Question

1-Explain six of the following terms with reference to the Mollusca. In your answer be as comprehensive as possible regarding each term and use illustrations. a) mantle cavity, b) radula c) ctenidia, d) Monoplacophora, e) trochophore, f) pulmonate gastropod, and g) torsion.

2-Describe how the gnathostomes developed a jaw, and discuss how this adaption has benefitted later chordate species.

Answer 1

1. Phylum Molluscs has many examples, such as octopus, clams, squids, snails, etc. The molluscs are soft from the inside, but on the outside, they have a hard shell or exoskeleton.

a) Mantle Cavity: Mantle is a soft tissue just beneath the hard shell of the animal body and it covers the body of the animal. The mantle also secreted sodium bicarbonate which helps in the formation of the hard shell. It is one of the main function of the mantle. The other function is that the mantle forms a cavity which is also known as the mantle cavity. This cavity lies between the animal body and the mantle tissue. Mantle cavity acts as a water pumping station for the aquatic molluscs.

Mantle cavity has all the organs in it, such as gills, reproductive, digestive and excretory systems. The water enters the mantle cavity which helps in the absorption of oxygen through gills. The excretory system helps in the removal of waste. Excretory pores are released into the mantle cavity which is then pumped out of the body of the animal. In some animals, such as cephalopods, the mantle cavity helps in locomotion.

b) Radula: Radula is a feeding organ which consists of chitins, which is a tooth-like substance made up of carbohydrates. It is located in the front of the mouth of the animal. Like tiny teeth works, molluscs also feed on food particles by scraping the food off from the surfaces and carrying it into their mouth.

c) Ctenidia: Ctenidia is the respiratory organ of the aquatic molluscs. It is present inside the mantle cavity and contains afferent and efferent blood vessels, nerves and it looks like a spiky comb-like structure. The axis of the ctenidia contains gills attached to it and the surface of the ctenidia are covered with cilia. The cilia help in the movement of the body in the water. The deoxygenated blood enters the ctenidia through the afferent blood vessel and the oxygenated blood travels from gills to the heart through the efferent blood vessel.

d) Monoplacophora: It has a cap-like shell and is bilaterally symmetrical. Their valves are distinct in appearance. The left and the right side will have distinct features. The shape of the shell depends on the type of species. Different types of species have different shapes and size of the shell such as they can be flat or curved, etc. The Monoplacophora has tentacles which are situated beside the mouth. They have a foot which is short and not strong. They are surrounded by a mantle cavity. The mantle cavity helps in separation of foot and mantle.  Monoplacophora is usually found in the deep sea attached to rocks. They eat everything like animals, plants and other small organisms.

e) Trocophores: Trocophores are characteristics of most molluscs. They are translucent and free-swimming larvae of many marine animals. They are ciliated and pear-shaped. The whole pear shape is surrounded by the ring of cilia which helps them to swim in the water.

f) Pulmonate Gastropod: They have lost their gills and breathe through lungs. The lungs are the sack-like structure which is a modification of the mantle cavity. They are highly vascularized. The exchange of gases takes place in the modified lungs due to the absence of gills. One of the examples of such animals is Snail. The snails are also found in the dry land due to which it is important for them to conserve more and more water. Hence, to conserve water, they convert the waste into uric acid and their excreta looks like that of birds.

g) Torsion: Torsion is a mechanism during which the visceral and pallial organs change their positions in most of the aquatic and terrestrial molluscs. The mantle cavity comes from posterior end to anterior end. The gills and kidneys move from left to right to help in breathing. During torsion, when the organs change places, the foot remains in the same place and all the rotation takes place below the neck so that the organs from the right and left side can change the position respectively.

2. Gnathostomes jaws: The opening and closing of the mouth to pump water into the anterior gills for respiration are believed to be the main reason behind the formation of jaws. This closing and opening mechanism of the mouth to pump water is supported by the arch which helps in catching the prey. This mechanism also helped in catching the larger prey because the jaws slowly developed to be more strong and tough due to the continuous opening and closing to pump water. This slowly helped in the development of strong and real jaws in the Gnathostomes.

This adaptation helped the later chordates in catching larger preys more effectively. The evolution also allowed the chordates to become great hunters and had options of food which the jawless animals could not consume. The grasping strength and the tearing of the food became easier for the chordates and hence, the options of food items also increased drastically for them.

The early Gnathostomes known were acanthodians and placoderms. Both these two species are extinct now. The latest example of Gnathostomes is Chondrichthyes and Osteichthyes. Chondrichthyes are cartilaginous fishes which consist of sharks, sawfishes, etc. Osteichthyes are bony fishes. There are approximately 30,000 species under this category.