But when it is active, the head and foot protrude from the shell and the animal crawls slowly. It lies above the head-foot complex and contains the main organs of the body. It is spirally coiled like the shell and fills all the whorls of the shell.
The edge of the mantle is thick and contains shell glands which secrete the shell. Present on the floor of the buccal cavity is a rounded elevation called the odontophore or tongue mass (Fig.6). The anterior end of the odontophore is round and known as the sub-radular organ.
The esophagus empties into the stomach, which lies on the left side of the visceral mass, just below the pericardium. The back part of the intestine is straight and thick-walled and is called the rectum.
A lamella in section
The lung sac is a modification of the mantle and is a closed cavity which depends on the dorsal wall of the mantle in the pulmonary chamber. The sac communicates with the pulmonary chamber by a large oval opening protected by two unequal valves (Fig. 11B). The siphon extends beyond the water level so that atmospheric air is drawn into the alveoli through it.
The lung sac now becomes greatly enlarged and occupies almost two-thirds of the lung chamber. The lung chamber is completely cut off from the branching chamber by the epitaenia, which contacts the roof of the mantle during this process (Fig. 11B). The alternate contraction and expansion of the mantle wall and the alveoli assist in the process of inspiration and expiration of air.
When Pila is on land, the respiratory tube does not form and the air enters the lung chamber directly through the expanded occipital lobe. During aestivation, Pila's life activities are at low ebb, and the air stored in the lung chamber appears to be the only source of oxygen for respiration.
- A fine cutaneous artery to the skin
- A thick oesophageal artery to the oesophagus
- A thick pallial artery to the left side of the mantle, left nuchal lobe and the osphradium
- Peri-intestinal sinus surrounding the terminal part of the intestine and the the genital duct. It collects blood from the digestive gland and the visceral organs
- Branchio-renal sinus lying outside the anterior renal chamber and leading into the afferent ctenidial vein. It receives blood from the roof of both the renal chambers
- Pulmonary sinus present in the wall of the pulmonary sac. It receives blood from the peri-visceral sinus
The heart is located on the left side of the visceral mass very close to the posterior end of the ctenidium (Figures 12 and 13). The pericardium is located between the pulmonary chamber and the posterior renal sac on the left side of the body gyrus (Figure 12). Thick pallial artery on the left side of the mantle, left nuchal lobe and osphradium.
The peri-intestinal sinus surrounding the terminal part of the intestine and the genital canal. Afferent ctenidial vein: Collects deoxygenated blood from the perivisceral sinus, rectum, and terminal part of the genital ducts and carries it to the ctenidium where the blood is oxygenated. The inner cavity of the front chamber is reduced due to the presence of multiple slats that protrude from both the roof and the floor.
The posterior renal ventricle lies on the left side of the rectum, tightly pressed against the pericardium and the digestive gland. There is only one osphradius, which lies suspended from the roof of the mantle cavity on the left side.
Osphradium
T.S. of Osphradium
Septibranchia (Poromya) 3. Eulamellibranchia (Anodonta )
Filibranchia (Arca)
The mantle or pallium is a thin delicate covering formed by the fold of the body wall that secretes the shell. The mantle has two large lateral folds that run the length of the body. In Pila, the free edge of the mantle is attached to the edge of the shell and is in contact with the region above the head.
The mantle consists of two lobes that correspond to the two valves of the shell and encloses the entire body. The simplest fusion is by the temporary appointment of the mantle edges of the two lobes. On the contrary, the exhaust siphon is narrower, smoother and is formed by the fusion of the two mantle lobes.
There is only a temporary merging of the edges of the sheath, leaving two holes: one in the front and the other in the back. This lack of fusion of the mantle edge is due to the presence of a well-developed byssus apparatus. The free mouth edge of the mantle fits loosely around the neck to form a collar, leaving a circular opening.
It is through this funnel that the water is pushed out of the mantle cavity. One end of the funnel opens with a wide opening into the mantle cavity, while the other end opens with a narrow opening to the outside. Like any other mollusk, the epithelial lining of the mantle separates the shell that is internal to most of them.
The ctenidia, osphradia and two nephridiopores lie on the front of the body behind the head. Hypothetical ancestral stage before twisting Shifting of the mantle cavity to the right side of the body. The anterior position of the mantle cavity after twisting may facilitate ventilation of the mantle cavity and gills.
COLLECTION OF SPAT OR BABY PEARL OYSTER
Today, pearls are produced artificially and Japan is one of the largest countries in the world that produces most of the pearls according to pearl culture technique. Mikimoto of Japan discovered a method to stimulate the pearl oysters to form pearls by artificially introducing foreign particles between the shell and mantle. The treated oysters are kept in perforated cages and lowered into the sea until pearls are formed.
The main difficulty encountered in this method was that the partially exposed pallial membrane was prone to infection and damage. In this method, a calcareous substance (core) is grafted with a piece of mantle to the body of the nacre.
REARING OF OYSTERS
SEEDING OR GRAFTING OPERATION
Mollusk shells are generally used to prepare the nucleus (the foreign particle inside the pearl is called the nucleus), as it has been found that the secretion of nacre occurs more around the nucleus. The shell is cut into small cubes and then ground into a spherical shape by placing them between two iron plates. This is used as a nucleus for placement into the oyster body that acts as the host.
A small strip of mantle tissue is taken from the edge of the mantle and cut into small squares. The outer surface of the mantle tissue is kept in contact with the nucleus to be inserted, as the nacre-secreting cells are found on the outer surface of the mantle. The graft tissue is inserted together with the artificially produced nucleus into the living tissue from another oyster.
An incision is made in the soft body of the oyster, through which a long and slender steel probe is inserted, and a tunnel is made in the soft body mass, taking the greatest care and avoiding the delicate organs such as the heart, liver, pancreas, kidney, stomach and gut. At the end of the tunnel is the scrotum, which consists of gonads and connective tissue. This is also the ideal place in the animal body to grow the cultured pearl because it is large enough to allocate the nucleus (and the future cultured pearl) and also receives nutrients from the body.
The operated oysters are placed in wire cages and suspended in the water from wooden rafts. Healing of the operated tissue takes about 6-7 days during which the cages are left undisturbed. Small pieces of mantle tissue will graft onto the surrounding tissue, grow around the sphere, and then begin to cover it with microscopic layers of nacre.
The breeding cages are checked periodically, all encrusting organisms are removed from the shell and the oysters are protected from parasitic attack. Pearl harvesting takes place from December to February, but varies depending on the climatic conditions of the area. Marine, freshwater and terrestrial forms in which the mantle cavity and internal organs are located anteriorly.
Similar to Mesogastropods in having a single monopectinate gill, one auricle and one nephridium,and in possessing a complex reproductive system
Similar to Mesogastropods in having a single monopectinate gill, one ear muscle and one nephridium, and possessing a complex reproductive system.
Shell with a short to very long siphonal canal
Radula bears three teeth in a transverse row (rachiglossate radula)
Complex osphradia with bipectinate folds
Example: Murex, Mitra, Voluta, Oliva etc
Class Gastropoda is the largest class of molluscs, including about 35,000 extant species and 15,000 fossil species. The visceral mass is sharply separated from the rest of the body and is covered with spicules. The mantle cavity is on the right side and becomes a vascularized chamber for gas exchange.
Snails with anus are present at the posterior end of the body, instead of laterally as in other pneumococci. Lung with two pairs of tentacles, the last pair of tentacles has eyes at the tip. They have large gills, which in some shells, in addition to gas exchange, have also taken on the function of collecting food.
Characteristics same as for Palaeotaxodonta
Characteristics same as Cryptodonta
Mantle margins are usually united posteriorly at one or more points, leading to the development of siphons. They are marine molluscs that burrow in sand in water depths of more than 6 meters. The animal is buried with a larger anterior end pointing downwards and the posterior end near the surface of the substrate.
Members of this class are adapted for swimming, with the exception of a few bottom-dwelling forms such as Octopus. The body is elongated along the dorsoventral axis, the functional anterior-posterior axis. The tentacles are present at the anterior end of the body and are homologous to the feet of other molluscs.
Members of this class have two pairs of gills and two pairs of nephridia.
