FLATWORMS 



In the order Polycladida the gut is many-branched and the body very 

 broad and much flattened (Fig. 11.11). Polyclads are exclusively marine and 

 are of interest chiefly because the life cycle includes a ciliated, free-swimming 

 larval stage which becomes an adult through a process of metamorphosis. 



Members of the order Acoela (Fig. 11.12) are small, plump worms, entirely 

 marine, which represent the simplest and apparently the most primitive type 

 of living Platyhelminthes. As implied by the name of the order, the acoels 

 do not have an enteron. There is usually an internal aggregation of gas- 

 trodermal cells surrounded by mesenchyme; food is brought from the mouth 

 into temporary spaces which open to accommodate it until intracellular di- 

 gestion breaks it down. Most of the specializations shown by the acoels involve 

 the reproductive system, which is often surprisingly complex; there is no ex- 

 cretory system. It seems reasonable to believe that the modern acoels are 

 most like the ancient animal type from which all Platyhelminthes have 

 evolved. 



The order Alloeocoela (Fig. 11.12) contains worms which are evidently 

 intermediate in most of their characteristics between acoels and triclads. 



Mouth 



Reproductive 

 complex 



Genital pore 



Brain with eye spots 



Gut, filled with food 



Penis 

 Penis glands 



77 — Pharynx 



Mouth- genital 

 opening 



Fig. 11.12. A, order Acoela: Anaperus gardineri. B, order Alloeocoela: Allostoma aiislria- 

 cnm. (Redrawn, after L. von Graff, from E. Bresslau in W. Kiikenthal and T. Krumbach, 



1933, Ilari/lhuch der ^ooloaie.) 



327 



