OBELIA 71 



portion of the water which fills this space is thus driven out with 

 considerable force through the central opening in the velum. As a 

 result, the animal is driven forward ; that is, aborally, at each con- 

 traction. As soon as the water has been forced out the contraction 

 ceases, the oral space again assumes normal size, and is filled with 

 water as at first. These rhythmic, swimming movements in the 

 medusa thus present a marked degree of coordination between the 

 nerve and muscle elements. 



Endoderm and Mesogloea. The endoderm cells form the 

 lining of the main enteric cavity in the manubrium as well as its 

 continuation in the radial and circular canals. These cells have 

 the same structure and function as in Hydra. The non-cellular, 

 secreted mesogloea, lying between the ectoderm and endoderm, has 

 the same general character in all Coelenterates, but in the medusa 

 and other jellyfish a very much greater amount is present than in 

 Hydra. It really forms the larger part of the body wall in these 

 types ; that is, the body wall may be said to consist of a thick layer 

 of mesogloea, covered on the outside by a thin layer of ectoderm 

 cells and on the inside by a thin layer of endoderm cells. 



While at first glance one fails to observe any great similarity 

 between the structure of Hydra and that of a free swimming jelly- 

 fish form, nevertheless, both are built upon the same fundamental 

 polyp plan, and the various parts of the body occupy practically 

 the same position in the two forms, the greatest difference between 

 the two being the relatively large amount of mesogloea present 

 in the medusa type. (W. f. 38.) 



C. Life Processes of Obelia 

 1. Metabolic Processes 



In the attached, colonial stage of Obelia, the processes of nutri- 

 tion, respiration, and excretion are so nearly like those of Hydra 

 that no special description is necessary. Any surplus of food 

 from the nutritive polyps goes by way of the coenosarc to the 

 support of the colony as a whole, particularly for the development 

 of the stalk and the reproductive polyps. 



In the free-swimming medusae there are certain methods of food 

 capture which should be noted. The jellyfish secures its food in 

 two ways. When it is moving through the water, the food which 

 comes in contact with the tentacles is captured and conveyed to 



