72 MANUAL OF ANIMAL BIOLOGY 



the mouth by them. These tentacles, owing to the sensory nerve 

 cells which are scattered through the ectoderm, are very sensitive 

 to stimuli, and small animals which come into contact with them 

 bring about a quick response. There is an explosion of the nemato- 

 cysts, and the prey, paralyzed by the poisoned stinging hairs, is 

 conveyed to the mouth by the tentacles. 



The jellyfish, when not swimming, sinks to the bottom and lies 

 with the oral surface up. In this position the tentacles and the oral 

 lobes of the manubrium are able to secure any bits of food that may 

 fall on them from the surface of the water. For example, bits of 

 clam meat may be dropped on the animal, and their capture and 

 ingestion easily seen. Any food secured by the animal is taken 

 into the enteric cavity through the mouth. Digestion as in Hydra 

 occurs both by intracellular and intercellular methods. A portion 

 of the food passes from the enteric cavity, or ' stomach,' at the base 

 of the manubrium, through the radial and circular canals where it is 

 absorbed, and thus the entire organism is nourished. 



2. Reproduction 



The reproduction of the colonial Obelia is entirely asexual, and 

 may take place either by a general growth of the hydrorhiza or by 

 the formation of numerous medusa buds in the gonangia. 



In the first type of reproduction mentioned, the hydrorhiza 

 of an Obelia colony grows along the substratum to which the colony 

 is attached. At certain intervals erect hydrocauli, which bear the 

 hydranths, are formed from the hydrorhiza. The nutritive 

 polyps thus formed furnish an additional supply of food material 

 so that the hydrorhiza can continue to grow. 



In the formation of medusa buds, Obelia exhibits a highly 

 interesting type of asexual reproduction which leads directly to the 

 consideration of the phenomenon of alternation of generations, 

 inasmuch as the free-swimming medusae, produced in the repro- 

 ductive polyps by asexual budding, develop into sexual individuals 

 possessing either ovaries or testes. The gonads of either sex are 

 attached, as previously stated, to the radial canals of the medusa, 

 and no external morphological differentiation is to be noted between 

 the two sexes. When sexually mature, the eggs break out of the 

 ovary of a female medusa directly into the sea water. Similarly, 

 from the testes of a male, active sperm are liberated. These swim 

 freely through the water to the eggs, and fertilization, followed by 



