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The biology of snails

The shell The body Reproduction Locomotion Nerve system A snail life:food,enemies and hibernation.

The body

The body of a snail consists of foot(sole),a head and a coiled visceral mass that is located in the shell. Movement takes place by means of expanding and extracting muscles in the foot.This is clearly visible when you place a snail on a glass surface,and look from below.Mucus glands located in the anterior(front)part of the foot secrete mucus. There are also mucus glands on the rest of the body protecting the snail against loss of water. Two pairs of tentacles are placed on the head,in prosobranch gastropods there is only one pair.The upper pair bears the eyes. The shell is secreted by a thick fold of skin,called the mantle. The shell is connected to the body throug a strong muscle which is attached to the columella.It has branches to the head and tentacles.Contracting this muscle enables the snail to withdraw in its shell. Inside the shell it forms the mantle cavity,which holds the heart,kidney and lung.In Prosobranchs the mantle cavity is open at the front,and in the roof is a plumate or a pectinate gill. In terrestrial Prosobranchs the gill has been lost. The mouth has a tongue called radula.In the top of the mouth is a hard ridge and food is being mashed between the radula and this ridge.The front teeth of the radula wear very fast but the radula grows from the backend. [picture:University of Dundee Museum Services]

Near the opening of the mouth salivary glands release digestive enzymes.The salivary glands secretions moisten the digested food and envelop them,thereby making easier it for the food to go into the oesophagus. The oesophagus ends in the stomach.The intestines release large quantities of a brown digestive juice in the stomach.The intestinal gland fills up most of the space in the visceral sac. The intestinal gland consists of smaller and bigger follicles. A steady back and forth movement of the digestive juices between stomach and intestines enhances the process of absorption of the food. The movement of the digestive juices is caused by the muscles of the intestinal gland and ciliae. The digested food flows over the livercells which absorb the food. The smaller intestine starts at the visceral sac it follows the edge of the kidney and enters the pulmonary cavity.It ends near the pneumostome(breathing pore). In the smaller intestine is on the point of exit from the visceral sac a deep groove.This groove which is coated with cilae takes over all non-absorbed solids and directs it into the small intestine. There the solids are compressed and enveloped with a layer of slime after which they leave the body. The kidney of a snail lays as a yellowish,trangular,organ in the rear segment of the lung.One of the sides of the kidney runs parallel to the intestine,the other side is arranged closely to the pericard.The kidney consists of two parts:the kidney cavity where the excrements are secreted and the primary urethra,in which the first part of the discharge of the excrements takes place. This goes then to a second segment,the secondary urethra,where it leaves the body. Snails living on the land are uricotelic,this means that to preserve water they excrete almost solid uric acid.Snails living in the water excrete ammonia, and are not uricotelic

Body:respiration and blood circulation The blood system in snails is open,with blood spaces and no veins. The pigment is colorless,and is called haemocyanin,which contains copper. The relative weak heart consists of a single thick walled ventricle and a single thin walled auricle.The blood takes oxygen from the lung and transports it to the auricle,and then to the ventricle. The ventricle releases blood with oxygen in the arteries,after which the blood goes to the tissues.The tissues take oxygen and food from the blood and excrete waste products.The blood flows back to the lung by means of blood spaces instead of veins. The so-called lung(pulmonary cavity) lays on the inside of the roof of the visceral-sac. Through the breathing-pore,the pneumostome,oxygen reaches the snails lung. When the pneumostome is open the roof and bottom of the pulmonary cavity are close together.When the bottom goes down the oxygen can flow into the lung. Then the pneumostome closes and the bottom goes up,pushing the oxygen in the body.It is comparable to the midriff in mammals.

Locomotion

Snails and slugs move by contracting and relaxing 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.Normally the sole is not divided,but some snails like Pomatias elegans have a sole which is split lenghthwise. Both halves can move seperatly,creating a sort of pedallike locomotion. Glands produce mucus which allows snails to move over rough or sharp material and crawl on vertical surfaces. It's also used for protection and navigation.Snail trails are broken while slug trails are continious.

Nerve system

The central nerve system of snails consists of paired nerveknots or ganglia.They have different functions. The cerebral ganglia for the senses in the head,the buccal ganglia for the mouth,the pedal ganglia for the muscles in the foot,the pleural ganglia for the mantle, and the intestinal ganglia for the organs. A seperate,not paired, ganglia is located under the intestines, this is named the visceral ganglion.The ganglia are connected lengthwise,and there are also connections across the body. The pallial cavity's nerve knots are called pleural ganglia.The lateral ganglia are called the parietal ganglia.

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