THE ANIMAL KINGDOM 7 



disguise a possibly primitive bilateral symmetry. But in some of the 

 ctenophores, which are the small jellyfishes whose enormous numbers 

 make the ocean phosphorescent at night, there are beginnings of bilateral 

 symmetry, especially in the tentacles. 



The importance of the group for the student of animal evolution lies 

 in the fact that the two-layered gastrula stage of the embryo, of all higher 

 forms, duplicates in all essentials the two-layered adult body of the simpler 

 coelenterates. Coelenterates, therefore, are taken to lie on or close to 

 the main trunk of the ancestral tree of higher animals. 



Taxonomists recognize some seven thousand species. 



Phylimi 4. PLATYHELMINTHES 



The platyhelminths or flatworms are well described by their name. 

 Included in the group are such diverse forms as liver flukes, planarians, 



CNTERON 



RHABOOCOELE "UJRBQJ-ARIAN 



VENTRAL NERVE 

 OVARY. 



•YOLK PRODUCING FOLLICLES 



lOR INTESTINE 

 ■PENIS 



OVIDUCT 



VAGINA 

 -GENITAL PORE 

 ■EGG IN UTERUS 



TESTI 



VAS DEFERENS .. ,,™,„„ 



TRICLAD TURBELLARIAN. 

 Pig. 5. — Diagrams of Rhabdocoel and Triclad Turbellarians, types of the Phylum 

 Platyhelminths. The group illustrates the beginnings of head and of bilateral sym- 

 metry. (Redrawn from Van Cleave, after von Graff.) 



tapeworms, and the nemerteans, which are nearly all marine. The 

 flattened bodies are bilaterally symmetrical, with a well-marked dorso- 

 ventral and antero-posterior differentiation. Nervous centers tend to 

 concentrate as a brain at the anterior end. There is no body-cavity, and 

 usually no anus except in nemerteans. The alimentary canal shows a 

 tendency to form a series of lateral diverticula, thus suggesting the begin- 

 nings of the coelomic pouches and gill-pouches of higher animals. 



Muscle bundles extend in various directions within the thickened 

 mesogloea. Although most flatworms have neither true circulation nor 

 blood vessels with walls, lacunar spaces filled with liquid occur, and are 



