A SEA ANEMONE 143 



Class : Anthozoa. Order : Zoantharia 

 A SEA ANEMONE (METRIDIUM) 



This animal, which is the largest sea anemone along the North 

 Atlantic coast, is often plentiful on rocks, shells, and docks in 

 shallow water. Place an expanded individual in a deep dish of 

 water and observe its shape, color, and method of attachment. 

 The upper end of the columnar body is called the disk ; in its 

 center is the elongate, sUtlike mouth, surrounded by the numer- 

 ous tentacles. The lower end of the animal is called the foot. It 

 is not permanently attached to the substratum ; the animal has 

 some locomotory powers and can slowly move from place to place. 



Study the form of the mouth. Note the thickened lips at each 

 angle of the mouth ; these form a ciliated groove, called the si- 

 phonoglyph, through which the genital products reach the out- 

 side. In some individuals only one siphonoglyph is present. 



Study the surface of the disk and the tentacles. The former is 

 frequently expanded and thrown into folds and lobes. The ten- 

 tacles are elongated diverticula of the disk and are hollow. They 

 are charged with nettle cells and are the principal organs of de- 

 fense and offense. They are also useful in feeding; after the 

 nettle cells have stung the small animals which constitute the 

 food of the sea anemone, the tentacles place them in its mouth. 

 The tentacles are not all the same size, those nearer the mouth 

 being the larger and the older. 



Note the character of the columnar body. It is pierced by 

 small pores through which long, white, glandular threads, armed 

 with nettle cells and called acontia, may be thrust when the 

 animal is irritated. 



Exercise 1. Draw the expanded animal, showing the column and the 

 disk, with the mouth and the tentacles. 



Internal Anatomy. Cut the animal into halves by a longitu- 

 dinal incision passing at right angles to the mouth and from the 

 disk to the foot. The mouth will be seen to open into a flattened 

 tube with more or less corrugated walls, called the gullet. Note 



