1G0 PHYSIOLOGY 



the animals, for it obscures them from their enemies 

 when it is poured into the water. Many molluscs 

 do an enormous amount of damage by boring w T ith a 

 file-like organ called the lingual ribbon into boats 

 and piles. Some of the larger forms like the squid 

 have been known to swamp boats and cause the loss 

 of life. 



Characteristics of the Group. — Like other animals 

 molluscs are made of living cells which are grouped in 

 well-defined tissues. They are irritable, they assimilate 

 food and oxygen, and they reproduce their kind. 

 Through the activity of the muscular and nervous sys- 

 tems they are able to move from place to place, and to 

 open and close the shell. Digestion is accomplished in 

 the alimentary canal with the help of the juices secreted 

 by the liver. The canal is a tube w T ith a single enlarge- 

 ment called the stomach. It is open to the outside at 

 both ends. As it is too long to pass directly through the 

 body it is twisted on itself. The great length of the 

 tube ensures a large digestive surface. 



Oxygen enters the body at a definite point through a 

 specially adapted membrane found on the surface of the 

 gills. Water carrying oxygen in solution passes over its 

 surface. By osmosis oxygen enters and carbon dioxide 

 passes off. 



Oxygen and digested food products are carried to 

 every part of the body by means of a well-developed cir- 

 culatory system. 



Wastes are given off through the activity of kidney- 

 like organs called nephridia which take the form of con- 

 voluted tubes open at both ends. One end opens to the 



