In this article we will discuss about the dissection and structures of different types of fishes.
1. Placoid Scale of Shark:
Placoid scales are found in elasmobranchs.
Preparation:
Cut a small piece of skin from the dorsal surface of a shark (Scoliodon). Put it in a hard glass test tube containing 5 to 10% KOH solution. Boil with constant stirring till the skin dissolves. Pour the contents of the test tube in a large watch glass. Allow to cool. The scales settle at the bottom. Decant the fluid. Repeat decantation with water till the last trace of KOH is removed.
Pipette a drop of water with the scales and put it on a slide. Remove the water with a piece of blotting paper and mount in glycerine. Staining, if required should be done in a small watch glass. Mount following routine procedure.
Structure:
(Fig. 29.1)
i. The scale has a base and a body.
ii. The basal plate is somewhat diamond shaped with a pulp cavity on the ventral surface, at the centre.
iii. The proximal end of the body attached to the basal plate is narrow. It widens distally.
iv. A few spines are present in the body which project a little beyond the distal margin.
2. Cycloid and Ctenoid Scales of Fishes:
These scales are present in teleosts or bony fishes.
Preparation:
Remove a few cycloid scales from a carp or a few ctenoid scales from a koi (Anabas) fish. Put them in a watch glass containing 10% KOH solution. Stir slowly with a needle till the covering epithelium dissolves. Wash thoroughly with water to remove the last trace of KOH. Make a temporary or stained permanent preparation, as required.
Structure:
(Fig. 29.2)
i. A thin, nearly rectangular plate of bone with a semicircular free border.
ii. Concentric rings representing annual growth present.
Cycloid scale:
The free end smooth.
Ctenoid scale:
The free end bears numerous short, bony spines.
3. Ampulla of Lorenzini (Dog Fish):
Scoliodon sp. (Dog-fish):
Ampullae of Lorenzini are identified by the large number of pores, arranged in groups, on the dorsal surface of the snout and the sides of the head of a shark (Scoliodon). On pressing the region a sticky fluid oozes out through the pores.
Preparation:
Cut a portion of the skin with underlying tissue from the dorsal surface of the head or snout with a group of such pores. Put it on a slide with a small amount of water. Ventrally, the ampulla is connected with fine nerves. Identify the ampulla in the tissue. Isolate it from the surrounding tissue with the help of two needles. Wash with water, stain with borax carmine in a watch glass, dehydrate, clear and mount.
Structure:
(Fig. 29.3)
i. A tubule with an opening at one end.
ii. The ampullary sac attached to the other end of the tubule.
iii. The sac is with eight radially arranged chambers around a central axis.
iv. The sac is connected with fine nerves.
4. Accessory Respiratory Organ of Magur Fish:
Clarias sp. (Magur fish):
The accessory organs are two, one in each branchial chamber.
Killing:
Clarias can be killed by any of the following methods:
i. By asphyxiation with chloroform.
ii. Striking the head with a hard object.
iii. Putting the fish in warm water. The method is unsatisfactory.
Preparation:
Fix the specimen on a dissecting tray in a lateral position with the help of large pins. Cut the operculum with a pair of stout scissors along the border of the branchial chamber, keeping the pointed arm outside. The branchial chamber, except the region occupied by the gills is enclosed in a highly vascularized membrane. Cut the membrane in the form of a cross (+), turn the flaps and pin those down. The arborescent organ is exposed.
Structure:
(Fig. 29.4)
It consists of two components:
i. A highly branched, vascularised arborescent organ associated with second and fourth gill arches and located on their dorsolateral aspect.
a. Due to profuse branching and deep red colour it appears like a beautiful flower.
ii. A heavily vascularised, membranous diverticulum of the branchial chamber enclosing the arborescent organ.
5. Accessory Respiratory Organ of Singi Fish:
Heteropneustes sp. (Singi fish):
The accessory respiratory organs are two, one on either side of the body. These are air tubes growing out from branchial chamber and extending to very near the tail being situated close to the vertebral column.
Killing:
The methods of killing Clarias are equally applicable to Heteropneustes.
Preparation:
Expose the branchial chamber of one side following the procedure adopted for Clarias. Give a deep incision with a sharp scalpel along the lateral line taking care it is not that deep to damage the air tube. Separate the muscles along the incision to expose the air tube. Working from the tail end trace the air tube up to its anterior end in the branchial chamber.
Structure:
(Fig. 29.5)
i. It is a long air tube extending from the posterodorsal corner of the branchial chamber to very near the tail.
ii. The anterior end communicates with the branchial chamber by a large opening.
iii. The posterior end is slightly narrow and closed.
iv. Four narrow ridges, made up of highly vascularised, small folds on the inner walls run along the whole length of the air tube.
6. Pituitary Gland of Rohu Fish:
Labeo sp. (Rohu fish):
In carps, the pituitary gland or hypophysis is an oval body attached to the ventral surface of the brain, just posterior to the crossing of optic nerves (Fig. 29.6A). It is connected to the brain by infundibulum and covered with Dura mater.
Collection of Pituitary Gland:
Two methods of collection are practiced:
A. Collection from whole fish:
The head of the fish is firmly hold with left hand in a dorsoventral position (Fig. 29.6B). To expose the brain a fairly large window is cut in the roof of the cranium with a stout scalpel or a sharp, pointed bone cutter. The fatty matters meninges around the brain is removed with a fine brush or cotton wool moistened with water. The residue of the fatty substances are washed away with clean water.
The medulla oblongata and the cranial nerves are cut close to their origin. The brain is lifted up from the anterior end, holding the olfactory lobe with a pair of fine forceps and removed from the cranium. In the process, the pituitary gland is left behind in the floor of the cranium. The membrane covering the gland is gently removed and the pituitary (Fig. 29.6B) is taken out with a blunt, bent needle.
B. Collection from severed head:
The foramen magnum is enlarged by cutting the supraoccipital bone with a bone cutter. The brain is either pushed forward with a narrow, blunt rod or pulled out with a pair of fine forceps, operating through the hole. The pituitary gland remains in its original position in the floor of the cranium. It is taken out with a blunt, bent needle.
Preservation of Pituitary Gland:
Pituitary gland is successfully preserved in any absolute solvent precipitant, viz. ethyl alcohol, isopropanol, acetone, etc. Immediately after collection the glands are put in the precipitant and the container tightly stoppered.
Later, the glands are transferred to containers with fresh precipitant, tightly stoppered and stored in a refrigerator at 4°C. Exposure to atmospheric temperature in transit or at the time of use, following preservation does not greatly affect the potency of the hormone in the gland.