172 PHYSIOLOGY 



Segmented Worms. — The segmented worms have al- 

 ready been fully described. Some of them are free and 

 some live in tubes. Some live in the water and have 

 well-developed heads bearing tentacles and eyes. Fleshy 

 outgrowths on the surface of the body are used for 

 swimming. The young undergo a metamorphosis simi- 

 lar to that of the molluscs. Leeches have sucking discs 

 and they have been used by physicians to draw blood 

 from the body when it was thought that a disease might 

 be cured by weakening the patient. 



Molluscoidea. — The molluscoidea were once thought 

 to be molluscs. They have an alimentary canal that is 

 open at both ends and there is a circle of tentacles around 

 the mouth. Some of them produce large colonies by 

 budding. Some have a bivalve shell through which a 

 stalk projects which is fastened to some support. All 

 of them reproduce sexually. 



