124 AN INTRODUCTION TO THE STUDY OF FOSSILS 



oesophagus into the large digestive cavity. The digestive cavity 

 constitutes the entire interior of the body, but is partly divided 

 into compartments by radial partitions {mesenteries, Fig. 46, me.) ; 

 each intermesenteric compartment is continuous with a hollow 

 tentacle. 



The polyp preys upon marine animals, — small crustaceans, 

 worms, etc. These it catches with its tentacles. Each tentacle 

 ends in a white knob and is speckled all over with white warts, 

 each one of which is a cluster of nettle-cells (nematocysts). 

 When one of these cells is touched it sends out a long barbed, 

 thread-like tube, through which poison is forced. This multi- 

 tude of nettle-cells, piercing the prey, paralyze it. The tentacles 

 then push this food into the mouth, through which it enters the 

 digestive cavity. The white cords (mesenteric filaments) 

 edging the mesenteries are crowded with nettle-cells ; these 

 cords, wrapping themselves about the prey, complete the killing, 

 while the many gland cells covering the cords pour out a digestive 

 fluid. 



After the digestion of the food, the waste is thrown out 

 through the mouth ; the digested portion mingled with the sea 

 water is forced by the contractions of the body through the 

 radial compartments and into the tentacles, the various cells 

 taking what they need for growth and repair. The entire body 

 acts thus as a huge blood-vessel, and since the digestive cavity 

 connects freely through the mouth with the water of the ocean, 

 the " blood " consists of sea water mingled with nutritive por- 

 tions of food. 



Respiration is performed mainly by the tentacles ; these being 

 thin and containing the body fluid or blood, permit a free inter- 

 change of the excess waste carbon dioxid, etc., of the body for 

 the free oxygen in the sea water. 



Well-developed muscles are present. There is a sphincter 

 muscle surrounding the body at the top which draws in the ten- 

 tacles and closes the mouth much as a string may close a bag by 

 gathering it together. Muscular layers line the mesenteries on 



