DISSECTION OF UNIO OR LAMELLIDENS
External features of Unio or Lamellidens
Unio is the most familiar specimen amongst laboratory dissections. Unio is found abundantly in lime-rich water and in the bottom of lakes, ponds, rivers and streams. Generally they are found half-embedded in mud or sand at the bottom.
Procedure : For external features of the shell, use well-expanded specimens preserved in formalin. Remove one of the shells by raising it upwards by a scalpel and cut carefully the anterior and posterior adductor muscles. Pull the shell backwards till the hinge is broken.
Note the following
- Size: 5 to 10 cm.
- Colour: Outer surface is rough and reddish brown, while the inner surface white, smooth and lustrous.
- Shape: The body is unsegmented, bilaterally symmetrical and compressed.
- Shell: It consists of two equal lateral valves and hence it is called as bivalve.
- Umbo: It is a swollen, knob-like structure, which represents the oldest part of the shell. The concentric lines of growth are found around the umbo. The lines of growth are found all around the surface of the shell.
- Hinge ligament : It is a tough dark brown and elastic cord, situated on the dorsal edge by which the two valves are united and hinged together. Sometimes, hinge is toothed in some specimens.
Inner view of shell of Unio or Lamellidens
The places of muscle insertion appear as rough areas in comparison to the smooth surface. Note the following muscle scars.
- At the anterior end three muscle scars are found.
- Scar of the anterior adductor muscle, helps in closing the shell valve.
- Scar of the anterior retractor muscle, helps in withdrawing the foot.
- Scar of the anterior protractor muscle, extends the foot.
- At the posterior end 2 scars are found :
- Scar of the posterior adductor muscle, closes the shell valves.
- Scar of the posterior retractor muscle, withdraws the foot.
- Hinge line is found on dorsal position near the junction of the two valves.
- Pallial line or extends from the anterior adductor to the posterior adductor muscle. It indicates the insertion of muscle fibres, which arise on the mantle edge connecting it to the shell.
General anatomy of Unio or Lamellidens
Take a large, nicely preserved specimen, wash it thoroughly to remove formalin smell. Lift the Unio by left hand and keep umbo towards the left side. Cut or scratch the portion of hinge ligament. Insert a scalpel through the inhalent siphon and cut the posterior adductor muscle. Similarly, cut the anterior adductor muscle and then lift the upper shell. Remove the mantle in order to expose the different soft parts and examine their disposition.
- Body: It is elongated and laterally compressed.
- Mantle: It forms a thin covering for entire body and consists of two symmetrical mantle lobes which are united posteriorly except at two places, where they form a small, smooth exhalent siphon and a large, papillated inhalent siphon. Mantle encloses mantle cavity.
- Muscles : Note the large anterior and posterior adductor muscles, small anterior retractor and protractor muscles and the posterior retractor muscle.
- Foot: It is large, muscular and axe-shaped mass, protruding ventrally.
- Labial palps : Two pairs are found just below the anterior protractor muscle.
- Gills or ctenidia : Two pairs are situated in posterior half. Each gill consists of two gill lamellae.
- Heart : It lies mid-dorsally in the visceral hump within pericardial cavity. The rectum passes through the heart.
- Kidneys: Two in number, one on each side of pericardium.
- Alimentary canal : It consists of mouth, oesophagus, stomach, intestine, rectum and anus large and greenish digestive gland surrounds the stomach.
Nervous system of Unio or Lamellidens
Take nicely preserved Unio and remove both the shell valves carefully by cutting the anterior and posterior adductor muscles. Now pin up the animal with the mantle fold with the foot facing upwards. Now, very carefully remove the gill lamellae of both the sides. Locate the posterior adductor muscle. at the base of which lies the X-shaped visceral ganglion.
Remove the peritoneal covering and visceral ganglion will be clearly seen. While removing the covering 2 cerebra-visceral connectives become visible. Trace this connective. Just below the anterior retractor muscle, cerebra-pleural ganglion is found. Locate the ganglion of other side. The cerebral commissure passes over the upper rim of the mouth. For pedal ganglia, cut the muscular part of the foot and see those ganglia in anterior one-third foot. Now trace the cerebra-pedal connectives. Expose the following ganglia, connectives and commissures.
- A visceral ganglion, found at the base of posterior adductor muscle.
- A pair of cerebro-pleural ganglia, situated at the base of anterior retractor muscle.
- A pair of pedal ganglia inside the foot.
- Cerebral commissure connecting cerebro-pleural ganglia.
- Cerebro-visceral connective, which connects cerebro-pleural and visceral ganglia.
- Cerebro-pedal connective, which connects cerebro-pleural and pedal ganglia.