Question

Explain structure and various types of weberian ossicles also expalin their arrangement and homology in fishes

Answer 1

Answer:

1. Structure of Weberian Ossicles:

The Weberian apparatus may consist of either four or three bony ossicles- (i) Claustrum, (ii) Scaphium, (iii) Intercalarium, and (iv) Tripus.

The last three form an articulated chain while the first piece of ascending order the claustrum, lies in front of the scaphium and is smallest anterior most piece. The claustrum either articulates with or forms a part of the neural arch of the first vertebra. The second piece scaphium is slightly larger, broad and compressed structure. A ventral peg-like process of the scaphium may be present and articulates with a depression in the centrum of the first vertebra.

The scaphium is connected to the intercalarium by means of an interossicular ligament.

The interclarium is connected to the tripus by means of another ligament.

The tripus is the largest piece and consists of three processes, i.e. anterior, middle and posterior.

The anterior process joins the interossicular ligament, while its middle process articulates between second and third vertebra. The posterior process is large and slightly curved and is known as transformator. It is connected with the wall of the gas bladder.

Explanation:

2. Connection of Weberian Ossicles with Internal Ear and Air Bladder:

In majority of the ostariophysian fishes, the air bladder is bichambered, though in most catfishes and few others, it is single chambered. It begins from some distance behind the internal ear or have extensions, which reach forward to the ear (e.g., Herring and its relatives) or may even penetrate the skull. The close proximity of bladder with the internal ear probably helps the fishes in better perception of sound.

The system of connection between the gas bladder and the internal ear through Weberian ossicles is similar in majority of the fish species.

The right and left sacculi of the membranous labyrinth are connected by means of a fluid filled transverse canal. From the middle of this canal, a blind diverticulum, the sinus endolymphaticus, projects posteriorly into a central fluid filled cavity, the sinus impar.

A small diverticulum of this cavity called the atrium sinus impar, containing perilymph, lies between the claustrum and scaphium of the Weberian ossicles. The chain of these ossicles are connected with the wall of the air bladder.

3.. Homology of Weberian Ossicles:

Weberian apparatus is a curious complex system in which every part has its own function. It is found in almost all the members of Osteriophysi, consisting of predominant fish species of the World’s freshwater. In no known fish, the Weberian apparatus appears to be non-functional or vestige. The homology and opinions are widely based on the morphology of Weberian apparatus in mature fishes.

Tripus is said to be derived from rib of the IIIrd vertebra.

1. Ossified ligament and ossified wall of the swim bladder.

2. Transverse process of IIIrd vertebra and ossified tunica externa of swim bladder.

3. Paraphysis and pleural rib of IIIrd vertebra and from the compound vertebra.

Intercalarium is believed to represent.

Neural arch and the transverse process of IInd vertebra.

1. Neural arch of IInd vertebra and ossified ligament.

2. Only neural arch of IInd vertebra.

3. Basi-dorsal of Ilnd vertebra together with the ossified ligament.

Scaphium has been variously considered as:

1. Neural arch of Ist vertebra.

2. Neural arch of IInd vertebra.

3. Rib of Ist vertebra.

4. Derived partly from neural arch of 1st vertebra and partly from the mesenchyma.

Claustrum is believed to represent.

1. The interspinous ossicle.

2. Modified spine of Ist vertebra.

3. Neural arch of Ist vertebra.

4. Intercalated cartilage.

5. Neural process of the Ist vertebra.

4. Functions of the Weberian Apparatus:

1. Auditory Function:

Frisch, 1932 and Stetter, 1938 suggested that Weberian apparatus appears to assist in auditory function. It transmits vibrations from swim bladder to endolymph of internal ear. Such vibrations cause otolith to strike the sensory organ and consequently impulse generates.

2. Sensitiveness to Sound Frequencies:

Frish, 1932 and Stetter, 1938 reported that fishes having connection between air bladder and internal ear through Weberian ossicles are sensitive to sound of higher and lower frequencies than in which such connection is not present.

3. Hydrostatic Function:

Hydrostatic function to Weberian apparatus was attributed by Dijkgraff (1950). Pulsation of swim bladder caused by change in its pressure is transmitted to internal ear through Weberian apparatus. The fishes respond to change in pressure by releasing gas from the bladder through the pneumatic duct. Weberian apparatus thus permits a better and more precise adjustment of swim bladder volume.