182 



THE INDIVIDUAL ORGANISM 



effect. The animals of this group may be said to have reached the tissue 

 level of construction. 



The small, contractile, many-armed animal known as Hydra (Fig. 

 13.5) is a good example of the group. It is saclike in construction, the 

 body wall being made up of two layers of cells surrounding a digestive 

 cavity; this cavity is the only one in the body and has but one opening, 

 the mouth. 



A step in advance of the coelenterate type of structure is shown by the 

 small, free-living flatworm, Planaria (Fig. 13.6), in which definite organs 



eye 



pharynx 



outh 



excretory 

 pore 



excretory 

 tube 



cephalic ganglion 



nerve cord 



peripheral 

 nerves 



Fig. 13.6. Diagrams of the fresh-water flatworm Planaria, showing (from left to right) the 

 digestive, excretory, and nervous systems. (From Parker and Haswell, Textbook of Zoology, 

 by permission The Macmillan Company.) 



are present. This worm resembles Hydra in having only a single cavity in 

 the body, with a single opening, the mouth. However, in addition to the 

 two cell layers corresponding to those that form the body wall in Hydra, 

 there is present in Planaria a third layer between them, which makes up 

 most of the bulk of the body and takes part in the formation of various 

 organs. Among the simple organs of Planaria are a muscular pharynx 

 for ingesting food, nerve cords and light-sensitive eyespots, excretory 

 apparatus, and reproductive organs. Even in this lowly worm there is a 

 suggestion of organ systems in the arrangement of the digestive, excre- 

 tory, and nervous structures. 



Organ-system level. Above the plane of the flatworms nearly all the 

 Metazoa have attained the organ-system level of construction. In these 

 higher Metazoa we not only find cells functioning individually, cell 



