Apple Snails or Pila Globosa Detailed Note, Study notes of Zoology

Download Apple Snails or Pila Globosa Detailed Note and more Study notes Zoology in PDF only on Docsity! Narrator : Md. Abdula Alsad BSc First Year Dept. Of Zoology University of Chittagong Topic : Mollusca (Pila globosa) Instructor : Dr. Syeda Ismat Ara Associate Professor Department of Zoology Apple Snail ○ Introduction to Pila ○ Habit and Habitat of Pila ○ Structure of Pila ○ Integumentary System of Pila ○ Body Cavity of Pila ○ Locomotion in Pila ○ Digestive System of Pila ○ Respiratory System of Pila ○ Circulatory System of Pila ○ Excretory System of Pila ○ Nervous System of Pila ○ Development of Pila Apple snail, Pila, is a freshwater snail and is quite abundant in fresh-water ponds and lakes. They are distributed in the Oriental and Ethiopian regions of the world. A few species of this genus are found in India, of which the most common species is Pila globosa. It is one of the largest freshwater molluscs.Pila globosa is abundantly found in ponds, pools, tanks, lakes and rice fields. They may also be found in freshwater streams, rivers and even in brackish water of low salinity. They are herbivorous and therefore, quite abundant in waters, having succulent aquatic vegetation. They are amphibious form being adapted for life in water as well as on land. For this they are provided with two fold respiratory adap tations. They respire in water by ctenidium and on land by pulmonary sac. Therefore, they possess double mode of respiration. During prolonged drought they undergo aestivation for a long time and during rains they return to normalcy. When dis turbed it withdraws itself into its spirally coiled shell and seals the Opening with its operculum. The body of Pila is enclosed in a thick spirally-coiled globular univalve shell. The shell has the form of an elongated cone coiled around a central axis in a spiral manner. A single revolution of the shell around the axis is called a whorl.The extreme top of the shell is called the apex (Fig. 1.80A), which is regarded as the oldest part of the body. Starting from the apex the other whorls — the penultimate whorl and body whorl are large so as to enclose the greater part of the body. The first whorl is smallest and the last one is largest (Fig. 1.81A). The last whorl contains a large aperture, which can be closed by a lid called operculum (Fig. 1.80B), which is attached to the posterior side of the foot. The operculum is a flat calcareous plate, formed as a cuticular secretion of a group of cells from the foot. The operculum has a Locomotion in Pila: The foot of Pila helps in locomotion. The flat sole of the foot helps Pila to move very slowly by creeping on the substratum. During movement the foot is protruded through the opening of the shell and this extension is brought about by the sudden influx of blood into it. The glands present in the foot produce a slimy secretion that helps the animal to glide on dry surface. The foot is provided with vertical, longitudinal and transverse muscles. During locomotion the wave-like contractions on its surface are produced by the contraction of the vertical muscles. The contraction of the transverse muscles drives the blood forward which causes the extension of the foot in front. During this process the longitudinal mus cles contract to pull the posterior end of the foot forward. Digestive System of Pila: Pila is herbivorous and it lives primarily on aquatic vegetation. Its digestive system comprises of a tubular digestive canal and digestive glands (Fig. 1.83). Digestive canal is made up of three distinct regions: (i) fore gut, (ii) mid gut and (iii) hind gut. The fore gut and hindgut develop from the embryonic ectodermal layer, while the mid gut is endodermal in origin. (i) Fore Gut: The fore gut includes the buccal mass and the esophagus. Mouth is a vertical slit which leads into the anterior end of the digestive tract which becomes greatly swelled to form an oval buccal cavity. The buccal cavity is enclosed by a strong thick- walled muscular structure called buccal mass. Many workers consider the buccal mass as the pharynx. The entrance of the mouth is guarded by a pair of chitinous jaws projecting from the roof of the buccal cavity). At the floor of the buccal cavity, is present a chitinous ribbon-like structure called radula or lingual ribbon. It is an elongated structure bearing trans verse rows of serrations. Each transverse row contains about seven teeth — two marginals and a lateral on either sides of a median rachidian tooth, forming the formula 2, 1, 1, 1, 2 = 7. The radula is movably placed by muscles upon a large outgrowth of the floor of the buccal cavity called tongue mass or odontophore, which is made up of muscle with cartilaginous support. The odontophore has an anteriorly placed subradular organ, which is more or less a rounded structure. It is divided into two by a median furrow. A small, pouch-like sublingual cavity is pre sent beneath the subradular organ. The radu la at the posterior end enters into a radular sac which supplies new teeth to the radula. The radula is pushed forward by muscles from behind and it works as a file for rasping food materials. Dorso-laterally in the anterior region of the roof of the buccal mass lies a pair of jaws. The jaws are flexible. Its ante rior cutting edge is truncated and serrated, bearing numerous small and two or three large teeth-like processes. The jaws help to cut the aquatic vegetation upon which Pila feeds. The buccal cavity receives the secre tion of two salivary glands, situated on its posterior side. The buccal cavity leads into a long, nar row oesophagus. The oesophagus, just after its origin from the buccal mass, gives out on each side, small outpushings called oeso phageal pouch. The oesophagus leads into the stomach. (ii) Mid Gut: The midgut consists of the stomach and the intestine. It is red in colour and is situated on the lower part of the visceral mass just below the pericardium. It is a large sac, bent on itself to form a ‘U’-tube, one limb of which receives the oesophagus and the other leads into the intestine. The portion at which the oesophagus ends is called the cardiac chamber, while the other end is called the pyloric chamber. The car diac chamber constitutes the main part of the stomach. The wall of the cardiac chamber is corrugated while that in the pyloric part exhibits transverse folds. The pyloric stomach is followed by a long narrow intestine that forms 2.5-3 coils. The posterior part of the intestine is nearly straight and turns to the anterior direction when it ends into the rectum. (iii) Hind Gut: It includes the rectum which is a thick-walled tube. It lies on the floor of the right side of the mantle cavity and finally opens to the exterior through a small anus. Anus is situated near the mouth within the right mantle opening. Digestive glands include the salivary gland and the hepatic gland/digestive gland liver. There are two salivary glands situated on each side of the oesophagus. The two ducts of the salivary glands run anteriorly to open into the buccal cavity. Their secretion consists of mucus and a starch digesting enzyme. The hepatic gland is black in colour and consti tutes the main bulk of the visceral hump. It gives out two ducts which unite to form a common duct that opens into the stomach. Respiratory System of Pila: Pila is amphibious in nature and exhibits a double mode of respiration. It is capable of absorbing oxygen dissolved in water by ctenidia or gills and is also capable of utiliz ing atmospheric oxygen by the pulmonary sac. Circulatory System of Pila: The circulatory system of Pila is well developed and has attained great complexity due to its double mode of respiration, Invol ving a gill as well as a lung. The circulatory system consists of the heart, the pericar dium, the arteries, the veins and the sinuses. The pericardium is a thin-walled rough ly ovoidal chamber lying dorsally on the left side of the body and extending anteriorly up to the stomach and the digestive gland. The pericardial chamber encloses the heart and the aortic ampulla. The heart is situated in the left-hand side of the visceral whorl very near to the poste rior end of the ctenidium. As the ctenidium lies in front of the heart, the animals are included under Prosobranchia. The heart consists of two chambers, an auricle and a ventricle (Fig. 1.85). The blood of Pila contains some colourless stellate amoeboid cells and a blue, copper containing respiratory pigment, called haemocyanin. The amoeboid cells are phagocytic in nature. Circulation of Blood: The vesicula seminalis is a somewhat curved, swollen structure with a blind, rounded pos terior prolongation and situated between the junction of the two parts of the vas deferens. It serves as a storehouse of the sperms. The penis is present in the form of a whip-like flagellum which is partially ensheathed by the penis sheath and situated on the right side of the body near the mantle opening. The penis sheath is a simple outgrowth from the inner surface of the mantle. The penis is capable of great extension. At the base of the penis sheath is an oval glandular thickening, the hypobranchial gland, consisting of tall cells containing small basal nuclei. Two types of spermatozoa are encoun tered in Pila: (i) Eupyrene type which is pear shaped and (ii) Oligopyrene type which is worm-like (Fig. 1.89B & C). The eupyrene type is functional and is capable of fertilizing the egg. Female reproductive system : The female reproductive system consists of ovary, oviduct, receptaculum seminalis, uterus, vagina and albumen gland (Fig. 1.89D). Development occurs out side the body of the female. The parents do not incubate or look after the eggs. Development of Pila is direct. The embryo floats in a central core of liquid albumen which is surrounded by a thick layer of whitish solid albumen. During development the visceral mass and the shell of the embryo becomes spirally coiled and the characteristic phenomena of torsion takes place. This results in the asymmetry of the body. The young snails hatching from the eggs are similar in form to the adult.