Chap. 24 



CTENOPHORES COMB JELLIES OR SEA WALNUTS 



495 



Fig. 24.2. Diagram of the digestive system of a ctenophore. 1, statocyst (sense 

 organ of balance); 2, anal pore; 4, aboral canal; 5, stomach; 6, transverse canal; 

 11, meridional canal; 12, pharynx; 22, mouth. Several labels omitted. (Courtesy, 

 Hyman: The Invertebrates, vol. 1. New York, McGraw-Hill Book Co., 1940.) 



mouth forward. The motion in each row usually begins with the last comb at 

 the aboral end and goes forward like a wave. Each beat is a strong backward 

 flap of the comb which drives the water out from under it and helps to push 

 the animal forward. If a ctenophore strikes head on against an object, the beat 

 is at once reversed. Experiments have shown that the movement of the combs 

 is controlled by the nerve cells that lie beneath the rows. At the rear, or aboral 

 pole, of the body there is an area of nerve and sensory cells. In the center of 

 this is a pit containing a sense organ, the statocyst, which holds a little cluster 

 of limestone particles supported by tufts of cilia that are connected with sen- 

 sory cells. This is believed to be a balancing or steering mechanism since any 

 turning of the body causes the limestone to rest more heavily on one or another 

 of the tufts. This would stimulate the sensory cells, and the stimulus carried to 

 the combs would cause them to beat faster on one side than the other. From 

 this polar area a nerve net extends through the body and is concentrated in 

 eight strands, one under each row of combs. 



On each side of the body is a sac into which the tentacles can be retracted. 

 The latter are often very long in proportion to the body and are the cteno- 

 phore's catch-traps for small animals. Their epidermis consists largely of glue 

 cells (colloblasts) each of which in action is a combination of a lasso and glue- 



