Invertebrates 



FIG. 25. 



CHAPTER VIII. 



ACTINOZOA. SUB-CLASS I. ZOANTHARIA : SEA 

 ANEMONES. 



Structure of a Sea Anemone. In many of the 

 rock-pools around our shores there are to be found 

 the exquisite forms which are the types of this 

 class, and which, like animated flowers, may be 

 seen expanding their sensitive petal-like tentacles 

 in search of the materials that constitute their prey. 

 On the retreat of the tide those that are left un- 

 covered by the water contract, and appear as little, 

 rounded, firm, gelatinous masses, attached to rocks 

 and stones by means of a flat suctorial disk. 



The body of one of 

 these when expanded is 

 somewhat cylindrical, hav- 

 ing a free extremity which 

 bears the mouth, and an 

 attached end, which is 

 usually capable of volun- 

 tary detachment. This 

 extremity is sometimes 

 called the foot. On mak- 

 ing a transverse section 

 the body appears^ like a 

 double tube ; the outer 

 tube is the body wall, the 

 inner bounds the stomach, 

 and between them is the body cavity into which the 

 stomach-sac opens below by a narrow aperture. 



Vertical section of common Sea ane- 

 mone, Actinia mesembryanthe- 

 mnm. 



m mouth, tnf primary mesentery 

 iti 1 secondary mesentery, e ecto- 

 derm, e 1 endoderm, / tentacle, t', 

 ovary, d disc of attachment, s 

 body-cavity. 



