336 



APPLIED BIOLOGY 



291. Coral-animals. Most important to man of all the 

 allies of Hydra are the animals whose skeletons form a large 

 part of many coral islands. (Look in a textbook of geography 

 for a list of coral islands.) In order to understand how coral- 

 animals form their hard skeletons, it is necessary to examine 

 a specimen of a sea-anemone; these resemble coral-animals 



in structure, but do not 

 secrete skeletons. 



(D) Living sea-anemones are 

 attached to rocks and piles be- 

 low low-tide mark. Schools 

 near the sea-shore could keep 

 small ones in a salt-water 

 aquarium ; but most schools 

 must depend upon museum 

 specimens preserved in forma- 

 lin. An entire sp'ecimea, one 

 split longitudinally, and one 

 cut transversely, will show the 

 essential points of structure. 



In an entire sea-anemone, 

 notice : the cylindrical body, 

 the base, the crown of small 

 tentacles, and the mouth in 

 the center of the crown of 

 tentacles. 



In a sea-anemone which has 



been split lengthwise, notice : the mouth opening into a long tube 

 (esophagus) extending down to near the base, and the delicate 

 membranes connecting this tube with the body-wall. 



In a specimen cut transversely, notice : the esophagus in the center, 

 the pairs of membranes (mesenteries) which extend from the esopha- 

 gus to the body-wall, and the pairs of shorter membranes which 

 extend inwards a short distance from the wall. All the spaces be- 

 tween the membranes extend up into the tentacles and down to 

 the opening of the esophagus (compare with the longitudinal section). 

 Foods taken in through the esophagus may pass anywhere between 

 the membranes, and be digested. In brief, the digestive cavity of 

 a sea-anemone is more complex than that of a hydra in that it is 

 subdivided by many pairs of membranous partitions. 



FIG. 104. Common sea-anemone con- 

 sisting of a cylindrical body with a 

 crown of numerous small tentacles 

 surrounding the mouth. (After Em- 

 erton.) 



