water circulates around the body, bringing in oxygen and
food, flushes out wastes, and carries reproductive products out into the environment. The gills
(ctenidia), which function in respiration, partially cover the visceral mass. They appear as thin
layers of striated tissue. The central part of the body suspended from the dorsal midline is the
visceral mass, which contains the visceral organs. It is continuous anteroventrally with the
muscular foot, which is used for locomotion. Outside the gills and adhering to the inside of the
valves is a thin sheet of tissue called the mantle. The space between the mantle and the visceral
mass is the mantle cavity. The mantle secretes the shell. The mantle functions in respiration.
The gills develop from the mantle. Dorsally, a thin membrane covers the pericardial cavity,
containing the heart. A true coelom is present but limited mainly to area around heart, and
perhaps the lumen around the gonads and part of the kidneys.
The clam has a hatchet-shaped foot that is located ventrally. It can be extended between the
valves using the pedal protractor muscles or withdrawn using the pedal retractor muscles. The
foot is used for burrowing and anchoring the clam, as well as in producing the slow locomotion
typical of clams. Blood is pumped into the foot, causing it to swell and to act as an anchor in the
mud or sand, and then longitudinal pedal retractor muscles contract to shorten the foot and pull
the animal forward.
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Water carrying dissolved oxygen enters the clam through the incurrent siphon and as it circulates
over the mantle and gills, gaseous exchange takes place.
The clam is a filter feeder. It feeds on microscopic particles and microorganisms (e.g., diatoms,
protozoans), which it extracts from the water. Water currents carrying food particles enter the
incurrent siphon. As the water passes through the gills, the food particles become trapped in
mucus. These mucous masses are transported by ciliary action toward the mouth. Ciliated flaps,
known as labial palps, attached to either side of the mouth direct the food-ladened mucus into
the mouth. From the mouth, the food passes into a short esophagus into the rounded stomach.
The saclike stomach is joined by ducts to the digestive gland or liver. A style sac, containing a
gelatinous rod, the crystalline style extends posteriorly from the stomach. The crystalline style
is kept whirling by means of cilia in the style sac. The gastric cilia and rotating style wind up the
mucus and food into a string and draw it along the esophagus to the stomach. As the crystalline
style is rotated against a toothlike projection known as the gastric shield, enzymes contained
within the rod are abraded off. These enzymes, which include amylase, cellulase, and probably
lipase, bring about the digestion of the food. A sorting region in the stomach sorts the food
particles by size. Small particles are directed into the digestive gland. Larger particles, usually
indigestible mineral particles, are directed to the intestine by a ciliated intestinal groove.
Extracellular digestion of carbohydrates and lipids begins in the stomach. Most protein digestion
occurs intracellularly within the digestive gland. The intestine receives undigested wastes and
forms feces. The intestine emerges from the stomach, loops through the visceral mass,
penetrates the pericardial cavity and is surrounded by the heart. After passing through the
pericardial cavity, the intestine becomes the rectum. It ends at the anus, where the waste is
released into the excurrent siphon.
The circulatory system is an open one. It consists of a dorsal heart, blood vessels, and open
spaces called sinuses. In an open circulatory system, the blood leaves the arteries and enters
open spaces, or sinuses in the body where it surrounds and bathes the organs, supplying them
with oxygen and removing wastes. In Molluscs, the coelom is greatly reduced. The main body
cavity is an open circulatory space or hemocoel. The three chambered heart lies within the
pericardial cavity. It is made up of two auricles (atria) and a single ventricle. The ventricle
folds around the intestine within the pericardial cavity.
The atria receive oxygenated blood from the gills by way of efferent ctenidial vessels.
The atria pump the blood into the ventricle. The ventricle pumps blood forward into an anterior
aorta that opens up into sinuses supplying oxygenated hemolymph to the foot and viscera
(except the kidneys and gills). The ventricle pumps hemolymph backward into a posterior
aorta delivering to the rectum and mantle. Blood oxygenated in the mantle or gills returns
directly to the atria, but that which circulates through other organs is collected in a vein to the
kidneys and then passes to the gills for oxygenation before returning to the heart.
The blood of molluscs contains various cells, including amebocytes, and is referred to as
hemolymph. It is responsible for picking up the products of digestion from the sites of
absorption and for delivering these nutrients throughout the body. It usually carries in solution
the copper-containing respiratory pigment hemocyanin.
Excretion is carried out by paired, tubular metanephridia often called kidneys, which remove
organic wastes from the blood and pericardial fluid. The paired kidneys are located beneath the
pericardial cavity. They are U-shaped, with one end opening up into the pericardial cavity, and
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the other by way of a nephridiopore into the suprabranchial chamber. Pericardial fluid passes
into the kidney. In the kidney, wastes are removed by ultrafiltration, useful metabolites are
reabsorbed, and the remaining urine passes through the nephridiopore into the suprabranchial
chamber.
The nervous system consists of three pairs of paired ganglia connected by nerves. The ganglia
include the cerebropleural ganglia beside the esophagus near the anterior adductor muscle, the
pedal ganglia in the foot, and the visceral ganglia below the posterior adductor muscle. The
anterior cerebropleural ganglia are connected by way of two pairs of nerve cords to the other
ganglia. One pair of nerve cords extends posterodorsally to the visceral ganglia, the other leads
ventrally to the pedal ganglia. The two cerebropleural ganglia are joined by a dorsal
commissure over the esophagus. The cerebropleural ganglia supplies nerves to the palps,
anterior adductor muscle, and mantle. The visceral ganglia serve the viscera (including the heart,
intestine, nephridia, and gonad), the mantle and siphons, posterior adductor muscle, posterior
pedal retractors, gills, and osphradia The pedal ganglia control the anterior pedal retractor
muscles, byssal retractor muscles, and the foot. Sense organs include light receptors, tactile
receptors, and statocysts for equilibrium. A thick patch of yellow epithelial cells known as the
osphradium in the mantle cavity monitors the water entering the mantle cavity for chemicals. If
potentially harmful chemicals are detected, ciliary beating and water intake can be
