108 INVERTEBRATE ZOOLOGY 



likewise resemble those of a ctenophore. Furthermore, in its 

 life history Coeloplana passes through larval stages which bear 

 ciliated combs (Fig. 59 B) of a form which have been thought 

 to be distinctive of the ctenophores. Ctenoplana closely resem- 

 bles Coeloplana except that it carries the rows of combs through 



A B c 



Fig. 60. — General organization in the three classes of Turbellaria. A, a 

 Rhabdocoel; B, a Polyclad; C, a Triclad. The nervous system is shown in 

 black; digestive system in cell outlines and nuclei; .4, testes closely stippled, 

 yolk gland coarsely stippled, ovary at posterior extremity on right; B and C, 

 male reproductive system stippled on left side of body, female organs on right. 

 {From von Graff). 



to its adult condition. Thus Coeloplana resembles a flatworm 

 and Ctenoplana a ctenophore, while each combines confusing 

 characteristics of both groups. 



Class Turbellaria 



The Turbellaria range in size from minute, microscopic forms 

 to some which attain several inches in length. The polyclads 

 and triclads are usually distinctly flattened and in outline 

 range from disc-shaped to lanceolate and long ribbonlike 



