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The Garden Snail

Classification:                       

Phylum Mollusca - soft bodied animals with chalky shells
Class Gastropoda - broad flat muscular foot
Genus Helix - refers to the helical coil of the shell
Species Aspersa - the common garden snail (cf the edible snail - Helix pomatia)

  

Nutrition:

  1. The garden snail is a herbivore (vegetarian)

  2. The mouth has a ribbon-like rasping tongue called a radula.

  3. The radula is covered with fine horny curved teeth.  The radula is rubbed back and forth against the leaf and tears off small pieces of vegetation in a similar manner to the action of a rasp.

  4. The crop contains symbiotic bacteria which digest the cellulose in the plant matter.

  5. Useful food material is absorbed in the intestine. 

  6. Undigested food passes out the anus.

 

Respiration and gas exchange:

  1. The snail has a simple lung.

  2. The opening to the lung can be open and closed by a muscular valve - the closing of the opening helps reduce water loss.

  3. The diaphragm is used to fill the lung with air and helps force oxygen into the blood

  4. O2 is carried away from the lungs in the blood.  In molluscs O2 is carried by haemocyanin (similar to haemoglobin in mammals).

  5. CO2  passes from the blood to the air in the lungs and is exhaled when the valve opens again.

 

Blood system:

  1. Snails have an open circulatory system with a simple two chambered heart.

  2. The heart pumps the blood into the aorta and then into smaller arteries.  From these it empties into the body cavity.  This cavity is called the haemocoel.

  3. The blood is bluish due the haemocyanin.

  4. The blood passes from the body cavity into vessels surrounding the lung where O2 is absorbed and CO2 is released from the blood.

  5. The blood re-enters the heart via the pulmonary vein.

 

Nervous system:

  1. Sense organs are eyes sensitive to light and dark - they are on the long tentacles.

  2. Chemosensors are found on the shorter tentacles.

  3. Snails have no hearing.

  4. Ganglia (singular = ganglion) contain groups of nerve cells.

  5. The cerebral ganglion acts as a 'brain'

  6. The pedal ganglion coordinates movement.

  7. The senses and ganglia are concentrated at the front of the body - this helps the snail to sense and react to danger as fast as possible!

 

Support and movement:

  1. The blood in the haemocoel helps support the internal organs.

  2. The shell is made of Calcium carbonate and provides camouflage, protection, and some support of internal organs.

  3. The snail moves by ripples of muscle contraction moving down its foot.  There is a gland behind the mouth which releases mucus to reduce friction and prevent damage to the foot.

  4. The shell grows from the outer rim where it joins to the mantle.  The shell is wound around a central support called the columella.

 

Excretion:

  1. Excretion is the removal of waste products from cell reactions.

  2. CO2 is excreted through the lung.

  3. A kidney-like organ removes nitrogen containing wastes and changes them to uric acid which is then excreted.

  4. Uric acid is insoluble and so does not use water when it is excreted - this is important to help conserve water.

Reproduction:

  1. Snails are hermaphrodites

  2. They have an ovotestis which produces both eggs and sperm cells.

  3. Fertilisation occurs after stimulation from a 'love-dart'.  Small packets (= spermatophores) containing the sperm are exchanged between snails.

  4. The ovotestes now produce eggs which are fertilised by the sperm in the spermatophores.  The eggs are laid on the ground or in litter.

 

Habitat, niche and environment:

  1. Habitat is the place where an organism lives. eg the snail lives in grass, hedges, gardens

  2. Niche is the role of an organism in its community.  The niche also includes the adaptive features the organism has which help it to survive and fulfil its role in the community. 
    eg The snail's niche is as a herbivore, it provides food for birds and hedgehogs, it is a pest to humans.  A few of its major adaptive features are a hard shell, rasping radula and mucus producing gland.

  3. Environment is the conditions in which the organism lives.  This can include the effects of both living and non-living things.
    eg.  The snail's environment is cool, dark and moist.

 

 

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