1. Pila globosa (the apple snail)
Dr. Sanjay Kumar Jha
Department of Zoology
P.G.G.C.G. – 42, Chandigarh

2. Systematic Position Class - Gastropoda Order - Prosobranchiata
Phylum - Mollusca
Class - Gastropoda
Order - Prosobranchiata
Suborder - Pectinibranchiata
Family - Pilidae
Genus - Pila
Species - globosa

3. Natural History
Pila has a wide distribution, found in the Oriental and Ethiopian Realms. The Oriental Realm/ region includes India, Pakistan, Bangladesh, Myanmar, Sri Lanka, Thailand, Malaysia, Indonasia, Vietnam, Philippines. The Ethiopian Realm comprises of Africa, Arabia, and Medagaskar.
Pila globosa is the common species of Northern India. It is commonly called as Apple Snail.
HABITAT: Pila is a freshwater mollusk. It generally inhabits ponds, tanks, lakes and rice fields. It may occur in rivers and streams also. It prefers clean water having soft and succulent vegetations such as, Valishneria and Pistia which composes its chief food.
HABITS: Pila is herbivorous in nature, creeping very slowly with its large foot, proverbial “Snail’s pace” covering merely about 50 mm. in a minute at full speed. It is an amphibious animal. Digestion is partly extra- and partly intracellular. Respiration with the gills (in water) and with the lungs (on land). When disturbed, it withdraws itself into the shell and closes its lid. It can withstand long period of drought by remaining dormant within its tightly closed shell buried in mud (Summer sleep or Aestivation). Sexes are separate and breeding takes place in rainy season. Fertilization is internal.

4. SHELL
STRUCTURE: The shell covering the body is somewhat globular in form, about 60 mm. wide and lemon-yellow in colour; univlaved (formed of single piece) and unilocular (undivided internally); and have a long, hollow cone wound or formed in a close spiral around a central vertical axis called the Columella. The various coils of the shell are termed as Whorls. There are 3 or 4 whorls in all (becoming larger from top to base) of the shell. The top of shell is called as Apex (oldest and 1st formed part of the shell). The lowermost whorl is the largest and is known as the Body whorl and the whorl next to it is called as Penultimate whorl. The body and penultimate whorls (except the body whorl) is called as spire. The line of contact between the two adjecent whorls is called as suture. The surface of the whorls is marked by faint vertical ridges, the Lines of growth. A few of the latter are more prominent and are called as Varices representing seasonal cessation of shell secretion or retarded growth.

5. SHELL (Contd.)
Mouth of the Shell: The shell opens out by a large, lunate-oblong aperture, the mouth. Margin of the mouth is called the peristome and its entire region has two lips – concave inner lip or columellar lip and convex outer lip. If the shell is held with the apex away from the observer, the mouth is towards the right, the shell of Pila is said to be Dextral (95%) otherwise rarely (5%) Pila’s mouth is towards the left and called as Sinistral. Operculum: The snail can completely withdraw into the shell, when disturbed, and close the mouth by a thin, flat lid, the operculum, borne on its foot. Outer and inner side has difference. Collumella: The collumella is the central axis of the shell which is hollow and twisted. It opens out below by a narrow aperture, the umbilicus. Shell with umbilicus is called as perforated or umbilicated shell.

6. Histology of the Shell: The shell is composed of following 3 layers:
SHELL (Contd.)
Histology of the Shell: The shell is composed of following 3 layers:
outer Periostracum,
middle Ostracum or Prismatic layer,
inner Hypostracum or Mother of Pearl.
The Periostracum is thin, pigmented, homogenous layer composed of a Quinone – a tanned, horny protein material called as conchiolin or conchin. It is protective in function. Ostracum is the thickest layer and is composed of alternating layers of Calcium carbonate and conchiolin running at right angles to the margin of mouth of the shell. The Hypostrachum also consists of similar alternating layers, but these layers run parallel to the margin of mouth rather than at right angle as in ostracum.

7. Body Form
The body of Pila is soft and differentiated ito three distinct regions, viz., anterior head, ventral foot and dorsal visceral mass. The head and foot only protrudes out of shell while creeping, while the visceral mass remains intact inside the shell with the help of strong columellar muscles arises form the foot. It also helps the withdrawal of the extended body parts when gets disturbed.
HEAD: The head is produced into a short snout that bears a pair of contractile, tapering processes called labial palps or 1st pair of tentacles. Mouth is present in the form of median vertical slit just beneath the bases of palps. Behind the snout, the head bears a pair of long, contractile, filamentous and tapering true or 2nd pair of tentacles. A short, cylindrical stalk, the ommatophore, bearing an eye at its tip is also present.

8. Body Form (Contd.)
FOOT: The foot is fairly large, highly extensile and triangular organ with its pointed apex directed backward. Its flat, smooth, ventral surface is called as Sole which helps the animal in creeping. Sole has two distinct parts – the anterior part propodium composed of muscles, and posterior part metapodium bearing operculum on its dorsal surface. Foot is organ of locomotion and attachment.
VISCERAL MASS: is the largest part of the body. It is exactly coiled like the shell to fit in it. Its greater part lies in the body whorl but also continued in the remaining whorls. It contains all the organ-systems in it.
MANTLE or PALLIUM: The visceral mass is covered by a thin, delicate, glandular membrane called the mantle or pallium (a characteristic molluscan feature). Anteriorly, the mantle is thick and pigmented which covers the head and its appendages (when in retracted state). Shell is also secreted by the mantle (periostracum and ostracum by its thick free edge and hypostracum by its entire dorsal surface). As the Pila grows in size, the shell also grows along with it (the mantle edge produces the new parts of the shell, while the mantle surface adds to the old portions of the shell). On either side of the head the mantle folds on itself to form highly contractile process, the nuchal lobes or pseudepipodium (right and left). The space formed between the mantle and body

9. Body Form (Contd.)
PALLIAL COMPLEX: All the organs present inside the mantle or pallial cavity is called as organs of pallial complex. It includes:-
Epitaenia, a prominent ridge extending from the (floor) base of right nuchal lobe to the posterior end of the cavity dividing the mantle cavity into two unequal regions (a small right branchial chamber and a large left pulmonary chamber).
Pulmonary sac or Lung is a large air containing sac or bag hanging from the roof of the pulmonary chamber and opening into it by a large pulmonary aperture or pneumostome.
Osphradium is a small, oval, leaf-like structure attached to the roof of the pulmonary chamber just behind the base of the left nuchal lobe.

10. Body Form (Contd.)
Ctenidium or gill is a long lamellar organ hanging form the mantle at the extreme right side of the branchial chamber.
Rectum is tube-like structure situated on the left side of gills on the floor of the branchial chamber. It opens into the branchial side of mantle cavity by an aperture called Anus just behind the right nuchal lobe.
Genital duct (vas deferens or vagina) lies on the left side of rectum, with its aperture a little behind the anus.
Anterior chamber of the kidney is a reddish mass projecting into the branchial chamber near the posterior end of the epitaenia. It opens by a small, slit-like renal aperture.
Penis and a Hypobranchial gland is two additional structures present in the male Pila. The penis arises from the edge of the mantle and gets connected with the genital duct only during copulation. The hypobranchial gland lies at the base of penis.
BODY CAVITY: Pila has both coelom and haemocoel (resembling both Annelids and Arthropods), but the coelom is highly reduced represented only by paricardium and cavity of the kidney. Blood is present in the spaces/sinuses in the connective-cum-muscular tissues.

11. Body Form (Contd.)
BODY WALL or INTEGUMENTS: The body wall consists of epidermis and dermis. The epidermis is a single layered epithelium. It contains unicellular mucus-secreting glands on the foot and shell-secreting glands on the mantle. The dermis consists of connective-cum-muscular tissue which merges with a similar tissues that fills the spaces between the viscera. LOCOMOTION: Locomotion is extremely slow in Pila which is performed by the foot. A series of rippling waves of contraction passes over the foot which forces the animal to move forward, dragging the rest of the body enclosed inside the shell along with it.

12. Digestive System
The digestive system is complete and consists of Alimentary canal and some Digestive glands.
Alimentary Canal: it starts from mouth and ends at anus (peculiarly anterior in position than normal). The alimentary canal consists of mouth, buccal mass, oesophagus (in foregut or stomodaeum), stomach and intestine (in midgut or mesenteron), rectum and anus (in hindgut or proctodaeum). Mouth is devoid of true lips. Buccal Mass is a large, thick-walled, highly muscular, pear-shaped organ embedded in the floor of mantle cavity and divided into 2 unequal parts: the anterior small vestibule and the posterior large buccal cavity by a pair prominent, dark-brown jaws having cutting edges and hangs from the roof. The floor of the buccal cavity is raised into a large laterally compressed platform called odontophore covered by subradular membrane. A narrow, cuticular ribbon-like Radula is also present in the buccal cavity bearing a close-set of teeth (marginal, lateral, and central) in transverse rows having Radular formula of 2:1:1:1:2.

13. Digestive System (Contd.)
Digestive Glands: are associated with the alimentary canal and help in digestion of food which includes the dorsal buccal glands, salivary glands and liver or hepatopancreas. Dorsal buccal glands are a pair of elongated glands situated in the roof of buccal cavity and secretes an accessory digestive fluid. Salivary glands are a pair of white, branching masses present behind the buccal mass on the sides of oesophageal pouches and secretes mucus and starch-splitting enzymes. Hepatopancreas is a large, conical, spirally coiled, dirty-green or brownish organ occupying the greater part of the coiled viscera and consisting of 3 main types of cells, viz., secretory cells (cellulose splitting enzymes), resorptive cells (digest proteins) and lime-containing cells (stores calcium phosphate). PHYSIOLOGY: It includes ingestion, digestion, absorption and egestion processes by Pila .

14. Respiratory System
Pila lives an amphibious life and thus exhibits two modes of respiration, i.e., Aquatic (branchial) carried on by ctenidium or gill in water and Aerial (pulmonary) carried on by a pulmonary sac or lung on land.
Ctenidium or Gill: Pila has a single ctenidium or gill attached to right dorsolateral wall of the branchial chamber of the mantle cavity. The gill is monopectinate (single row of lamellae attached to the ctenidial axis). (see diagram).
Pulmonary sac or Lung: The pulmonary sac or lung is large sac developed in the roof of the pulmonary chamber and formed by the modification of mantle itself. It has thin, moist, highly vascular and muscular walls communicating with the pulmonary chamber by a large, oval aperture called pneumostome guarded by two unequal flaps.
PHYSIOLOGY: (a) Aquatic Respiration in water.
(b) Aerial respiration in dirty water, on coming to land or during aestivation.

15. Respiratory System (Contd.)
Mechanism of Aquatic Respiration: The gill of Pila is situated on the right side of the body, but its blood, nerve supply and position of ospharidium indicates that originally it belongs to the left side. It shifts to the right side during development (Torsion effect). Aquatic respiration chiefly takes place in the water. The head and foot of the animal was fully extended. The two nuchal lobes forms two channels by folding their margins upward. The left channel is larger and deeper acting as Inhalent siphon whereas the right channel is smaller and serves as Exhalent siphon. Cilia in the gill lamellae sets up water current that sucks the water from external medium into the pulmonary chamber via., left nuchal lobe. Ospharidium, located at the entrance checks the chemical nature of water which stops the water circulation if it founds it dirty or toxic. From the pulmonary chamber the water crosses the depressed epitaenia near the posterior end and enters the branchial chamber, where, it washes the gills and exchange of gases takes place there through the thin epithelium of the gill lamellae. The befouled water then passed out of the right nuchal lobe and during this process, the opening of pulmonary sac is kept closed. Mechanism of Aerial Respiration: Pila resorts to aerial respiration – (i) at regular intervals during favourable conditions; (ii) when the surrounding water becomes dirty and deficient in oxygen; (iii) when it comes on the land; and (iv) during aestivation. During aerial respiration still from the water, the pila folds the margin of left nuchal lobe to form tube or siphon or trumpet funnel. The pulmonary chamber is cutoff from the branchial chamber by tightly pressing of epitaenia against the roof of mantle. The pneumostome becomes almost circular and apposed to the base of siphon or trumpet. The expansion and contraction of pulmonary sac bring inspiration and expiration. Exchange of gases takes place through the thin wall of the pulmonary sac.

16. Circulatory System Heart of Pila
Circulatory system of Pila is open type. It is quite complicated due to dual mode of respiration. It comprises the pericardium, heart, arteries, sinuses, veins and blood. (1) Pericardium: It is more or less ovoid, thin-walled sac situated vertically but somewhat obliquely on the left side of the body whorl, just behind the pulmonary sac between the two renal chambers (dorsally) and junction of oesophagus and stomach (ventrally). The cavity of pericardium is true coelom as it communicates with the posterior renal chamber. It encloses heart. (2) Heart: It comprises of two chambers: an auricle and a ventricle, connected by auriculoventricular aperture guarded by semilunar valves (which allows the blood to flow in one direction i.e. towards ventricle). The auricle is thin walled and receives oxygenated and purified blood from two veins, viz. efferent ctenidial vein from gills and efferent pulmonary vein from lungs while it gets deoxygenated blood from post. renal chamber via. efferent renal vein. The ventricle has a thick, muscular wall and receive mixed type of blood from auricle and sends it to whole body through large artery (Aortic trunk). A pair of semilunar valves is also present in aortic trunk also.
Heart of Pila

17. Circulatory System (Contd.)
(3) Arteries: The aortic trunk immediately divides into two branches, viz. the anterior Cephalic Aorta and the posterior Visceral Aorta. The cephalic aorta is dilated near its base into a sac, the aortic ampulla which helps in proper distribution of blood in the head region of pila. The cephalic aorta supplies blood to pericardium, skin, oesophagus, left and right side of mantle, left & right nuchal lobes, radular sac, eyes, tentacles, foot and to copulatory organs in males. The visceral aorta extends backward into the visceral mass and gives off arteries to peircardium of its side, skin, digestive glands, stomach, intestine, kidney, hepatopancreas and gonads.
(4) Sinuses & Veins: The dexoygenated blood from the various parts of the body collects in small spaces called the lacunae which later on joins to form larger spaces termed the sinuses. Both of them lacks a definite wall and constitutes the Haemocoel. Sinuses are of 4 main types, viz. perivisceral, periintestinal, branchiorenal and pulmonary sinus. The cephalic aorta sends its blood to perivisceral and pulmonary sinus while visceral aorta circulates its blood via. periintestinal and branchiorenal sinus (Chart).
(5) Blood: The blood consists of plasma and corpuscles. The plasma is light-blue in colour due to respiratory pigment, haemocyanin dissolved in it. Corpuscles are colourless, amoeboid and of several types. They are called leucocytes.
Function: Blood transports food , O2, CO2 and nitrogenous wastes to desired places. It also keeps the tissues moist and protect the animal from microorganisms.

18. Excretory System
The excretory organ of Pila comprises of single large renal organ or Kidney. It is actually on the left side of the body, the right kidney being disappeared or modified into genital duct. The coelom of kidney is in communication with the coelom of pericardial cavity and to the exterior. It consists of two distinct chambers: anterior and posterior. (i) Anteior Renal Chamber: a small, ovoid, reddish sac situated in front of the pericardium and projecting into the branchial chamber of the mantle cavity near the posterior end of epitaenia. It communicates with posterior renal chamber at one end and on other with the branchial chamber by an oblique slit called nephridiopore. The roof and floor of ARC has efferent & afferent renal sinus and is also lamellated. (ii) Posterior Renal Chamber: is a large, hooked, brownish or greyish sac situated behind the anterior renal chamber between the rectum and the pericardium communicating at one end with the anterior renal chamber and at the other end with the pericardium by an aperture called nephrostome present as perforation on the renopericardial septum separating post. renal chamber from the pericardium. Roof of the chamber is highly vascularised by aff. and eff. renal sinus branches.

19. Excretory System (Contd.)
Physiology of Excretion: The auricular wall of the heart forms the pericardial fluid by ultrafilteration of blood across the auricular wall. This pericardial fluid, is thus called as Primary Urine which later on passes through nephrostome into metanephridial system, the Kidney. In the kidney the primary urine is modified by selective reabsorption and secretion of certain wastes from the blood across the kidney wall. The resulting final urine is then discharged through ranal aperture (nephridiopore) into the branchial chamber from where it is removed by the outgoing current of water through the right nuchal lobe. Pila, being a freshwater mollusk, maintains a relatively low level of blood salts and excretes mainly hypertonic urine by reabsorption of salts to loose a good amount of water from the body.

20. Figure of Nervous system of Pila
The nervous system of Pila is well developed and comprises of two main parts – C.N.S., and P.N.S. It shows asymmetry like other organ-systems due to twisting of visceral mass.
1. Central Nervous System: it includes the ganglia and their commissures (connections between similar ganglia) and connectives (connections between dissimilar ganglia). The main ganglia includes – cerebral, buccal, pedal, pleural, supra- and infra-intestinal and visceral. Of these supra- and infra-intestinal ganglia are unpaired and rest are paired ganglions.
2. Peripheral Nervous System: it consists of nerves arising from C.N.S. and innervates various parts of body:
Each cerebral ganglion gives off – 2 nerves to the skin of snout, 2 nerves to tantacles, 2 nerves to buccal mass, 1 nerve to eye and 1 nerve to the statocysts.
Figure of Nervous system of Pila

21. Nervous System (Contd.)
Each buccal ganglion sends off several nerves to the buccal mass, salivary gland, oesophageal pouch and greater part of oesophagus.
Each pedal ganglion sends a large number of nerves to the foot.
The left pleural ganglion provides nerves to the parietal wall, left nuchal lobe, ospharidium, mantle and columellar muscles.
The right pleural ganglion sends nerves to the parietal wall, epitaenia, right nuchal lobe, copulatory organ and columellar muscles.
The supra-intestinal ganglion gives off a stout nerve that supplies nerves to mantle and ctenidium.
Each visceral ganglion gives off (i) a right nerve to renal organs, reproductive organs and to the intestine; (ii) a left nerve to the pericardium, stomach, liver and reproductive organs; (iii) and a few small nerves to the neighbouring organs.
The supra-intestinal visceral connective supplies nerves to the pulmonary sac, pericardium, ctenidium and parietal wall of the body of Pila.

22. A Statocyst (in section) of Pila
Sensory System
The sense organs of Pila include tentacles, statocysts, ospharidium and eyes.
Tentacles: Pila has a pair of short, conical labial palps or first pair of tentacles at the anterior end of the snout and a pair of long, filamentous true or 2nd pair of tentacles on their sides. Both the pairs of tentacles act as the tactile organs.
Statocysts: Pila has a pair of statocysts, one embedded in a pit near the pedal ganglion of its side. Each statocyst is a spherical capsule surrounded by a hard, leathery, protective covering of single layer epidermal connective tissue cells. These are sensory in nature and innervated by nerves from cerebral ganglion. The cavity of capsule is full of a fluid having a calcareous particles called Statoconia (an organ of equilibrium).
A Statocyst (in section) of Pila

23. Sensory System (Contd.)
Ospharidium: A single ospharidium hangs from the roof of the pulmonary chamber close to its entrance, oval in shape about 6 mm. in length. It is a bipectinate organ, consisting of slightly raised central axis bearing 14 conical fleshy leaflets on its either side. It is a connective-cum-nervous tissue having rich blood supply and contains sensory and gland cells along with ciliary base. It functions as an organ to test physico-chemical nature of water entering through left nuchal lobe in the pulmonary chamber.
Eyes: Pila has a pair of small, black eyes, each situated on a short stalk, the ommatophore, arising outside the true tentacle of its side. Eye consists of egg-shaped optic vesicle embedded in connective tissue below the skin. The wall of vesicle is lined by pigmented retinal cells (having slender supporting cells and broad visual cells) with a hyaline, gelatinous lens present in its optic cavity. The optic vesicle is covered over by thin transparent covering, inner cornea or pellucida interna on inner side and with pellucida externa on its outer side.
T.S. Ospharidium
V.S. Eye of Pila

24. Male Reproductive System
The Pila is a dioecious animal with clear sexual dimorphism. Males have a smaller shell with less dilated body whorl with better developed and functional penis.
Male Reproductive System: The male reproductive system comprises of a testis, vas deferens, a copulatory organ and a hypobranchial gland.
Testis: is a flat, cream-coloured, triangular organ, covered by a thin membrane and situated on the digestive gland (liver or hepatopancreas) in the upper part of the visceral mass.
Vas Deferens: Fine tubules from the testis called vas efferentia, united to form a large Vas deferens which leaves the testis from its posterior end. It is differentiated into 3 regions – the proximal narrow tubular part, the middle small, sac like vasicula seminalis and the distal broad glandular part which runs forward along the left side of the rectum which ends in male genital aperture by a claw-like genital pepilla.
Copulatory Organ: It is a long, stout, slightly curved tapering organ called Penis projecting from the mantle edge in front of the anus.
Hypobranchial Gland: is and oval thickening with pleated surface at the base of penis sheaths of unknown function.
SPERM: Pila produces two types of sperms:-
Eupyrene - with twisted nucleus and a single axial filament and functional.
Oligopyrene - with a broad, curved nucleus and with many axial filaments and are sluggish and non-functional.
Copulatory Organ
Male Reproductive System

25. Reproductive System (Contd.)
Female Reproductive System: The female reproductive system comprises an ovary, oviduct receptaculum seminis, uterus, vagina and copulatory organ or hypobrabchial gland.
Ovary: is smaller than testis. It is flat dark, conical organ lying on the digestive gland in the upper part of the visceral mass. It is composed of a large number of rounded lobules or acini that unite to form small lobules, which in turn unite to form a large tube, the oviduct.
Oviduct : It leaves the ovary near its middle. It descends along the inner border of the digestive gland and enters the posterior renal chamber and opens into receptaculum seminis.
Receptaculum Seminis: is a smallest bean-shaped sac lying within the posterior renal chamber attached to the uterus. It serves to store spermatozoa received from male during copulation.
Uterus: is a large, pyriform sac situated in the body whorl outside the body whorl outside the renal organ. It continued anteriorly into a tubular vagina.
Vagina: The enters the branchial chamber of the mantle cavity and runs forward along the left side of rectum and opens by small slit like female genital aperture.
Copulatory Organ: similar to the male, but highly reduced.
Female Reproductive System

26. Breeding and development
Breeding in Pila takes place in the rainy season. It involves three processes: copulation, fertilization and laying. Copulation: Copulation may occur in water on moist land. It takes about 3-4 hrs. The male and female snails come together facing one another with their right nuchal lobes lie opposite to one another. The penis of the male elongates along with its sheath and its proximal end comes in contact with the male genital aperture and the distal end passes into the mantle cavity of the female and finally to its genital aperture. Now the seminal fluid is shed into the vagina of the female where it migrates to be stored into the receptaculum seminis. After that, both separates. Fertilization: Fertilization is internal in Pila. It takes place in the uterus, where both ova and spermatozoa pass, the former from the ovary and the latter from the receptaculum seminis. Laying: Egg laying starts 2 or 3 days after copulation. The eggs are laid in sheltered places (holes and crevices) or moist land near water. A single female lays eggs at a time. The eggs are rounded, about the size of the pea seeds and covered by whitish shell. They contain a good deal of food. No parental care. Development: The egg undergoes spiral cleavage, total and unequal, very similar in the early stages to that of Nereis. There is no larval stage. A tiny young snail similar to the adult in form, hatches from the egg.