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ESSENTIALS OF ZOOLOGY 



enteron have many lateral, blind extensions or pockets called diver- 

 ticula which greatly increase the surface exposure of the organ and 

 project among most of the other tissues of the body. The whole 

 arrangement represents a complicated gastrovascular cavity whose 

 wall is endodermal. 



The excretory system is new to our study and is composed of a set 

 of tubules which relate themselves to all parts of the body. There 

 are two principal, longitudinal, coiled tubules, one along each side 

 of the body, which receive many small branches and open by minute 

 pores located just posterior to the eyespots, and by several other 

 pores along the length. All of the smaller branch-tubules have at 

 their blind ends a flame cell which is hollow and contains a mass of 



Nucleus 



Cilia 



^^ Excretory tubule 



Pig. 118. — Flame cell of planaria. 



long cilia that are continually beating in a direction toward the 

 tubule, the movements appearing something like a flickering flame. 

 The cellular walls of the tubules as Avell as the flame cells arise in 

 the mesoderm. Under strict definition, some authors object to calling 

 this arrangement a system. 



Another mesoderm organization is the muscular system. It is com- 

 posed of an outer circular layer just under the epidermis ; an outer 

 longitudinal layer just medial to the circular layer ; oblique bundles 

 of fibers; and at the medial margin of the mesoderm is another ir- 

 regular, internal, longitudinal layer just medial to a circular layer. 

 By the alternate activity of these layers the animal is capable of 

 great extension and contraction, 



