clam

Respiratory System

Most species of clam have SEPARATE SEXES. There are both male and female clams.

The GONADS or reproductive organs (TESTES or OVARIES) are located dorsally near the pericardial cavity. Eggs and sperm are released into the mantle cavity. Fertilization in clams depends on the species: MARINE (ocean dwelling) clams – EXTERNAL FERTILIZATION Eggs and sperm are released into the mantle cavity and leave by excurrent siphon FRESHWATER clams- INTERNAL FERTILIZATION Sperm enter mantle cavity through the incurrent siphon; developing larva are discharged through the excurrent siphon and grow to adults.

Circulatory system

Clams have an OPEN circulatory system, meaning that the circulatory fluid (HEMOLYMPH) does not remain in vessels. It is collected from the gills, pumped thorough the heart, and released directly into spaces in the tissues. Open circulation is NOT AS EFFICIENT as a closed system because nutrients and oxygen are not pumped directly to organs. In addition, high oxygen and low oxygen blood can mix allowing fewer nutrients and oxygen to reach the cells.

Nervous system

Clams have NO CEPHALIZATION. There is NO distinct head area. Instead of a pair of cerebral ganglia in the head connected to one ventral nerve cord like an earthworm, a clam’s nervous system consists o 3 PAIRS of GANGLIA throughout the body
connected by TWO PAIRS of long NERVE CORDS. Nerve cells in the ganglia control the muscles involved in locomotion and feeding, and process sensory info about light, touch, and chemicals (food) in the water. Clams follow the same body plan seen in earthworms and other invertebrates with a DORSAL HEART and a VENTRAL NERVE CORD.

Excretory system

The excretory organ in clams is the KIDNEY. This organ collects nitrogen waste produced by body cells from the break down of proteins and excretes it into the mantle cavity where it is removed with water exiting through the excurrent siphon.
The kidney also maintains the balance of water and ions in the body (OSMOREGULATION).

body plan

The first dissection shows a clam which has been opened by cutting the large adductor muscles, which attach to both valves and thus function to close the shell. On the inside of the shell that has been separated, note the places where the adductor muscles attach. A thin sheet of tissue, the mantle (most of which has been cut away) covers the animal's body and secretes the shell. On the separated valve, note the line along which the mantle was attached. The mouth is hidden under a pair of labial palps above the adductor muscle to the left; therefore the anterior end of the animal is to the left. On the ventral (bottom) side is the large muscular foot, which attaches to the shell via smaller retractor muscles. The foot contains portions of the digestive and reproductive organs, and also serves as an appendage for burrowing into the mud. The region dorsal to the foot is called the visceral mass.

Digestive System

The second dissection on display shows a clam that has been cut along the midline of the body. The food particles that are filtered from the water passing through the ctenidia are carried anteriorly by water currents and guided to the mouth by the labial palps. A short esophagus leads to the irregularly-shaped stomach, which is embedded in the greenish digestive gland (1iver). From the stomach, the intestine extends through the pericardial cavity to the rectum, which is located dorsal to the excurrent siphon. The gonads from a yellowish mass among the loops of the intestine. Their ducts, which will not be seen here, open into the suprabranchial chamber.

Integumentary system

A hard outer shell and a soft body. It uses a muscular foot for movement. The skin is the largest organ in the body: 12-15% of body weight, with a surface area of 1-2 meters. Skin is continuous with, but structurally distinct from mucous membranes that line the mouth, anus, urethra, anv vagina. Two distinct layers occur in the skin: the dermis, and epidermis. The basic cell type of the epidermis is the keratinocyte, which contain keratin, a fibrous protein. Basal cells are the innermost layer of the epidermis. melanocyte produce the pigment melanin, and are also in the inner layer of the epidermis. The dermis is a connective tissue layer under the epidermis, and contains nerve endings, sensory receptors, capiliaries, and elastic fibers.

Habit, diet, life cycle distinguishes and reproduction:

• The life cycle of the hard clam includes a pelagic larval phase and relatively sedentary benthic juvenile and adult phases.
• Hard clams begin their adult lives as males, often become females with greater maturity, and require individuals of both sexes for reproduction.
• Clams develop functional male gonads during the first or second year of life.
• Spawning cycles are affected mainly by water temperature and the availability of food, and thus vary according to latitude. Spawning often occurs in "pulses" and may continue for months, but usually there are one or more distinct spawning peaks.
• In the Chesapeake Bay region, spawning usually commences when temperatures rise above 20-23 degrees C (68-73 degrees Fahrenheit.
• Female fecundity is high, and individuals can release 16 million to 24 million eggs per spawn, although laboratory studies often have recorded values of only 1-3 million eggs, and only a few will reach maturity.
• Individuals may release as many as 60 million eggs during one season.
• Eggs are 70 to 73 microns in diameter and are surrounded by a gelatinous membrane. Eggs and sperm of adults are expelled in the water current and fertilization occurs externally in the water column.
• In the Bay area, the most significant growth occurs in spring and fall, when abundant food and optimum water temperatures coincide. Growth decreases in summer and stops altogether in winter. Growth rate also decreases with age, and when this happens, clams become thicker, rather than increasing in shell length.
• Most larval stages of the hard clam swim toward light (or opposite force of gravity), so most are concentrated in the surface waters and are dispersed by wind, waves, and current.
• The pediveliger stage is the final larval stage before settlement and metamorphosis to juveniles. At this stage the organism has a foot that extends from its shell.
• Once the hard clam settles to Bay bottom, it uses its foot to dig into the mud, and secretes a calcium carbonate shell that increases in size as the clam’s internal organs grow.
• The most growth takes place in temperatures between 10 C (50 F) and 25 C (77 F). A cross-section of a hard clam’s shell usually reveals a clam’s age. Hard clams may live for more than 30 years, and occasionally reach 50 years.
• Clam is distinguished as mollusk. Because it has an outer shell and has a soft body. It has a muscular foot that it uses for movement.
• Calm eat small plants and animas called plankton.

The pictures are found in:

1. http://images.google.com/images
2. http://sps.k12.ar.us/massengale/crayfish_dissection.htm