[Classification][Physidae][[Hydrobiidae] [Assimineidae] [Vivipariidae] [Bithyniidae] [Valvatiidae]
[Acroloxoidae][  Lymnaeidae][ Planorbidae][Pomacea][Pila][Lanistes][Saulea] [Afropomus] [Marisa][Asolene]

Freshwater snails

Welcome to the new introduction page of the freshwater snails.This part of the site
has information about the biology of freshwatersnails.
On different pages you will find the species within their Superfamilies and
Families.

Biology
Breathing
Nervous system
Blood circulation
Reproduction
The shell

Introduction to the freshwatersnails

You will find freshwatersnails all over the world,from freezing conditions to hot,sunny areas in the tropics.There are many different kinds of habitats:
large lakes,flowing streams,calm ponds,brackish pools or bodies of water which turn into muddy areas in the dry season.
Freshwatersnails have adapted to all those conditions.
Freshwatersnails fall in two categories: Prosobranchs or Pulmonates.
The Prosobranchs have gills in their mantle cavity and an operculum on the
back of their foot.They have separate sexes.
The Pulmonates have no gills and are hermaphroditic.

Biology of Ampulariidae

The body of the snail can be divided in three parts:head,foot and visceral mass.The body
is attached to the shell with a strongly developed muscle:the columellar muscle.
This starts from the foot and is attached to the columella.The visceral mass is 
covered by a skin and forms the pallium or the mantle.It forms a cover over the anterior part of the animal and envelopes the head when it is 
withdraw in the shell.
The edge of the mantle is thickened and has the glands that secrets the shell.
The headbears on both sides a labial palp,or the first pair of the tentacles.
Behind he snout are the second pair of tentacles,these structures are filamentous.
Thick at the base but ending in thin threads.They are about 2 inches long when
fully extended.On small,blunt stalks,the ommatophores,are the eyes placed.
The mantle cavity,also called the visceral sac,contains the heart,kidney and the other
organs.It is closed in pulmonate snails except for the breathing pore,the pneumostome.
The roof is covered with a network of blood vessels and forms a lung.
In prosobranch snails it is open at the front and on the left is a pectinate gill.
Many freshwater snails have a secondary gill near the pneumostome.
The digestive proces starts with the mouth,food is rasped with radula.
This consists of very small teeth and never stops growing from the back.
The feeding canal has three sections:the fore-gut,the mid-gut and the hind-gut.
The mouth is located at the end of the snout.It has no true lips,just plicate edges.
The first part of the feeding canal is formed by the buccal mass,the next part
is the oesophagus.The oesophagus enters the visceral mass into the stomach.
In the visceral mass the oesophagus runs through the right side of the pericardium
along the edge of the digestive gland. 
The stomach has three chambers:
the posterior chamber(gizzard),the anterior chamber(vestibule) and the style sac.
Enzymes and slime are added to the food in the posterior chamber,in the anterior
chamber big food parts are filtered out.The food is transported to the intestine,
where it is taken up through the walls.
The intestine ends in the anus.

Freshwatersnails move by expanding and extracting muscles in the foot.
There are two sets of muscle fibers,each performs a different task.When moving forward 
one set contracts pulling the snail from the front and pushing it off toward the back.
At the same time the second set pulls the outer surface of the sole forward.


Breathing
The Prosobranchs have a gill which is attached to the roof of the mantle cavity.
 
the lung                                              the gill
The lung is situated on the left side of the mantle cavity and is filled with air.
Like landsnails they have a pneumostome which is open when they leave the water.
The snails in the Family Ampullariidae have a siphon which enables them to take
in air while submerged.This is very long in the genus Pomacea,and much shorter in the
other genera.

The Pulmonates having no gills need to come to the watersurface to get oxygen.
Pondsnails(Lymnaeidae)can fill their mantle cavity with water and are able to extract oxygen from the water.Although Lymnaea truncatula
and L.palustris which live in marshes are usually found living out of the water.
Lymnaea peregra,a common species,can use different methods depending in which conditions
they live.Snails living near the edge of lakes come sometimes to the surface to get air,
other snails living further have a bubble of gas in the mantle cavity,and snails living
in the deepest part of lakes have a mantle cavity which is always filled with water.
The tentacles of Pondsnails have many fine arteries which function as a sort of gill.

Ramshorn snails(Planorbis)have blood with haemoglobin,this helps the snail getting oxygen from the water.Especially in poluted conditions.

Nervous system


The nervous system of freshwatersnails(like landsnails)consists of nerveknots
called ganglia.From the head two pairs of connectives stretch through the body.
The pedal connective goes to the food and the pleuroviscal connective to the
visceral mass.In contrast to the other ganglia,the viseral mass ganglion is
not paired.The pedal ganglia coordinate locomotion,the cerebral ganglia are located
in the head,the pleural ganglia coordinate lung and mantle function,the visceral
ganglia coordinate kidney and other organ functions.

Blood circulation
The heart of a snail consist of a single ventricle which has a thin wall and a thick
walled auricle.The auricle receives blood rich in oxygen from the lung.This goes to
the ventricle,which releases it in the arteries.From there it flows freely to the
rest of the tissues,which take oxygen and food from it.
The ampulla compensates the bloodpressure in the aorta when the ventricle contracts.
The heart and the ampulla are enclosed in the pericard which connects to the posterior
kidney through a canal.The posterior kidney receives the urine from the pericard.
When the blood flows through the anterior kidney it goes to the mantle vein which
takes it back to the gill and lung.From there it goes back to the heart.
The anterior aorta leads from the heart to the head and foot,the posterior aorta
connects with one artery to the digestive organs and one artery to the rest of
the organs.
All parts of the body are being served with blood from the ventricle through the major 
and visceral aortae.The major aorta takes the blood to the head,part of the mantle,
the buccal mass,the oesophagus,the copulatory organ and the columellar muscle.
The visceral aorta takes blood to the whole of the visceral mass.Blood is collected
form the body into two main sinuses:perivisceral and the peri-intestinal.
From there the blood goes into the ctenidium,pulmonary sac or into the renal organ.
During underwater breathing most of the blood from the peri-visceral sinus goes into the
ctenidium and is oxygenated there.But during breathing above water this blood goes into the pulmonary sac for aeration.
This blood reaches the auricle through the ctenidial or the pulmonary vein.
Blood from the peri-intestinal sinus goes into the anterior or posterior renal chamber.
Blood going into the anterior renal chamber loses its waste products there and then passes into the ctenidium 
where it is oxygenated or into the posterior renal chamber.
The posterior renal chamber gets blood from the peri-intestinal sinus directly or through
the anterior renal chamber.There the blood loses its nitrogenous content and enters the
auricle without being oxygenated.The auricle receives non-oxygenated blood from the
posterior renal chamber and oxygenated blood from the ctenidium and the pulmonary sac.
This mixed blood goes into the ventricle where it is transferred to all the body-parts.
The blood circulation takes also care of stretching the tentacles and the penis.

H = Head - cephalic hemocoel
F = Foot hemocoel
E = Oesophagus
Vm= Visceral mass (hemocoel) 
aK = anterior (front) Kidney
pK = posterior (back) Kidney
A = Ampulla Au = Auricle
V = Ventricle
Aa = Aorta anterior
Ap = Aorta posterior 
vS = Visceral sinus
fS = Foot sinus