Transcript PHYLUM : mollusca

PILA GLOBOSA
(THE APPLE SNAIL)
Dr. Sanjay Kumar Jha
Department of Zoology
P.G.G.C.G. – 42, Chandigarh
SYSTEMATIC POSITION
Phylum Class
Order SuborderFamily Genus Species -
Mollusca
Gastropoda
Prosobranchiata
Pectinibranchiata
Pilidae
Pila
globosa
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.
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.
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.
SHELL (CONTD.)
Histology of the Shell: The shell is
composed of following 3 layers:
a) outer Periostracum,
b) middle Ostracum or Prismatic layer,
c) 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.
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.
a) 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.
BODY FORM (CONTD.)
b) 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.
c) 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
BODY FORM (CONTD.)
PALLIAL COMPLEX: All the organs present
inside the mantle or pallial cavity is called
as organs of pallial complex. It includes:1) 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).
2) 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.
3) 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.
BODY FORM (CONTD.)
4) Ctenidium or gill is a long lamellar organ hanging form the mantle at
the extreme right side of the branchial chamber.
5) 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.
6) Genital duct (vas deferens or vagina) lies on the left side of rectum,
with its aperture a little behind the anus.
7) 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.
8) 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.
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 shellsecreting 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.
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 ribbonlike 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.
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 .
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.
1) 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).
2) 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.
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.
CIRCULATORY SYSTEM
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
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.
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.
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.
NERVOUS SYSTEM
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:
a) 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
NERVOUS SYSTEM (CONTD.)
b) Each buccal ganglion sends off several nerves to the buccal mass, salivary
gland, oesophageal pouch and greater part of oesophagus.
c) Each pedal ganglion sends a large number of nerves to the foot.
d) The left pleural ganglion provides nerves to the parietal wall, left nuchal
lobe, ospharidium, mantle and columellar muscles.
e) The right pleural ganglion sends nerves to the parietal wall, epitaenia,
right nuchal lobe, copulatory organ and columellar muscles.
f) The supra-intestinal ganglion gives off a stout nerve that supplies nerves
to mantle and ctenidium.
g) 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.
SENSORY SYSTEM
The sense organs of Pila include tentacles,
statocysts, ospharidium and eyes.
1) 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.
2) 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
SENSORY SYSTEM (CONTD.)
3)
4)
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
REPRODUCTIVE SYSTEM
The Pila is a dioecious animal with clear sexual dimorphism. SPERM: Pila produces two types
Males have a smaller shell with less dilated body whorl with
of sperms:better developed and functional penis.
a) Eupyrene with twisted
Male Reproductive System: The male reproductive system
nucleus and a single axial
comprises of a testis, vas deferens, a copulatory organ and a
filament and functional.
hypobranchial gland.
(i)
Testis: is a flat, cream-coloured, triangular organ, covered b) Oligopyrene - with a broad,
by a thin membrane and situated on the digestive gland
curved nucleus and with many
(liver or hepatopancreas) in the upper part of the visceral
axial filaments and are sluggish
mass.
and non-functional.
(ii)
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
(iii) Copulatory Organ: It is a long, stout, slightly curved
tapering organ called Penis projecting from the mantle
edge in front of the anus.
Male Reproductive
(iv) Hypobranchial Gland: is and oval thickening with pleated
System
surface at the base of penis sheaths of unknown function.
REPRODUCTIVE SYSTEM (CONTD.)
Female Reproductive System: The female reproductive system
comprises an ovary, oviduct receptaculum seminis, uterus,
vagina and copulatory organ or hypobrabchial gland.
(a) 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.
(b) 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.
(c) 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.
(d) 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.
(e) 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.
(f) Copulatory Organ: similar to the male, but highly reduced.
Female Reproductive System
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 200-800
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.