Biology

Mr. Weinkle

Clam Dissection

Background Information

Kingdom Animalia Phylum Mollusca Class Bivalvia

These organisms include the clams, oysters, mussels and scallops. They have shells divided into two halves, hinged at the mid-dorsal line. Two sets of adductor muscles holds the shell tightly together to protect the animal. When the shell is open the animal may extend its foot for digging or anchoring. Bivalves lack a distinct head or the radula. Most of these animals are filter feeders. They trap food particles in the mucus that coats their gills and then use cilia to transport the particles to the mouth. Water flows into the mantle cavity through the incurrent siphon, passes over the gills and then exits through the excurrent siphon. While most of these animals are slow moving and may spend their entire lives buried in mud, some species attach themselves to docks, rocks or other stationary objects. The scallops may be quite active using their shells to flap along the sea bottom.

Purpose: To investigate the anatomy and behavior of the Bivalve Mollusc and compare it to the other animals including ourselves.

Materials:  Clam, dissecting pan, scalpel or scissors, teasing pins, absorbent paper.

Procedure: Read the handouts carefully. They are your guides to a successful dissection.  Draw the clam from the outside (top and side views). Measure the shell and describe in notes and drawings (front, top and side views). Make several drawings of the animal. Once the shell is open, be sure to observe the animal carefully. Take note of the mantle and the yellowish gills with their faint striations. Take notes about any textures, shapes and other observable qualities. Label ALL drawings. 

Leave the animal within the valve (shell) during the dissection.

The dissection:

·        Pry the two valves apart and carefully cut through the large posterior adductor muscle below and behind the hinge.  (The instructor may do this for you). Then cut through the anterior adductor muscle.  These two muscles keep the shell closed.  Notice the mantle, a white membrane that lines each shell.  Examine the posterior end where the edges of the mantle meet to form the pigmented siphons.  Notice the papillae on the incurrent siphon through which water and food enter as well as the smooth edges of the dorsal excurrent siphon through which water leaves the mollusc.

·        Turn back the mantle flap. Scrape some cells from the edge of the mantle and from the gills.  Look for the ciliated cells with the dissecting microscopes.  Notice that the gills of each side unite, forming a channel above the gills from which the dorsal excurrent siphon leads. The lower incurrent siphon leads to the lower body cavity.

·        Under the hinge, locate the heart.  The mollusc heart has 3 chambers. Carefully, cut into the pericardial cavity and find the yellowish ventricle, which may still be pulsing.  A tube, the intestine, leads through the ventricle but has no connection with it.  Look for an auricle on each side of the ventricle.  Trace the anterior aorta (above the intestine) carrying blood forward and the posterior aorta (below the intestine).

·        Posterior to the heart and in front of the posterior adductor muscle are dark kidneys. 

·        TRY to trace the digestive system.  Locate the mouth (surrounded by labial palps) just back of the anterior adductor muscle and in front of and above the foot.  Behind the anterior adductor muscle find the dark colored digestive gland that surrounds the stomach.  From here, trace the intestine through the heart and toward the excurrent siphon.

·        Carefully cut away the muscle of the foot to expose the ovary or testis in the posterior dorsal part.  Directly under the posterior adductor muscle, try to find the yellowish visceral ganglia encased by a thin membrane. From these ganglia, you may trace the nerves forward to the cerebral ganglia, close to the labial palps.  Continue to follow the nerves into a pair of orange foot ganglia deeply embedded in the foot forward of the gonad.

Results and Observations: All finished drawings and notes should be accurate and neat. For the final lab report, go over your notes and rewrite them for accuracy. Label your drawings.

Conclusions:

(Answer these questions)

What distinguishes this animal from others you have observed?

Can you observe any body symmetry? Describe it.

Compare the anatomy and organs of the clam with that of the annelid and yourself.

What organs are present in both human and mollusc bodies?

How is the heart connected to the respiratory organs?

How is the digestive system similar to yours? How is it different?

Be thorough and complete in your answers.