Virtual Dissection - Clam

Overview images of clam, left mantle removed

1 – This is the clam's foot, a muscular organ used for digging. Two retractor muscles withdraw the foot into the shell.
2 – This is the anterior adductor muscle, a major muscle for closing the valves.
3 – This is the posterior adductor muscle, a major muscle for closing the valves.
4 – This is the visceral mass, a thickened region extending from the foot dorsally to the pericardial cavity and bordered by the mouth and siphons. The visceral mass contains the organs of digestion and reproduction.
5 – These are the siphons. The upper arrow points to the excurrent siphon, the lower arrow, the incurrent siphon.
6 – These are the gills. Two pairs of gills are found on each side of the clam.
7– These are the labial palpi. The palpi form the boundry of the mouth on their anterior end. They are covered with heavily ciliated cells and direct food toward the mouth.
8 – The arrow shows the ventral margin of the right mantle. Note that the right mantle joins the reminants of the left mantle to form the incurrent and excurrent siphons.
9 – The arrow points to the pericardial cavity, covered with a thin, dark membrane.


Overview image of clam, left valve removed

1 – This is the clam's left mantle. The mantle secretes the shell and is attached to it along the pallial line seen on the inner surface of an empty valve. Note: the left mantle was detached from the left valve when it was removed. The black arrows show the border of the left mantle.
2 – This is the anterior adductor muscle, a major muscle for closing the valves.
3 – This is the posterior adductor muscle, a major muscle for closing the valves.
4 – This is the pericardial cavity, a region covered with a thin, dark membrane that contains the heart, kidney, etc.
5 – This is the margin of the right mantle. The right and left mantles join together to form the incurrent and excurrent siphons.
6 – This is location of the incurrent and excurrent siphons.


Image of inner surface of valve

1 – This is the inner surface of the clam's left valve. In the dissection you performed, this valve was removed for you.
2 – This is the posterior adductor muscle, a major muscle for closing the valves. The sea food we call scallops are the adductor muscles of the bivalve known as the pecten.
3 – This is the anterior adductor muscle, a major muscle for closing the valves. To open a clam, a thin knife is slid between the valves and the two adductor muscles are cut.
4 – This is the hinge area of the shell. A hinge ligament holds the valves together. Interlocking teeth in this area prevent the valves from side slipping when closed.
5 – Small teeth border the margin of each valve. These teeth prevent the valves from sliding laterally when the shell is closed.
6 – The arrow points to the posterior shell region where the incurrent and excurrent siphons are positioned.
7 – This structure indicated is the umbo of the shell. This is the oldest part of the shell.
8 – This narrow line (called the pallial line) on the inner surface of the shell connecting the two adductor muscles is the region where the mantle was attached to the shell. An indentation in this line marks the location of the two siphons.


Image of intact clam

1 – This is the clam's left valve. In the dissection you performed, this valve was removed for you.
2 – This indicates the anterior or head end of the clam.
3 – This indicates the posterior or tail end of the clam.
4 – This indicates the dorsal or upper surface of the clam.
5 – This indicates the ventral or lower surface of the clam.
6 – This structure indicated is the umbo of the shell. This is the oldest part of the shell.
7 – This faint line indicated on the surface of the shell is a growth ring. Notice how all growth rings emanate from the umbo.


Clam
(Venus mercenaria)
Kingdom: Animalia
Phylum: Mollusca
Class: Pelecypoda
Order: Eulamellibranchia
Family: Veneridae
Genus: Venus
Species: mercenaria

Adductor muscle(s)
Removal of the mantle shows the underlying soft body parts, a prominent feature of which are the adductor muscles in dimyarian species (clams and mussels) or the single muscle in monomyarian species (oysters and scallops). In clams and mussels the two adductor muscles are located near the anterior and posterior margins of the shell valves. The large, single muscle is centrally located in oysters and scallops. The muscle(s) close the valves and act in opposition to the ligament and resilium, which spring the valves open when the muscles relax. In monomyarian species the divisions of the adductor muscle are clearly seen. The large, anterior (striped) portion of the muscle is termed the "quick muscle" and contracts to close the valves shut; the smaller, smooth part, known as the "catch muscle," holds the valves in position when they have been closed or partially closed. Some species that live buried in the substrate (e.g. clams) require external pressure to help keep the valves closed since the muscles weaken and the valves open if clams are kept out of a substrate in a tank.
Gills
The prominent gills or ctenidia are a major characteristic of lamellibranchs. They are large leaf-like organs that are used partly for respiration and partly for filtering food from the water. Two pairs of gills are located on each side of the body. At the anterior end, two pairs of flaps, termed labial palps, surround the mouth and direct food into the mouth.
Foot
At the base of the visceral mass is the foot. In species such as clams it is a well developed organ that is used to burrow into the substrate and anchor the animal in position. In scallops and mussels it is much reduced and may have little function in adults but in the larval and juvenile stages it is important and is used for locomotion. In oysters it is vestigial. Mid-way along the foot is the opening from the byssal gland through which the animal secretes a thread-like, elastic substance called "byssus" by which it can attach itself to a substrate. This is important in species such as mussels and some scallops enabling the animal to anchor itself in position.
Digestive system
The large gills filter food from the water and direct it to the labial palps, which surround the mouth. Food is sorted and passed into the mouth. Bivalves have the ability to select food filtered from the water. Boluses of food, bound with mucous, that are passed to the mouth are sometimes rejected by the palps and discarded from the animal as what is termed "pseudofaeces". A short oesophagus leads from the mouth to the stomach, which is a hollow, chambered sac with several openings. The stomach is completely surrounded by the digestive diverticulum (gland), a dark mass of tissue that is frequently called the "liver". An opening from the stomach leads to the much-curled intestine that extends into the foot in clams and into the gonad in scallops, ending in the rectum and eventually the anus. Another opening from the stomach leads to a closed, sac-like tube containing the crystalline style. The style is a clear, gelatinous rod that can be up to 8 cm in length in some species. It is round at one end and pointed at the other. The round end impinges on the gastric shield in the stomach. It is believed it assists in mixing food in the stomach and releases enzymes that assist in digestion. The style is composed of layers of mucoproteins, which release digestive enzymes to convert starch into digestible sugars. If bivalves are held out of water for a few hours the crystalline style becomes much reduced and may disappear but it is reconstituted quickly when the animal is replaced in water.
Circulatory system
Bivalves have a simple circulatory system, which is rather difficult to trace. The heart lies in a transparent sac, the pericardium, close to the adductor muscle in monomyarian species. It consists of two irregular shaped auricles and a ventricle. Anterior and posterior aorta lead from the ventricle and carry blood to all parts of the body. The venous system is a vague series of thin-walled sinuses through which blood returns to the heart.
Nervous system
The nervous system is difficult to observe without special preparation. Essentially it consists of three pairs of ganglia with connectives (cerebral, pedal and visceral ganglia).
Urogenital system
Sexes of bivalves can be separate (dioecious) or hermaphroditic (monoecious). The gonad may be a conspicuous, well defined organ as in scallops or occupy a major portion of the visceral mass as in clams. The gonad is generally only evident during the breeding season in oysters when it may form up to 50% of the body volume. In some species such as scallops, the sexes can be readily distinguished by eye when the gonad is full since the male gonad is white in colour and the female is red, even in hermaphroditic species. Colour of the full gonad may distinguish the sexes in some species such as mussels. In other species, microscopic examination of the gonad is required to determine the sex of the animal. A small degree of hermaphrodism may occur in dioecious species.
Protandry and sex reversal may occur in bivalves. In some species there is a preponderance of males in smaller animals indicating that either males develop sexually before females or that some animals develop as males first and then change to females as they become larger. In some species, e.g. the European flat oyster, Ostrea edulis, the animal may spawn originally as a male in a season, refill the gonad with eggs and spawn a second time during the season as a female.
The renal system is difficult to observe in some bivalves but is evident in such species as scallops where the two kidneys are two small, brown, sac-like bodies that lie flattened against the anterior part of the adductor muscle. The kidneys empty through large slits into the mantle chamber. In scallops, eggs and sperm from the gonads are extruded through ducts into the lumen of the kidney and then into the mantle chamber.


CITES:
Prepared by the BioG 101-104 Course Staff.Comments to Jon C. Glase: jcg6@cornell.eduAll contents © 2000 Cornell University. All rights reserved.Revised: April 5, 2000URL: http://biog-101-104.bio.cornell.edu/
http://www.fao.org/docrep/007/y5720e/y5720e07.htm