1 84 



MANUAL OF ELEMENTARY ZOOLOGY 



definition which of the repeated structures we call indi- 

 viduals. 



Other kinds of polyps and medusas are known than those 

 coeienterata wn i c h are represented by Hydra and Obelia. 

 Sea-anemones (Actiniaria) are polyps (Fig. 397) 

 of rather complex structure, the edge of the mouth being 

 tucked in to form a gullet lined with ectoderm, while folds 

 or mesenteries of endoderm stretch across the enteron from 



7KZLS? 



Fig. 116. — A diagram of a vertical 

 section of a sea-anemone. 



ect., ectoderm of tentacle ; ect.g., ectoderm 

 lining gullet; end., endoderm; ent., 

 enteron ;g., gullet ; gon. , gonad ; in./., 



^I mesenterial filament ; mus., longitudinal 

 or "retractor" muscle; mus'., oblique 

 or "parietal" muscle ; st.l., mesoglea ; 

 ten., tentacle; i°mes., primary mesen- 

 tery. 



the body - wall, some of them {primary mesenteries) 

 reaching the gullet. The testes and ovaries are developed 

 from the endoderm of the mesenteries, and their products 

 are shed through the mouth. The ectoderm is ciliated, 

 and the mesoglea contains cells of various kinds derived 

 from the ectoderm and endoderm. 1 At the ends of the 



1 Though these cells do not form a third layer or " mesoderm " such 

 as is found in the higher animals (p. 230), yet they foreshadow that 

 element of the mesoderm which is known during development as 

 "mesenchyme" (p. 495). 



